feat: add support for PKCE (#686)

internal/app/api/v0/handlers/endpoint_authentication.go

@@ -24,9 +24,9 @@ type AuthenticationService interface {
 	// PlainLogin authenticates a user with a username and password.
 	PlainLogin(ctx context.Context, username, password string) (*domain.User, error)
 	// OauthLoginStep1 initiates the OAuth login flow.
-	OauthLoginStep1(_ context.Context, providerId string) (authCodeUrl, state, nonce string, err error)
+	OauthLoginStep1(_ context.Context, providerId string) (authCodeUrl, state, nonce, codeVerifier string, err error)
 	// OauthLoginStep2 completes the OAuth login flow and logins the user in.
-	OauthLoginStep2(ctx context.Context, providerId, nonce, code string) (*domain.User, string, error)
+	OauthLoginStep2(ctx context.Context, providerId, nonce, code, codeVerifier string) (*domain.User, string, error)
 	// OauthProviderLogoutUrl returns an IdP logout URL for the given provider if supported.
 	OauthProviderLogoutUrl(providerId, idTokenHint, postLogoutRedirectUri string) (string, bool)
 }
@@ -231,7 +231,7 @@ func (e AuthEndpoint) handleOauthInitiateGet() http.HandlerFunc {
 			return
 		}
 
-		authCodeUrl, state, nonce, err := e.authService.OauthLoginStep1(context.Background(), provider)
+		authCodeUrl, state, nonce, codeVerifier, err := e.authService.OauthLoginStep1(context.Background(), provider)
 		if err != nil {
 			slog.Debug("failed to create oauth auth code URL",
 				"provider", provider, "error", err)
@@ -247,6 +247,7 @@ func (e AuthEndpoint) handleOauthInitiateGet() http.HandlerFunc {
 		authSession := e.session.GetData(r.Context())
 		authSession.OauthState = state
 		authSession.OauthNonce = nonce
+		authSession.OauthCodeVerifier = codeVerifier
 		authSession.OauthProvider = provider
 		authSession.OauthReturnTo = returnTo
 		e.session.SetData(r.Context(), authSession)
@@ -323,7 +324,7 @@ func (e AuthEndpoint) handleOauthCallbackGet() http.HandlerFunc {
 
 		loginCtx, cancel := context.WithTimeout(context.Background(), 30*time.Second) // avoid long waits
 		user, idTokenHint, err := e.authService.OauthLoginStep2(loginCtx, provider, currentSession.OauthNonce,
-			oauthCode)
+			oauthCode, currentSession.OauthCodeVerifier)
 		cancel()
 		if err != nil {
 			slog.Debug("failed to process oauth code",
@@ -362,6 +363,7 @@ func (e AuthEndpoint) setAuthenticatedUser(r *http.Request, user *domain.User, o
 
 	currentSession.OauthState = ""
 	currentSession.OauthNonce = ""
+	currentSession.OauthCodeVerifier = ""
 	currentSession.OauthProvider = oauthProvider
 	currentSession.OauthReturnTo = ""
 	currentSession.OauthIdToken = idTokenHint

internal/app/api/v0/handlers/web_session.go

@@ -26,11 +26,12 @@ type SessionData struct {
 	Lastname  string
 	Email     string
 
-	OauthState    string
-	OauthNonce    string
-	OauthProvider string
-	OauthReturnTo string
-	OauthIdToken  string
+	OauthState        string
+	OauthNonce        string
+	OauthCodeVerifier string
+	OauthProvider     string
+	OauthReturnTo     string
+	OauthIdToken      string
 
 	WebAuthnData string
 
@@ -80,16 +81,17 @@ func (s *SessionWrapper) DestroyData(ctx context.Context) {
 
 func (s *SessionWrapper) defaultSessionData() SessionData {
 	return SessionData{
-		LoggedIn:       false,
-		IsAdmin:        false,
-		UserIdentifier: "",
-		Firstname:      "",
-		Lastname:       "",
-		Email:          "",
-		OauthState:     "",
-		OauthNonce:     "",
-		OauthProvider:  "",
-		OauthReturnTo:  "",
-		OauthIdToken:   "",
+		LoggedIn:          false,
+		IsAdmin:           false,
+		UserIdentifier:    "",
+		Firstname:         "",
+		Lastname:          "",
+		Email:             "",
+		OauthState:        "",
+		OauthNonce:        "",
+		OauthCodeVerifier: "",
+		OauthProvider:     "",
+		OauthReturnTo:     "",
+		OauthIdToken:      "",
 	}
 }

internal/app/auth/auth.go

@@ -47,6 +47,11 @@ const (
 	AuthenticatorTypeOidc  AuthenticatorType = "oidc"
 )
 
