reject empty oauth2 state

oauth2/sessions.go

@@ -52,5 +52,9 @@ func (s *SessionStateStore) VerifyState(r *http.Request, expected string) (bool,
 		return false, err
 	}
 
+	if state == "" {
+		return false, nil
+	}
+
 	return subtle.ConstantTimeCompare([]byte(expected), []byte(state)) == 1, nil
 }

oauth2/sessions_test.go

@@ -0,0 +1,41 @@
+// Copyright 2026 Palantir Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package oauth2
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/alexedwards/scs"
+	"github.com/alexedwards/scs/stores/memstore"
+)
+
+// An empty state means no OAuth flow was initiated.
+// VerifyState must reject it before comparing state values,
+// since the comparison treats two empty strings as a match.
+func TestSessionStateStore_VerifyState_RejectsEmptyState(t *testing.T) {
+	store := &SessionStateStore{Sessions: scs.NewManager(memstore.New(0))}
+
+	r := httptest.NewRequest(http.MethodGet, "/callback?state=", nil)
+
+	ok, err := store.VerifyState(r, r.FormValue("state"))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if ok {
+		t.Fatal("VerifyState accepted an empty/unstored state")
+	}
+}