Require authentication for OAuth token refresh (#21421)

According to the OAuth spec
https://datatracker.ietf.org/doc/html/rfc6749#section-6 when "Refreshing
an Access Token"

> The authorization server MUST ... require client authentication for
confidential clients


Fixes #21418

Co-authored-by: Gusted <williamzijl7@hotmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/web/auth/oauth.go

@@ -48,6 +48,7 @@ const (
 // TODO move error and responses to SDK or models
 
 // AuthorizeErrorCode represents an error code specified in RFC 6749
+// https://datatracker.ietf.org/doc/html/rfc6749#section-4.2.2.1
 type AuthorizeErrorCode string
 
 const (
@@ -68,6 +69,7 @@ const (
 )
 
 // AuthorizeError represents an error type specified in RFC 6749
+// https://datatracker.ietf.org/doc/html/rfc6749#section-4.2.2.1
 type AuthorizeError struct {
 	ErrorCode        AuthorizeErrorCode `json:"error" form:"error"`
 	ErrorDescription string
@@ -80,6 +82,7 @@ func (err AuthorizeError) Error() string {
 }
 
 // AccessTokenErrorCode represents an error code specified in RFC 6749
+// https://datatracker.ietf.org/doc/html/rfc6749#section-5.2
 type AccessTokenErrorCode string
 
 const (
@@ -98,6 +101,7 @@ const (
 )
 
 // AccessTokenError represents an error response specified in RFC 6749
+// https://datatracker.ietf.org/doc/html/rfc6749#section-5.2
 type AccessTokenError struct {
 	ErrorCode        AccessTokenErrorCode `json:"error" form:"error"`
 	ErrorDescription string               `json:"error_description"`
@@ -129,6 +133,7 @@ const (
 )
 
 // AccessTokenResponse represents a successful access token response
+// https://datatracker.ietf.org/doc/html/rfc6749#section-4.2.2
 type AccessTokenResponse struct {
 	AccessToken  string    `json:"access_token"`
 	TokenType    TokenType `json:"token_type"`
@@ -663,6 +668,30 @@ func AccessTokenOAuth(ctx *context.Context) {
 }
 
 func handleRefreshToken(ctx *context.Context, form forms.AccessTokenForm, serverKey, clientKey oauth2.JWTSigningKey) {
+	app, err := auth.GetOAuth2ApplicationByClientID(ctx, form.ClientID)
+	if err != nil {
+		handleAccessTokenError(ctx, AccessTokenError{
+			ErrorCode:        AccessTokenErrorCodeInvalidClient,
+			ErrorDescription: fmt.Sprintf("cannot load client with client id: %q", form.ClientID),
+		})
+		return
+	}
+	// "The authorization server MUST ... require client authentication for confidential clients"
+	// https://datatracker.ietf.org/doc/html/rfc6749#section-6
+	if !app.ValidateClientSecret([]byte(form.ClientSecret)) {
+		errorDescription := "invalid client secret"
+		if form.ClientSecret == "" {
+			errorDescription = "invalid empty client secret"
+		}
+		// "invalid_client ... Client authentication failed"
+		// https://datatracker.ietf.org/doc/html/rfc6749#section-5.2
+		handleAccessTokenError(ctx, AccessTokenError{
+			ErrorCode:        AccessTokenErrorCodeInvalidClient,
+			ErrorDescription: errorDescription,
+		})
+		return
+	}
+
 	token, err := oauth2.ParseToken(form.RefreshToken, serverKey)
 	if err != nil {
 		handleAccessTokenError(ctx, AccessTokenError{

tests/integration/oauth_test.go

@@ -299,10 +299,11 @@ func TestAccessTokenExchangeWithBasicAuth(t *testing.T) {
 		"client_secret": "inconsistent",
 	})
 	req.Header.Add("Authorization", "Basic ZGE3ZGEzYmEtOWExMy00MTY3LTg1NmYtMzg5OWRlMGIwMTM4OjRNSzhOYTZSNTVzbWRDWTBXdUNDdW1aNmhqUlBuR1k1c2FXVlJISGpKaUE9")
+	resp = MakeRequest(t, req, http.StatusBadRequest)
 	parsedError = new(auth.AccessTokenError)
 	assert.NoError(t, json.Unmarshal(resp.Body.Bytes(), parsedError))
 	assert.Equal(t, "invalid_request", string(parsedError.ErrorCode))
-	assert.Equal(t, "client_id in request body inconsistent with Authorization header", parsedError.ErrorDescription)
+	assert.Equal(t, "client_secret in request body inconsistent with Authorization header", parsedError.ErrorDescription)
 }
 
 func TestRefreshTokenInvalidation(t *testing.T) {
@@ -329,7 +330,33 @@ func TestRefreshTokenInvalidation(t *testing.T) {
 	// test without invalidation
 	setting.OAuth2.InvalidateRefreshTokens = false
 