+const (
+	pkceMethodS256  = "S256"  // SHA-256 hashing
+	pkceMethodPlain = "plain" // plain text
+)
+
 // AuthenticatorOauth is the interface for all OAuth authenticators.
 type AuthenticatorOauth interface {
 	// GetName returns the name of the authenticator.
@@ -70,6 +75,10 @@ type AuthenticatorOauth interface {
 	GetAllowedUserGroups() []string
 	// GetLogoutUrl returns an IdP logout URL if supported by the provider.
 	GetLogoutUrl(idTokenHint, postLogoutRedirectUri string) (string, bool)
+	// PKCEAuthCodeOptions returns PKCE options for the authorization request and the verifier for the token exchange.
+	PKCEAuthCodeOptions() ([]oauth2.AuthCodeOption, string)
+	// PKCETokenOptions returns PKCE options for the token exchange.
+	PKCETokenOptions(verifier string) []oauth2.AuthCodeOption
 }
 
 // AuthenticatorLdap is the interface for all LDAP authenticators.
@@ -448,30 +457,34 @@ func (a *Authenticator) passwordAuthentication(
 
 // OauthLoginStep1 starts the oauth authentication flow by returning the authentication URL, state and nonce.
 func (a *Authenticator) OauthLoginStep1(_ context.Context, providerId string) (
-	authCodeUrl, state, nonce string,
+	authCodeUrl, state, nonce, codeVerifier string,
 	err error,
 ) {
 	oauthProvider, ok := a.oauthAuthenticators[providerId]
 	if !ok {
-		return "", "", "", fmt.Errorf("missing oauth provider %s", providerId)
+		return "", "", "", "", fmt.Errorf("missing oauth provider %s", providerId)
 	}
 
 	// Prepare authentication flow, set state cookies
 	state, err = a.randString(16)
 	if err != nil {
-		return "", "", "", fmt.Errorf("failed to generate state: %w", err)
+		return "", "", "", "", fmt.Errorf("failed to generate state: %w", err)
 	}
 
+	// Generate PKCE code verifier and challenge if enabled. Otherwise, options will be empty.
+	authCodeOptions, codeVerifier := oauthProvider.PKCEAuthCodeOptions()
+
 	switch oauthProvider.GetType() {
 	case AuthenticatorTypeOAuth:
-		authCodeUrl = oauthProvider.AuthCodeURL(state)
+		authCodeUrl = oauthProvider.AuthCodeURL(state, authCodeOptions...)
 	case AuthenticatorTypeOidc:
 		nonce, err = a.randString(16)
 		if err != nil {
-			return "", "", "", fmt.Errorf("failed to generate nonce: %w", err)
+			return "", "", "", "", fmt.Errorf("failed to generate nonce: %w", err)
 		}
 
-		authCodeUrl = oauthProvider.AuthCodeURL(state, oidc.Nonce(nonce))
+		authCodeOptions = append(authCodeOptions, oidc.Nonce(nonce))
+		authCodeUrl = oauthProvider.AuthCodeURL(state, authCodeOptions...)
 	}
 
 	return
@@ -531,13 +544,16 @@ func isAnyAllowedUserGroup(userGroups, allowedUserGroups []string) bool {
 
 // OauthLoginStep2 finishes the oauth authentication flow by exchanging the code for an access token and
 // fetching the user information.
-func (a *Authenticator) OauthLoginStep2(ctx context.Context, providerId, nonce, code string) (*domain.User, string, error) {
+func (a *Authenticator) OauthLoginStep2(
+	ctx context.Context,
+	providerId, nonce, code, codeVerifier string,
+) (*domain.User, string, error) {
 	oauthProvider, ok := a.oauthAuthenticators[providerId]
 	if !ok {
 		return nil, "", fmt.Errorf("missing oauth provider %s", providerId)
 	}
 
