Prevent user enumeration

web/users.go

@@ -65,24 +65,31 @@ func (u *userAuthRoundtrip) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 
 	user, pass, auth := r.BasicAuth()
 	if auth {
-		if hashedPassword, ok := c.Users[user]; ok {
-			cacheKey := hex.EncodeToString(append(append([]byte(user), []byte(hashedPassword)...), []byte(pass)...))
-			authOk, ok := u.cache.get(cacheKey)
-
-			if !ok {
-				// This user, hashedPassword, password is not cached.
-				u.bcryptMtx.Lock()
-				err := bcrypt.CompareHashAndPassword([]byte(hashedPassword), []byte(pass))
-				u.bcryptMtx.Unlock()
-
-				authOk = err == nil
-				u.cache.set(cacheKey, authOk)
-			}
-
-			if authOk {
-				u.handler.ServeHTTP(w, r)
-				return
-			}
+		hashedPassword, validUser := c.Users[user]
+
+		if !validUser {
+			// The user is not found. Use a fixed password hash to
+			// prevent user enumeration by timing requests.
+			// This is a bcrypt-hashed version of "fakepassword".
+			hashedPassword = "$2y$10$QOauhQNbBCuQDKes6eFzPeMqBSjb7Mr5DUmpZ/VcEd00UAV/LDeSi"
+		}
+
+		cacheKey := hex.EncodeToString(append(append([]byte(user), []byte(hashedPassword)...), []byte(pass)...))
+		authOk, ok := u.cache.get(cacheKey)
+
+		if !ok {
+			// This user, hashedPassword, password is not cached.
+			u.bcryptMtx.Lock()
+			err := bcrypt.CompareHashAndPassword([]byte(hashedPassword), []byte(pass))
+			u.bcryptMtx.Unlock()
+
+			authOk = err == nil
+			u.cache.set(cacheKey, authOk)
+		}
+
+		if authOk && validUser {
+			u.handler.ServeHTTP(w, r)
+			return
 		}
 	}
 

web/users_test.go

@@ -83,3 +83,48 @@ func TestBasicAuthCache(t *testing.T) {
 	close(start)
 	wg.Wait()
 }
+
+// TestBasicAuthWithFakePassword validates that we can't login the "fakepassword" used in
+// to prevent user enumeration.
+func TestBasicAuthWithFakepassword(t *testing.T) {
+	server := &http.Server{
+		Addr: port,
+		Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.Write([]byte("Hello World!"))
+		}),
+	}
+
+	done := make(chan struct{})
+	t.Cleanup(func() {
+		if err := server.Shutdown(context.Background()); err != nil {
+			t.Fatal(err)
+		}
+		<-done
+	})
+
+	go func() {
+		ListenAndServe(server, "testdata/tls_config_users_noTLS.good.yml", testlogger)
+		close(done)
+	}()
+
+	login := func() {
+		client := &http.Client{}
+		req, err := http.NewRequest("GET", "http://localhost"+port, nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+		req.SetBasicAuth("fakeuser", "fakepassword")
+		r, err := client.Do(req)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if r.StatusCode != 401 {
+			t.Fatalf("bad return code, expected %d, got %d", 401, r.StatusCode)
+		}
+	}
+
+	// Login with a cold cache.
+	login()
+	// Login with the response cached.
+	login()
+}