diff --git a/server/env.ts b/server/env.ts
index bba2e35bdab0..0506a214bd8d 100644
--- a/server/env.ts
+++ b/server/env.ts
@@ -536,6 +536,22 @@ export class Environment {
   @IsOptional()
   public DEFAULT_EMAIL_DOMAIN = environment.DEFAULT_EMAIL_DOMAIN ?? "askii.ai";
 
+  /**
+   * Root domain of the foss-server-bundle deployment (set by
+   * `foss-server-bundle/platform.sh`). All app subdomains hang off this
+   * (e.g. `docs.${PLATFORM_DOMAIN}`, `pm.${PLATFORM_DOMAIN}`).
+   *
+   * Used by `/auth/portal-logout?next=…` as the redirect allowlist: the
+   * endpoint will follow a `next` URL whose host matches `PLATFORM_DOMAIN`
+   * or any of its subdomains, and reject everything else. Unset →
+   * endpoint still clears cookies, just won't follow any redirect.
+   */
+  @IsOptional()
+  public PLATFORM_DOMAIN = (environment.PLATFORM_DOMAIN ?? "")
+    .toLowerCase()
+    .replace(/^\.+/, "")
+    .trim();
+
   /**
    * A boolean switch to toggle the rate limiter at application web server.
    */
diff --git a/server/routes/auth/index.test.ts b/server/routes/auth/index.test.ts
index 9f29fc5a97d1..1e87471d97c7 100644
--- a/server/routes/auth/index.test.ts
+++ b/server/routes/auth/index.test.ts
@@ -1,3 +1,4 @@
+import env from "@server/env";
 import { buildUser, buildCollection } from "@server/test/factories";
 import { getTestServer } from "@server/test/support";
 import { JWT_COOKIE_TTL_DAYS } from "@server/utils/authentication";
@@ -81,3 +82,151 @@ describe("auth/redirect", () => {
     expect(ageMs).toBeLessThan(expectedMs + 60_000);
   });
 });
+
+describe("auth/portal-logout", () => {
+  let originalPlatformDomain: string;
+
+  beforeEach(() => {
+    originalPlatformDomain = env.PLATFORM_DOMAIN;
+  });
+
+  afterEach(() => {
+    env.PLATFORM_DOMAIN = originalPlatformDomain;
+  });
+
+  const setPlatformDomain = (domain: string) => {
+    env.PLATFORM_DOMAIN = domain;
+  };
+
+  /**
+   * Matches `<name>=;` followed (anywhere up to the next cookie boundary)
+   * by `path=/`. Regression guard for the path-scoping bug: without
+   * `path=/`, Set-Cookie defaults to `/auth/portal-logout`, which fails
+   * to shadow the original `path=/` cookie — the browser keeps the JWT
+   * and "logout" silently does nothing.
+   */
+  const cookieClearedAtRoot = (name: string) =>
+    new RegExp(`${name}=;[^,]*\\bpath=/(?:[;,]|$)`, "i");
+
+  it("should clear accessToken and lastSignedIn cookies at path=/ on every call", async () => {
+    setPlatformDomain("");
+    const res = await server.get("/auth/portal-logout", { redirect: "manual" });
+    expect(res.status).toEqual(200);
+    const setCookie = res.headers.get("set-cookie") ?? "";
+    // Both cookies cleared AND scoped to path=/ so they actually shadow
+    // the original login cookies in the browser.
