diff --git a/server/src/__tests__/authz-company-access.test.ts b/server/src/__tests__/authz-company-access.test.ts
index 0928d62bbd2..b4b713592ad 100644
--- a/server/src/__tests__/authz-company-access.test.ts
+++ b/server/src/__tests__/authz-company-access.test.ts
@@ -46,6 +46,35 @@ describe("assertCompanyAccess", () => {
     expect(() => assertCompanyAccess(req, "company-1")).toThrow("Viewer access is read-only");
   });
 
+  it("rejects writes when membership details are present but omit the target company", () => {
+    const req = makeReq({
+      method: "POST",
+      actor: {
+        type: "board",
+        userId: "user-1",
+        source: "session",
+        companyIds: ["company-1"],
+        memberships: [],
+      },
+    });
+
+    expect(() => assertCompanyAccess(req, "company-1")).toThrow("User does not have active company access");
+  });
+
+  it("allows legacy board actors that only provide company ids", () => {
+    const req = makeReq({
+      method: "POST",
+      actor: {
+        type: "board",
+        userId: "user-1",
+        source: "session",
+        companyIds: ["company-1"],
+      },
+    });
+
+    expect(() => assertCompanyAccess(req, "company-1")).not.toThrow();
+  });
+
   it("rejects signed-in instance admins without explicit company access", () => {
     const req = makeReq({
       method: "GET",
diff --git a/server/src/routes/authz.ts b/server/src/routes/authz.ts
index 5efa350ecff..ec6c49621dc 100644
--- a/server/src/routes/authz.ts
+++ b/server/src/routes/authz.ts
@@ -51,16 +51,12 @@ export function assertCompanyAccess(req: Request, companyId: string) {
     }
     const method = typeof req.method === "string" ? req.method.toUpperCase() : "GET";
     const isSafeMethod = ["GET", "HEAD", "OPTIONS"].includes(method);
-    if (!isSafeMethod && !req.actor.isInstanceAdmin) {
-      const memberships = req.actor.memberships ?? [];
-      const membership = memberships.find((item) => item.companyId === companyId);
-      if (!membership) {
+    if (!isSafeMethod && !req.actor.isInstanceAdmin && Array.isArray(req.actor.memberships)) {
+      const membership = req.actor.memberships.find((item) => item.companyId === companyId);
+      if (!membership || membership.status !== "active") {
         throw forbidden("User does not have active company access");
       }
-      if (membership?.status !== "active") {
-        throw forbidden("User does not have active company access");
-      }
-      if (membership?.membershipRole === "viewer") {
+      if (membership.membershipRole === "viewer") {
         throw forbidden("Viewer access is read-only");
       }
     }