-	refreshReq := NewRequestWithValues(t, "POST", "/login/oauth/access_token", map[string]string{
+	req = NewRequestWithValues(t, "POST", "/login/oauth/access_token", map[string]string{
+		"grant_type": "refresh_token",
+		"client_id":  "da7da3ba-9a13-4167-856f-3899de0b0138",
+		// omit secret
+		"redirect_uri":  "a",
+		"refresh_token": parsed.RefreshToken,
+	})
+	resp = MakeRequest(t, req, http.StatusBadRequest)
+	parsedError := new(auth.AccessTokenError)
+	assert.NoError(t, json.Unmarshal(resp.Body.Bytes(), parsedError))
+	assert.Equal(t, "invalid_client", string(parsedError.ErrorCode))
+	assert.Equal(t, "invalid empty client secret", parsedError.ErrorDescription)
+
+	req = NewRequestWithValues(t, "POST", "/login/oauth/access_token", map[string]string{
+		"grant_type":    "refresh_token",
+		"client_id":     "da7da3ba-9a13-4167-856f-3899de0b0138",
+		"client_secret": "4MK8Na6R55smdCY0WuCCumZ6hjRPnGY5saWVRHHjJiA=",
+		"redirect_uri":  "a",
+		"refresh_token": "UNEXPECTED",
+	})
+	resp = MakeRequest(t, req, http.StatusBadRequest)
+	parsedError = new(auth.AccessTokenError)
+	assert.NoError(t, json.Unmarshal(resp.Body.Bytes(), parsedError))
+	assert.Equal(t, "unauthorized_client", string(parsedError.ErrorCode))
+	assert.Equal(t, "unable to parse refresh token", parsedError.ErrorDescription)
+
+	req = NewRequestWithValues(t, "POST", "/login/oauth/access_token", map[string]string{
 		"grant_type":    "refresh_token",
 		"client_id":     "da7da3ba-9a13-4167-856f-3899de0b0138",
 		"client_secret": "4MK8Na6R55smdCY0WuCCumZ6hjRPnGY5saWVRHHjJiA=",
@@ -337,24 +364,24 @@ func TestRefreshTokenInvalidation(t *testing.T) {
 		"refresh_token": parsed.RefreshToken,
 	})
 
-	bs, err := io.ReadAll(refreshReq.Body)
+	bs, err := io.ReadAll(req.Body)
 	assert.NoError(t, err)
 
-	refreshReq.Body = io.NopCloser(bytes.NewReader(bs))
+	req.Body = io.NopCloser(bytes.NewReader(bs))
-	MakeRequest(t, refreshReq, http.StatusOK)
+	MakeRequest(t, req, http.StatusOK)
 
-	refreshReq.Body = io.NopCloser(bytes.NewReader(bs))
+	req.Body = io.NopCloser(bytes.NewReader(bs))
-	MakeRequest(t, refreshReq, http.StatusOK)
+	MakeRequest(t, req, http.StatusOK)
 
 	// test with invalidation
 	setting.OAuth2.InvalidateRefreshTokens = true
-	refreshReq.Body = io.NopCloser(bytes.NewReader(bs))
+	req.Body = io.NopCloser(bytes.NewReader(bs))
-	MakeRequest(t, refreshReq, http.StatusOK)
+	MakeRequest(t, req, http.StatusOK)
 
 	// repeat request should fail
-	refreshReq.Body = io.NopCloser(bytes.NewReader(bs))
+	req.Body = io.NopCloser(bytes.NewReader(bs))
-	resp = MakeRequest(t, refreshReq, http.StatusBadRequest)
+	resp = MakeRequest(t, req, http.StatusBadRequest)
-	parsedError := new(auth.AccessTokenError)
+	parsedError = new(auth.AccessTokenError)
 	assert.NoError(t, json.Unmarshal(resp.Body.Bytes(), parsedError))
 	assert.Equal(t, "unauthorized_client", string(parsedError.ErrorCode))
 	assert.Equal(t, "token was already used", parsedError.ErrorDescription)