+    expect(setCookie).toMatch(cookieClearedAtRoot("accessToken"));
+    expect(setCookie).toMatch(cookieClearedAtRoot("lastSignedIn"));
+  });
+
+  it("should 302 to next when host is in the allowlist", async () => {
+    setPlatformDomain("foss.arbisoft.com");
+    const target = "https://pm.foss.arbisoft.com/auth/portal-sign-out/";
+    const res = await server.get(
+      `/auth/portal-logout?next=${encodeURIComponent(target)}`,
+      { redirect: "manual" }
+    );
+    expect(res.status).toEqual(302);
+    expect(res.headers.get("location")).toEqual(target);
+    // cookies still cleared at path=/
+    const setCookie = res.headers.get("set-cookie") ?? "";
+    expect(setCookie).toMatch(cookieClearedAtRoot("accessToken"));
+  });
+
+  it("should match subdomains of allowlist entries", async () => {
+    setPlatformDomain("foss.arbisoft.com");
+    const target = "https://docs.foss.arbisoft.com/done";
+    const res = await server.get(
+      `/auth/portal-logout?next=${encodeURIComponent(target)}`,
+      { redirect: "manual" }
+    );
+    expect(res.status).toEqual(302);
+    expect(res.headers.get("location")).toEqual(target);
+  });
+
+  it("should reject next on a host outside the allowlist", async () => {
+    setPlatformDomain("foss.arbisoft.com");
+    const target = "https://evil.example/steal";
+    const res = await server.get(
+      `/auth/portal-logout?next=${encodeURIComponent(target)}`,
+      { redirect: "manual" }
+    );
+    expect(res.status).toEqual(200);
+    // still clears cookies at path=/ — non-redirect rejection isn't an error path
+    const setCookie = res.headers.get("set-cookie") ?? "";
+    expect(setCookie).toMatch(cookieClearedAtRoot("accessToken"));
+  });
+
+  it("should enforce dot-boundary on suffix matches", async () => {
+    // "foss.arbisoft.com.evil" must NOT match "foss.arbisoft.com".
+    setPlatformDomain("foss.arbisoft.com");
+    const target = "https://foss.arbisoft.com.evil.example/x";
+    const res = await server.get(
+      `/auth/portal-logout?next=${encodeURIComponent(target)}`,
+      { redirect: "manual" }
+    );
+    expect(res.status).toEqual(200);
+  });
+
+  it("should reject every next when PLATFORM_DOMAIN is unset", async () => {
+    setPlatformDomain("");
+    const res = await server.get(
+      "/auth/portal-logout?next=https%3A%2F%2Ffoss.arbisoft.com%2F",
+      { redirect: "manual" }
+    );
+    expect(res.status).toEqual(200);
+  });
+
+  it("should reject malformed next values", async () => {
+    setPlatformDomain("foss.arbisoft.com");
+    const res = await server.get(
+      "/auth/portal-logout?next=not-a-url",
+      { redirect: "manual" }
+    );
+    expect(res.status).toEqual(200);
+  });
+
+  it("should reject next with non-http(s) schemes", async () => {
+    // javascript:, data:, file:, etc. parse fine via new URL() but must
+    // never be a valid next-hop — `<a href="javascript:…">` style would
+    // execute in the user's browser if we 302'd to it.
+    setPlatformDomain("foss.arbisoft.com");
+    for (const target of [
+      "javascript:alert(1)",
+      "data:text/html,<script>alert(1)</script>",
+      "file:///etc/passwd",
+    ]) {
+      const res = await server.get(
+        `/auth/portal-logout?next=${encodeURIComponent(target)}`,
+        { redirect: "manual" }
+      );
+      expect(res.status).toEqual(200);
+    }
+  });
+
+  it("should accept both http and https for allowlisted hosts", async () => {
+    // http allowed for local-dev / localhost flows; https for production.
+    // Beyond the two are rejected by the scheme gate.
+    setPlatformDomain("foss.arbisoft.com");
+    for (const target of [
+      "https://docs.foss.arbisoft.com/x",
+      "http://docs.foss.arbisoft.com/x",
+    ]) {
+      const res = await server.get(
+        `/auth/portal-logout?next=${encodeURIComponent(target)}`,
+        { redirect: "manual" }
+      );
+      expect(res.status).toEqual(302);
+      expect(res.headers.get("location")).toEqual(target);
+    }
+  });
+
+  it("should not 302 when next is missing", async () => {
+    setPlatformDomain("foss.arbisoft.com");
+    const res = await server.get("/auth/portal-logout", { redirect: "manual" });
+    expect(res.status).toEqual(200);
+    // cookies still cleared at path=/
+    const setCookie = res.headers.get("set-cookie") ?? "";
+    expect(setCookie).toMatch(cookieClearedAtRoot("accessToken"));
+  });
+});
diff --git a/server/routes/auth/index.ts b/server/routes/auth/index.ts
index d0d6be6c52b1..bcbc2c034d17 100644
--- a/server/routes/auth/index.ts
+++ b/server/routes/auth/index.ts
@@ -3,6 +3,7 @@ import { addDays } from "date-fns";
 import Koa from "koa";
 import bodyParser from "koa-body";
 import Router from "koa-router";
+import env from "@server/env";
 import { AuthenticationError } from "@server/errors";
 import authMiddleware from "@server/middlewares/authentication";
 import coalesceBody from "@server/middlewares/coaleseBody";
@@ -93,6 +94,92 @@ router.get("/redirect", authMiddleware(), async (ctx: APIContext) => {
   );
 });
 
+/**
+ * Returns true iff `url` is a safe redirect target:
+ *   - scheme is http or https (rejects javascript:, data:, etc.)
