fix(oidc): trust Entra's graph.microsoft.com userinfo out of the box

Microsoft Entra's discovery document advertises userinfo_endpoint on graph.microsoft.com — a host distinct from the login.microsoftonline.com issuer — so discover_oidc's cross-host guard rejected it and disabled OIDC unless the operator set trusted_endpoint_hosts. Add login.microsoftonline.com to the built-in KNOWN_TRUSTED_OAUTH_ENDPOINT_HOSTS allow-list (mirroring the Google entry) so Azure AD OIDC works with no extra configuration. Surfaced by the live obo integration test.

docs/oidc.md

@@ -77,17 +77,17 @@ IdP from redirecting the token-exchange POST (which carries
 being aimed at internal services.
 
 A few public IdPs legitimately split endpoints across hostnames. Google
-is the canonical example:
+and Microsoft Entra ID are the canonical examples:
 
-| Field | Hostname |
-|-------|----------|
-| issuer | `accounts.google.com` |
-| token_endpoint | `oauth2.googleapis.com` |
-| jwks_uri | `www.googleapis.com` |
-| userinfo_endpoint | `openidconnect.googleapis.com` |
+| IdP | Issuer host | Cross-host endpoint(s) |
+|-----|-------------|------------------------|
+| Google | `accounts.google.com` | `oauth2.googleapis.com`, `www.googleapis.com`, `openidconnect.googleapis.com` |
+| Microsoft Entra | `login.microsoftonline.com` | `graph.microsoft.com` (userinfo) |
 
-Google's set is built in — operators using `https://accounts.google.com`
-need no extra configuration.
+Both sets are built in — operators using `https://accounts.google.com` or
+`https://login.microsoftonline.com/<tenant>/v2.0` need no extra
+configuration. (Entra's discovery document advertises `userinfo_endpoint`
+on `graph.microsoft.com`, distinct from the issuer host.)
 
 For other IdPs whose discovery document references a non-issuer host,
 extend the allow-list explicitly:

tests/test_oidc.py

@@ -910,6 +910,40 @@ class TestValidateDiscoveredEndpoint:
 
         asyncio.run(_run())
 
+    def test_discover_accepts_entra_userinfo_on_graph(self):
+        """discover_oidc accepts Entra's cross-host userinfo on graph.microsoft.com
+        via the built-in allow-list, so Azure AD OIDC works out of the box with no
+        trusted_endpoint_hosts override."""
+        tenant = "11111111-1111-1111-1111-111111111111"
+        config = _make_config(
+            issuer=f"https://login.microsoftonline.com/{tenant}/v2.0",
+            authorization_endpoint="",
+            token_endpoint="",
+            userinfo_endpoint="",
+            jwks_uri="",
+        )
+        discovery_doc = {
+            "authorization_endpoint": f"https://login.microsoftonline.com/{tenant}/oauth2/v2.0/authorize",
+            "token_endpoint": f"https://login.microsoftonline.com/{tenant}/oauth2/v2.0/token",
+            "userinfo_endpoint": "https://graph.microsoft.com/oidc/userinfo",
+            "jwks_uri": f"https://login.microsoftonline.com/{tenant}/discovery/v2.0/keys",
+        }
+        mock_response = MagicMock()
+        mock_response.json.return_value = discovery_doc
+        mock_response.raise_for_status = MagicMock()
+
+        async def _run():
+            client = _mock_async_client(lambda url: _async_return(mock_response))
+            with (
+                patch("socket.getaddrinfo", return_value=self._PUBLIC_ADDR),
+                patch("httpx.AsyncClient", return_value=client),
+            ):
+                result = await discover_oidc(config)
+            assert result.enabled is True
+            assert result.userinfo_endpoint == "https://graph.microsoft.com/oidc/userinfo"
+
+        asyncio.run(_run())
+
     def test_discover_rejects_http_endpoint(self):
         """discover_oidc returns enabled=False when an endpoint is http:// in prod."""
         config = _make_config(

turnstone/core/oauth_ssrf.py

@@ -42,6 +42,20 @@ KNOWN_TRUSTED_OAUTH_ENDPOINT_HOSTS: dict[str, frozenset[str]] = {
             "openidconnect.googleapis.com",
         }
     ),
+    # Microsoft Entra ID (commercial cloud). The tenant issuer is
+    # login.microsoftonline.com/<tenant>/v2.0, but its discovery document
+    # advertises userinfo_endpoint on graph.microsoft.com — a distinct host.
+    # Without this entry, discovery rejects the cross-host userinfo and
+    # disables OIDC, so every Azure AD deployment would need a manual
+    # trusted_endpoint_hosts override to log in. (Sovereign clouds —
+    # login.microsoftonline.us / *.chinacloudapi.cn — use their own graph
+    # hosts and can be added the same way.)
+    "login.microsoftonline.com": frozenset(
+        {
+            "login.microsoftonline.com",
+            "graph.microsoft.com",
+        }
+    ),
 }