Improve OIDC error handling

providers/oidc.go

@@ -206,12 +206,15 @@ func (p *OIDCProvider) CreateSessionFromToken(ctx context.Context, token string)
 func (p *OIDCProvider) createSession(ctx context.Context, token *oauth2.Token, refresh bool) (*sessions.SessionState, error) {
 	idToken, err := p.verifyIDToken(ctx, token)
 	if err != nil {
+		switch err {
+		case ErrMissingIDToken:
+			// IDToken is mandatory in Redeem but optional in Refresh
+			if !refresh {
+				return nil, errors.New("token response did not contain an id_token")
+			}
+		default:
 			return nil, fmt.Errorf("could not verify id_token: %v", err)
 		}
-
-	// IDToken is mandatory in Redeem but optional in Refresh
-	if idToken == nil && !refresh {
-		return nil, errors.New("token response did not contain an id_token")
 	}
 
 	ss, err := p.buildSessionFromClaims(idToken)

providers/provider_data.go

@@ -129,9 +129,12 @@ type OIDCClaims struct {
 func (p *ProviderData) verifyIDToken(ctx context.Context, token *oauth2.Token) (*oidc.IDToken, error) {
 	rawIDToken := getIDToken(token)
 	if strings.TrimSpace(rawIDToken) != "" {
+		if p.Verifier == nil {
+			return nil, ErrMissingOIDCVerifier
+		}
 		return p.Verifier.Verify(ctx, rawIDToken)
 	}
-	return nil, nil
+	return nil, ErrMissingIDToken
 }
 
 // buildSessionFromClaims uses IDToken claims to populate a fresh SessionState

providers/provider_data_test.go

@@ -137,23 +137,33 @@ func TestProviderData_verifyIDToken(t *testing.T) {
 
 	testCases := map[string]struct {
 		IDToken       *idTokenClaims
+		Verifier      bool
 		ExpectIDToken bool
 		ExpectedError error
 	}{
 		"Valid ID Token": {
 			IDToken:       &defaultIDToken,
+			Verifier:      true,
 			ExpectIDToken: true,
 			ExpectedError: nil,
 		},
 		"Invalid ID Token": {
 			IDToken:       &failureIDToken,
+			Verifier:      true,
 			ExpectIDToken: false,
 			ExpectedError: errors.New("failed to verify signature: the validation failed for subject [123456789]"),
 		},
 		"Missing ID Token": {
 			IDToken:       nil,
+			Verifier:      true,
 			ExpectIDToken: false,
-			ExpectedError: nil,
+			ExpectedError: ErrMissingIDToken,
+		},
+		"OIDC Verifier not Configured": {
+			IDToken:       &defaultIDToken,
+			Verifier:      false,
+			ExpectIDToken: false,
+			ExpectedError: ErrMissingOIDCVerifier,
 		},
 	}
 
@@ -170,12 +180,13 @@ func TestProviderData_verifyIDToken(t *testing.T) {
 				})
 			}
 
-			provider := &ProviderData{
+			provider := &ProviderData{}
-				Verifier: oidc.NewVerifier(
+			if tc.Verifier {
+				provider.Verifier = oidc.NewVerifier(
 					oidcIssuer,
 					mockJWKS{},
 					&oidc.Config{ClientID: oidcClientID},
-				),
+				)
 			}
 			verified, err := provider.verifyIDToken(context.Background(), token)
 			if err != nil {

providers/provider_default.go

@@ -22,6 +22,14 @@ var (
 	// code
 	ErrMissingCode = errors.New("missing code")
 
+	// ErrMissingIDToken is returned when an oidc.Token does not contain the
+	// extra `id_token` field for an IDToken.
+	ErrMissingIDToken = errors.New("missing id_token")
+
+	// ErrMissingOIDCVerifier is returned when a provider didn't set `Verifier`
+	// but an attempt to call `Verifier.Verify` was about to be made.
+	ErrMissingOIDCVerifier = errors.New("oidc verifier is not configured")
+
 	_ Provider = (*ProviderData)(nil)
 )