+ *   - hostname is `PLATFORM_DOMAIN` itself or a subdomain of it
+ *
+ * Suffix match enforces a dot boundary: `foss.arbisoft.com` matches
+ * `foss.arbisoft.com` and `*.foss.arbisoft.com` but NOT
+ * `foss.arbisoft.com.evil.example`. Closes the obvious open-redirect
+ * surface the endpoint would otherwise expose.
+ *
+ * When `PLATFORM_DOMAIN` is unset, every `next` is rejected — the
+ * endpoint still clears the cookie, it just won't follow a redirect.
+ */
+function isAllowedSignOutNext(url: string): boolean {
+  let parsed: URL;
+  try {
+    parsed = new URL(url);
+  } catch {
+    return false;
+  }
+  // Reject non-http(s) schemes outright — javascript: and data: parse fine
+  // but should never be a valid next-hop. https is required in production;
+  // http is allowed for dev/localhost flows.
+  if (parsed.protocol !== "https:" && parsed.protocol !== "http:") {
+    return false;
+  }
+  const host = parsed.hostname.toLowerCase();
+  if (!host) {
+    return false;
+  }
+  const platformDomain = env.PLATFORM_DOMAIN;
+  if (!platformDomain) {
+    return false;
+  }
+  return host === platformDomain || host.endsWith("." + platformDomain);
+}
+
+/**
+ * GET /auth/portal-logout?next=<absolute_url>
+ *
+ * Clears the accessToken + lastSignedIn cookies and 302-redirects to
+ * `next`. Designed for the foss-server-bundle portal's "Log out of all
+ * apps" redirect chain — the portal navigates the browser through each
+ * app's logout URL, each step clearing its own cookies while the browser
+ * is on that app's own domain (so the Set-Cookie scope is correct).
+ *
+ * CSRF-exempt by design: the portal cannot share Outline's CSRF token
+ * cross-origin, so the existing POST + CSRF flow isn't usable here. The
+ * residual risk is force-logout (an attacker embeds an `<img>` and the
+ * victim's session ends). Low impact — the only state lost is the
+ * cookie, and ForwardAuth re-auths on the next request.
+ *
+ * Open-redirect protection: `next` is validated against `PLATFORM_DOMAIN`
+ * (suffix-match on a dot boundary). Unset `PLATFORM_DOMAIN` rejects every
+ * `next` — cookies are still cleared, the endpoint just returns 200
+ * instead of 302.
+ */
+router.get("/portal-logout", async (ctx: APIContext) => {
+  const epoch = new Date(0);
+  // path:/ is load-bearing — without it Koa defaults the Set-Cookie path
+  // to the request URL (`/auth/portal-logout`), which doesn't shadow the
+  // original `accessToken` cookie's path:/ scope. The browser keeps the
+  // JWT. Matches the cookie-clear shape used by the auth() catch block
+  // at server/middlewares/authentication.ts:114-117.
+  ctx.cookies.set("accessToken", "", {
+    sameSite: "lax",
+    expires: epoch,
+    path: "/",
+  });
+  // lastSignedIn is non-HttpOnly because the frontend reads it.
+  ctx.cookies.set("lastSignedIn", "", {
+    httpOnly: false,
+    sameSite: "lax",
+    expires: epoch,
+    path: "/",
+  });
+
+  const nextRaw = String(ctx.query.next ?? "").trim();
+  if (nextRaw && isAllowedSignOutNext(nextRaw)) {
+    ctx.redirect(nextRaw);
+    return;
+  }
+
+  ctx.body = { ok: true };
+});
+
 app.use(bodyParser());
 app.use(coalesceBody());
 app.use(verifyCSRFToken());