fix(security): don't mint new ID tokens on refresh; confirm offline_access gating

idpyoidc already gates refresh-token issuance on the offline_access scope
(verified by test), but the refresh-token grant was configured to also mint
fresh ID tokens. Drop id_token from the refresh_token grant's supports_minting
so refreshing yields only access (and a rotated refresh) token; ID tokens come
from authentication. Refresh-token rotation is retained.

Refs: porchlight-553

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/porchlight/oidc/provider.py

@@ -67,7 +67,10 @@ def _build_server_config(settings: Settings) -> dict:
                             "expires_in": 3600,
                         },
                         "refresh_token": {
-                            "supports_minting": ["access_token", "refresh_token", "id_token"],
+                            # Rotate refresh tokens (mint a new one) and mint
+                            # access tokens, but do NOT mint fresh ID tokens on
+                            # refresh — re-authentication should issue ID tokens.
+                            "supports_minting": ["access_token", "refresh_token"],
                             "expires_in": 86400,
                         },
                     },

tests/test_oidc/test_e2e_flow.py

@@ -33,8 +33,8 @@ async def _setup_rp_and_user(app: FastAPI) -> None:
         "redirect_uris": [(REDIRECT_URI, {})],
         "response_types_supported": ["code"],
         "token_endpoint_auth_method": "client_secret_basic",
-        "scope": ["openid", "profile", "email"],
+        "scope": ["openid", "profile", "email", "offline_access"],
-        "allowed_scopes": ["openid", "profile", "email"],
+        "allowed_scopes": ["openid", "profile", "email", "offline_access"],
         "client_salt": secrets.token_hex(8),
     }
     oidc_server.keyjar.add_symmetric(CLIENT_ID, CLIENT_SECRET)
@@ -263,3 +263,69 @@ async def test_pkce_wrong_verifier_rejected(client: AsyncClient) -> None:
         },
     )
     assert res.status_code != 200, f"PKCE verifier mismatch should be rejected, got {res.status_code}: {res.text}"
+
+
+async def _authorize_with_offline_access(client: AsyncClient, state: str, nonce: str) -> str:
+    """Run the auth-code flow requesting offline_access, return the auth code."""
+    scope = "openid profile offline_access"
+    # offline_access requires prompt=consent per the OIDC spec.
+    await client.get(
+        "/authorization",
+        params={
+            "response_type": "code",
+            "client_id": CLIENT_ID,
+            "redirect_uri": REDIRECT_URI,
+            "scope": scope,
+            "state": state,
+            "nonce": nonce,
+            "prompt": "consent",
+        },
+        follow_redirects=False,
+    )
+    token = await get_csrf_token(client)
+    await client.post(
+        "/login/password",
+        data={"username": "alice", "password": "testpass"},
+        headers={"HX-Request": "true", "X-CSRF-Token": token},
+    )
+    complete = await client.get("/authorization/complete", follow_redirects=False)
+    assert "/consent" in complete.headers["location"]
+    token = await get_csrf_token(client)
+    consent = await client.post(
+        "/consent",
+        data={"action": "allow", "scope": ["openid", "profile", "offline_access"]},
+        headers={"X-CSRF-Token": token},
+        follow_redirects=False,
+    )
+    return parse_qs(urlparse(consent.headers["location"]).query)["code"][0]
+
+
+async def test_refresh_token_issued_only_with_offline_access(client: AsyncClient) -> None:
+    app = client._transport.app  # type: ignore[union-attr]
+    await _setup_rp_and_user(app)
+
+    # Without offline_access: no refresh token.
+    code = await _login_and_authorize(client, secrets.token_urlsafe(8), secrets.token_urlsafe(8))
+    no_offline = await _exchange_token(client, code)
+    assert "refresh_token" not in no_offline
+
+
+async def test_refresh_grant_does_not_mint_id_token(client: AsyncClient) -> None:
+    app = client._transport.app  # type: ignore[union-attr]
+    await _setup_rp_and_user(app)
+
+    code = await _authorize_with_offline_access(client, secrets.token_urlsafe(8), secrets.token_urlsafe(8))
+    token_data = await _exchange_token(client, code)
+    assert "refresh_token" in token_data, "offline_access should yield a refresh token"
+
+    # Use the refresh token; the response must not contain a new ID token.
+    auth_header = b64encode(f"{CLIENT_ID}:{CLIENT_SECRET}".encode()).decode()
+    res = await client.post(
+        "/token",
+        data={"grant_type": "refresh_token", "refresh_token": token_data["refresh_token"]},
+        headers={"Authorization": f"Basic {auth_header}", "Content-Type": "application/x-www-form-urlencoded"},
+    )
+    assert res.status_code == 200, res.text
+    refreshed = res.json()
+    assert "access_token" in refreshed
+    assert "id_token" not in refreshed