diff --git a/apps/api/.env.example b/apps/api/.env.example
index 6bed24003b4..586ae43fc43 100644
--- a/apps/api/.env.example
+++ b/apps/api/.env.example
@@ -79,3 +79,8 @@ API_KEY_RATE_LIMIT="60/minute"
 # MPASS_BYPASS_PATHS=/god-mode,/api/instances
 # Required for 3-layer logout (Django → oauth2-proxy → Cognito)
 # MPASS_SIGNOUT_URL=https://foss-auth.local.moneta.dev/oauth2/sign_out?rd=https%3A%2F%2Fcognito.example.com%2Flogout
+# Root domain of the foss-server-bundle deployment. Used by
+# /auth/sign-out/?next= as the redirect allowlist — only URLs whose
+# host equals PLATFORM_DOMAIN or is a subdomain of it are followed.
+# Set automatically by foss-server-bundle/platform.sh.
+# PLATFORM_DOMAIN=foss.arbisoft.com
diff --git a/apps/api/plane/authentication/urls.py b/apps/api/plane/authentication/urls.py
index bc736087fcb..47f443be84d 100644
--- a/apps/api/plane/authentication/urls.py
+++ b/apps/api/plane/authentication/urls.py
@@ -33,7 +33,10 @@
 # GiteaOauthInitiateSpaceEndpoint, GiteaCallbackSpaceEndpoint,
 
 urlpatterns = [
-    # signout — kept active (used by frontend 3-layer logout)
+    # signout — POST for in-app logout, GET (?next=) for the foss-bundle
+    # portal's "Log out of all apps" redirect chain. Both methods flush
+    # the Django session via the same django.contrib.auth.logout() call;
+    # GET is CSRF-exempt with PLATFORM_DOMAIN-bounded ?next= validation.
     path("sign-out/", SignOutAuthEndpoint.as_view(), name="sign-out"),
     path("spaces/sign-out/", SignOutAuthSpaceEndpoint.as_view(), name="space-sign-out"),
     # csrf token — kept active (Django forms need it)
diff --git a/apps/api/plane/authentication/views/app/signout.py b/apps/api/plane/authentication/views/app/signout.py
index 65aa29aef4e..3774ea0d5e9 100644
--- a/apps/api/plane/authentication/views/app/signout.py
+++ b/apps/api/plane/authentication/views/app/signout.py
@@ -2,18 +2,24 @@
 # SPDX-License-Identifier: AGPL-3.0-only
 # See the LICENSE file for details.
 
+# Standard library
+from urllib.parse import urlparse
+
 # Django imports
 from django.views import View
 from django.contrib.auth import logout
 from django.conf import settings
-from django.http import HttpResponseRedirect
+from django.http import HttpResponseBadRequest, HttpResponseRedirect
 from django.utils import timezone
+from django.utils.decorators import method_decorator
+from django.views.decorators.csrf import csrf_exempt
 
 # Module imports
 from plane.authentication.utils.host import user_ip, base_host
 from plane.db.models import User
 
 
+@method_decorator(csrf_exempt, name="dispatch")
 class SignOutAuthEndpoint(View):
     def post(self, request):
         try:
@@ -36,3 +42,62 @@ def post(self, request):
             return HttpResponseRedirect(mpass_signout_url)
 
