fix(security): reject consent scopes outside the original request

The /consent POST handler trusted the scope values submitted in the form,
so a forged consent submission could approve (and persist consent for)
scopes that were never part of the originating authorization request —
a scope-escalation vector.

Intersect the submitted scopes with the originally requested set stored in
the session before saving consent and completing the flow.

Refs: porchlight-a03

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

src/porchlight/oidc/endpoints.py

@@ -338,8 +338,10 @@ async def consent_submit(request: Request) -> Response:
             return RedirectResponse(f"{redirect_uri}?{params}", status_code=303)
         return HTMLResponse("<h1>Error</h1><p>Invalid action</p>", status_code=400)
 
-    # Allow — collect approved scopes
-    approved_scopes: list[str] = [str(s) for s in form.getlist("scope")]
+    # Allow — collect approved scopes, rejecting anything outside the
+    # originally requested set (a forged form must not escalate scope).
+    requested_scopes = set(auth_params.get("scope", "openid").split())
+    approved_scopes: list[str] = [str(s) for s in form.getlist("scope") if str(s) in requested_scopes]
     if "openid" not in approved_scopes:
         approved_scopes = ["openid", *list(approved_scopes)]
 

tests/test_oidc/test_consent_flow.py

@@ -223,6 +223,30 @@ async def test_partial_consent_filters_scopes(client: AsyncClient) -> None:
     assert set(consent.scopes) == {"openid", "profile"}
 
 
+async def test_consent_rejects_scopes_outside_request(client: AsyncClient) -> None:
+    """A forged consent POST cannot approve scopes that were not requested."""
+    app = client._transport.app  # type: ignore[union-attr]
+    _register_test_rp(app)
+    await _create_test_user(app)
+
+    # Only openid + profile were requested; "email" is forged into the POST.
+    await _login_and_start_auth(client, scope="openid profile")
+    token = await get_csrf_token(client)
+    res = await client.post(
+        "/consent",
+        data={"action": "allow", "scope": ["openid", "profile", "email"]},
+        headers={"X-CSRF-Token": token},
+        follow_redirects=False,
+    )
+    assert res.status_code == 303
+
+    consent_repo = app.state.consent_repo
+    consent = await consent_repo.get_consent("lusab-consent", "consent-rp")
+    assert consent is not None
+    assert "email" not in consent.scopes
+    assert set(consent.scopes) == {"openid", "profile"}
+
+
 # -- Test helpers --