fix(oidc): reject backslash paths in the return_to sanitizer (#1359)

The OIDC `return_to` sanitizer rejects values starting with `//` and
then relies on `urlsplit` to catch anything carrying a netloc. A value
such as `/\host` has no netloc, so it is stored in the session and used
as the post-login redirect target and browsers resolve the backslash as
a path separator, which lands the user outside the app after a
successful login.

`_normalize_return_to` now also rejects values whose path contains a
backslash. That matches the frontend sanitizer in `authRedirect.ts`,
which parses with `URL` and already discards those forms, so the two
ends agree again. The check covers the path only, so query and fragment
backslashes still round-trip, and it also catches the script-root case
where `/app/\host` strips to `/\host`.

## Verification

- New cases in `tests/core/test_oidc_routes.py` cover the rejected
forms, including under a script root, and confirm `/`, `/settings` and
`/search?q=x#frag` are unaffected. They fail on current main and pass
here.
- Full suite (3155), ruff, ruff format, basedpyright, vulture green.

shelfmark/core/oidc_routes.py

@@ -109,7 +109,7 @@ def _normalize_return_to(raw_return_to: object) -> str | None:
         return None
 
     parsed = urlsplit(value)
-    if parsed.scheme or parsed.netloc:
+    if parsed.scheme or parsed.netloc or "\\" in parsed.path:
         return None
 
     script_root = request.script_root.rstrip("/")

tests/core/test_oidc_routes.py

@@ -182,6 +182,56 @@ class TestOIDCLoginEndpoint:
         with client.session_transaction() as sess:
             assert "oidc_return_to" not in sess
 
+    @patch("shelfmark.core.oidc_routes._get_oidc_client")
+    def test_login_ignores_backslash_return_to(self, mock_get_client, client):
+        fake_client = Mock()
+        fake_client.authorize_redirect.return_value = redirect("https://auth.example.com/authorize")
+        mock_get_client.return_value = (fake_client, MOCK_OIDC_CONFIG)
+
+        resp = client.get("/api/auth/oidc/login?return_to=%2F%5Cevil.example.com%2Fphish")
+
+        assert resp.status_code == 302
+        with client.session_transaction() as sess:
+            assert "oidc_return_to" not in sess
+
+    @pytest.mark.parametrize(
+        "raw",
+        [
+            "/\\evil.example.com",
+            "/\\evil.example.com/phish",
+            "/\\\\evil.example.com",
+            "/\\/evil.example.com",
+        ],
+    )
+    def test_normalize_return_to_rejects_backslash_paths(self, app, raw):
+        from shelfmark.core.oidc_routes import _normalize_return_to
+
+        with app.test_request_context("/api/auth/oidc/login"):
+            assert _normalize_return_to(raw) is None
+
+    def test_normalize_return_to_rejects_backslash_under_script_root(self, app):
+        from shelfmark.core.oidc_routes import _normalize_return_to
+
+        with app.test_request_context(
+            "/shelfmark/api/auth/oidc/login",
+            environ_overrides={"SCRIPT_NAME": "/shelfmark"},
+        ):
+            assert _normalize_return_to("/shelfmark/\\evil.example.com") is None
+
+    @pytest.mark.parametrize(
+        ("raw", "expected"),
+        [
+            ("/", "/"),
+            ("/settings", "/settings"),
+            ("/search?q=x#frag", "/search?q=x#frag"),
+        ],
+    )
+    def test_normalize_return_to_keeps_local_paths(self, app, raw, expected):
+        from shelfmark.core.oidc_routes import _normalize_return_to
+
+        with app.test_request_context("/api/auth/oidc/login"):
+            assert _normalize_return_to(raw) == expected
+
 
 class TestOIDCCallbackEndpoint:
     def test_normalize_claims_returns_empty_dict_for_invalid_mapping(self):