         return HttpResponseRedirect(base_host(request=request, is_app=True))
+
+    def get(self, request):
+        """
+        Cross-origin redirect-chain entry-point for the foss-server-bundle
+        portal's "Log out of all apps" flow.
+
+        Clears the Django session via django.contrib.auth.logout(), then
+        302s to ?next= (validated against PLATFORM_DOMAIN to prevent open
+        redirect). The chain works because the browser is on this app's
+        own domain when this endpoint runs — so Django's Set-Cookie scope
+        is correct.
+
+        CSRF-exempt (set at class level): no token is shared cross-origin
+        with the portal. Residual risk is force-logout (attacker embeds
+        <img src="…/sign-out"> ending the victim's session); low impact —
+        only the session itself is lost, and re-auth via ForwardAuth is
+        automatic.
+        """
+        logout(request)
+
+        next_url = (request.GET.get("next") or "").strip()
+        if next_url:
+            if not self._is_allowed_next(next_url):
+                return HttpResponseBadRequest(
+                    "next= target host is not a subdomain of PLATFORM_DOMAIN"
+                )
+            return HttpResponseRedirect(next_url)
+
+        # No ?next= — fall back to MPASS_SIGNOUT_URL so a manual hit still
+        # chains through oauth2-proxy + Cognito.
+        mpass_signout_url = getattr(settings, "MPASS_SIGNOUT_URL", None)
+        if mpass_signout_url:
+            return HttpResponseRedirect(mpass_signout_url)
+        return HttpResponseRedirect(base_host(request=request, is_app=True))
+
+    @staticmethod
+    def _is_allowed_next(url):
+        """True iff URL's host equals PLATFORM_DOMAIN or is a subdomain.
+
+        Suffix match enforces a dot boundary: foss.arbisoft.com matches
+        foss.arbisoft.com and *.foss.arbisoft.com but NOT
+        foss.arbisoft.com.evil. Unset PLATFORM_DOMAIN → False (every
+        ?next= rejected).
+        """
+        platform_domain = (
+            getattr(settings, "PLATFORM_DOMAIN", "") or ""
+        ).strip().lower().lstrip(".")
+        if not platform_domain:
+            return False
+
+        try:
+            host = urlparse(url).hostname
+        except ValueError:
+            return False
+        if not host:
+            return False
+
+        host = host.lower()
+        return host == platform_domain or host.endswith("." + platform_domain)
diff --git a/apps/api/plane/settings/common.py b/apps/api/plane/settings/common.py
index 6d23c23a8d7..d7c23634142 100644
--- a/apps/api/plane/settings/common.py
+++ b/apps/api/plane/settings/common.py
@@ -62,6 +62,12 @@
 # mPass proxy auth
 MPASS_BYPASS_PATHS = [p.strip() for p in os.environ.get("MPASS_BYPASS_PATHS", "").split(",") if p.strip()] or None
 DEFAULT_EMAIL_DOMAIN = os.environ.get("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. Used
+# by /auth/portal-sign-out/ as the ?next= redirect allowlist: only URLs
+# whose host equals PLATFORM_DOMAIN or is a subdomain of it are followed.
+# Unset → endpoint still flushes the Django session, just won't redirect.
+PLATFORM_DOMAIN = (os.environ.get("PLATFORM_DOMAIN", "") or "").strip().lower().lstrip(".")
 
 # SMB portal hostname segment (landing / logout redirects) vs default Plane workspace slug
 SMB_NAME = os.environ.get("SMB_NAME")
diff --git a/apps/api/plane/tests/unit/views/test_signout.py b/apps/api/plane/tests/unit/views/test_signout.py
index 6ba375e0988..44e2af67e0c 100644
--- a/apps/api/plane/tests/unit/views/test_signout.py
+++ b/apps/api/plane/tests/unit/views/test_signout.py
@@ -129,3 +129,125 @@ def test_falls_back_to_base_host_when_save_raises_and_no_mpass_url(
 
         assert response.status_code == 302
         assert response["Location"] == _BASE_HOST
+
+
+# GET /auth/sign-out/?next=<url> — portal logout-chain entry point
+# ────────────────────────────────────────────────────────────────
+
+
+def _make_get_request(factory: RequestFactory, qs: str = "") -> MagicMock:
+    url = "/auth/sign-out/" + (f"?{qs}" if qs else "")
+    req = factory.get(url)
+    req.user = MagicMock(id="user-uuid-1234")
+    return req
+
+
+@pytest.mark.unit
+class TestSignOutAuthEndpointGet:
+    """The GET variant: portal-driven logout chain.
+
+    Same `_flush_session()` body as POST. Differences:
+      - ?next= allowlist validated against PLATFORM_DOMAIN (host equality
+        or dot-bounded subdomain).
+      - CSRF-exempt (set at the class level via @method_decorator).
+      - Falls back to MPASS_SIGNOUT_URL or base_host when ?next= is
+        missing / invalid.
+    """
+
+    @patch("plane.authentication.views.app.signout.base_host", return_value=_BASE_HOST)
+    @patch("plane.authentication.views.app.signout.logout")
+    @patch("plane.authentication.views.app.signout.User")
+    @patch("plane.authentication.views.app.signout.settings")
+    def test_redirects_to_allowlisted_next(
+        self, mock_settings, mock_user_cls, mock_logout, mock_base_host, factory, view
+    ):
+        mock_settings.PLATFORM_DOMAIN = "foss.arbisoft.com"
+        mock_user_cls.objects.get.return_value = MagicMock()
+        next_url = "https://docs.foss.arbisoft.com/auth/sign-out/"
+
+        response = view.get(_make_get_request(factory, f"next={next_url}"))
+
+        mock_logout.assert_called_once()  # GET still flushes the session
+        assert response.status_code == 302
+        assert response["Location"] == next_url
+
+    @patch("plane.authentication.views.app.signout.base_host", return_value=_BASE_HOST)
+    @patch("plane.authentication.views.app.signout.logout")
+    @patch("plane.authentication.views.app.signout.User")
+    @patch("plane.authentication.views.app.signout.settings")
+    def test_rejects_next_on_disallowed_host(
+        self, mock_settings, mock_user_cls, mock_logout, mock_base_host, factory, view
+    ):
+        mock_settings.PLATFORM_DOMAIN = "foss.arbisoft.com"
+        mock_user_cls.objects.get.return_value = MagicMock()
+
+        response = view.get(
+            _make_get_request(factory, "next=https://evil.example/steal")
+        )
+
+        mock_logout.assert_called_once()  # session still flushed
+        assert response.status_code == 400
+
+    @patch("plane.authentication.views.app.signout.base_host", return_value=_BASE_HOST)
+    @patch("plane.authentication.views.app.signout.logout")
+    @patch("plane.authentication.views.app.signout.User")
+    @patch("plane.authentication.views.app.signout.settings")
+    def test_suffix_match_enforces_dot_boundary(
+        self, mock_settings, mock_user_cls, mock_logout, mock_base_host, factory, view
+    ):
+        """foss.arbisoft.com.evil must not match foss.arbisoft.com."""
+        mock_settings.PLATFORM_DOMAIN = "foss.arbisoft.com"
+        mock_user_cls.objects.get.return_value = MagicMock()
+
+        response = view.get(
+            _make_get_request(factory, "next=https://foss.arbisoft.com.evil/x")
+        )
+
+        assert response.status_code == 400
+
+    @patch("plane.authentication.views.app.signout.base_host", return_value=_BASE_HOST)
+    @patch("plane.authentication.views.app.signout.logout")
+    @patch("plane.authentication.views.app.signout.User")
+    @patch("plane.authentication.views.app.signout.settings")
+    def test_empty_platform_domain_rejects_all_next(
+        self, mock_settings, mock_user_cls, mock_logout, mock_base_host, factory, view
+    ):
+        mock_settings.PLATFORM_DOMAIN = ""
+        mock_user_cls.objects.get.return_value = MagicMock()
+
+        response = view.get(
+            _make_get_request(factory, "next=https://docs.foss.arbisoft.com/x")
+        )
+
+        assert response.status_code == 400
+
+    @patch("plane.authentication.views.app.signout.base_host", return_value=_BASE_HOST)
+    @patch("plane.authentication.views.app.signout.logout")
+    @patch("plane.authentication.views.app.signout.User")
+    @patch("plane.authentication.views.app.signout.settings")
+    def test_no_next_falls_back_to_mpass_url(
+        self, mock_settings, mock_user_cls, mock_logout, mock_base_host, factory, view
+    ):
+        mock_settings.PLATFORM_DOMAIN = "foss.arbisoft.com"
+        mock_settings.MPASS_SIGNOUT_URL = _MPASS_URL
+        mock_user_cls.objects.get.return_value = MagicMock()
+
+        response = view.get(_make_get_request(factory))
+
+        mock_logout.assert_called_once()
+        assert response.status_code == 302
+        assert response["Location"] == _MPASS_URL
+
+    @patch("plane.authentication.views.app.signout.base_host", return_value=_BASE_HOST)
+    @patch("plane.authentication.views.app.signout.logout")
+    @patch("plane.authentication.views.app.signout.User")
+    @patch("plane.authentication.views.app.signout.settings")
+    def test_malformed_next_is_rejected(
+        self, mock_settings, mock_user_cls, mock_logout, mock_base_host, factory, view
+    ):
+        mock_settings.PLATFORM_DOMAIN = "foss.arbisoft.com"
+        mock_user_cls.objects.get.return_value = MagicMock()
+
+        response = view.get(_make_get_request(factory, "next=:::garbage"))
+
+        assert response.status_code == 400