-	oauth2Token, err := oauthProvider.Exchange(ctx, code)
+	oauth2Token, err := oauthProvider.Exchange(ctx, code, oauthProvider.PKCETokenOptions(codeVerifier)...)
 	if err != nil {
 		return nil, "", fmt.Errorf("unable to exchange code: %w", err)
 	}

internal/app/auth/auth_oauth.go

@@ -30,6 +30,8 @@ type PlainOauthAuthenticator struct {
 	sensitiveInfoLogging bool
 	allowedDomains       []string
 	allowedUserGroups    []string
+	usePKCE              bool
+	pkceMethod           string
 }
 
 func newPlainOauthAuthenticator(
@@ -62,6 +64,14 @@ func newPlainOauthAuthenticator(
 	provider.sensitiveInfoLogging = cfg.LogSensitiveInfo
 	provider.allowedDomains = cfg.AllowedDomains
 	provider.allowedUserGroups = cfg.AllowedUserGroups
+	provider.usePKCE = cfg.UsePKCE == nil || *cfg.UsePKCE
+	provider.pkceMethod = cfg.PKCEMethod
+	if provider.pkceMethod == "" {
+		provider.pkceMethod = pkceMethodS256
+	}
+	if provider.usePKCE && provider.pkceMethod != pkceMethodS256 && provider.pkceMethod != pkceMethodPlain {
+		return nil, fmt.Errorf("unsupported PKCE method %q, allowed: S256, plain", provider.pkceMethod)
+	}
 
 	return provider, nil
 }
@@ -83,6 +93,32 @@ func (p PlainOauthAuthenticator) GetLogoutUrl(_, _ string) (string, bool) {
 	return "", false
 }
 
+// PKCEAuthCodeOptions returns PKCE options for the authorization request and the verifier for the token exchange.
+func (p PlainOauthAuthenticator) PKCEAuthCodeOptions() ([]oauth2.AuthCodeOption, string) {
+	if !p.usePKCE {
+		return nil, ""
+	}
+
+	verifier := oauth2.GenerateVerifier()
+	if p.pkceMethod == pkceMethodPlain {
+		return []oauth2.AuthCodeOption{
+			oauth2.SetAuthURLParam("code_challenge", verifier),
+			oauth2.SetAuthURLParam("code_challenge_method", pkceMethodPlain),
+		}, verifier
+	}
+
+	return []oauth2.AuthCodeOption{oauth2.S256ChallengeOption(verifier)}, verifier
+}
+
+// PKCETokenOptions returns PKCE options for the token exchange.
+func (p PlainOauthAuthenticator) PKCETokenOptions(verifier string) []oauth2.AuthCodeOption {
+	if !p.usePKCE || verifier == "" {
+		return nil
+	}
+
+	return []oauth2.AuthCodeOption{oauth2.VerifierOption(verifier)}
+}
+
 // RegistrationEnabled returns whether registration is enabled for the OAuth authenticator.
 func (p PlainOauthAuthenticator) RegistrationEnabled() bool {
 	return p.registrationEnabled

internal/app/auth/auth_oauth_test.go

@@ -0,0 +1,61 @@
+package auth
+
+import (
+	"testing"
+
+	"golang.org/x/oauth2"
+)
+
+func TestPlainOauthAuthenticatorPKCES256Options(t *testing.T) {
+	authenticator := PlainOauthAuthenticator{usePKCE: true, pkceMethod: "S256"}
+
+	options, verifier := authenticator.PKCEAuthCodeOptions()
+	if verifier == "" {
+		t.Fatal("expected verifier")
+	}
+
+	values := authCodeValues(t, options)
+
+	if values.Get("code_challenge") == "" {
+		t.Fatal("expected code_challenge")
+	}
+	if values.Get("code_challenge_method") != "S256" {
+		t.Fatalf("expected S256 challenge method, got %q", values.Get("code_challenge_method"))
+	}
+
+	tokenOptions := authenticator.PKCETokenOptions(verifier)
+	if len(tokenOptions) != 1 {
+		t.Fatalf("expected one token option, got %d", len(tokenOptions))
+	}
+}
+
+func TestPlainOauthAuthenticatorPKCEPlainOptions(t *testing.T) {
+	authenticator := PlainOauthAuthenticator{usePKCE: true, pkceMethod: "plain"}
+
+	options, verifier := authenticator.PKCEAuthCodeOptions()
+	values := authCodeValues(t, options)
+
+	if values.Get("code_challenge") != verifier {
+		t.Fatalf("expected plain challenge %q, got %q", verifier, values.Get("code_challenge"))
+	}
+	if values.Get("code_challenge_method") != "plain" {
+		t.Fatalf("expected plain challenge method, got %q", values.Get("code_challenge_method"))
+	}
+}
+
+func TestPlainOauthAuthenticatorPKCEDisabled(t *testing.T) {
+	authenticator := PlainOauthAuthenticator{usePKCE: false, pkceMethod: "S256"}
+
+	options, verifier := authenticator.PKCEAuthCodeOptions()
+	if len(options) != 0 {
+		t.Fatalf("expected no auth code options, got %d", len(options))
+	}
+	if verifier != "" {
+		t.Fatalf("expected empty verifier, got %q", verifier)
+	}
+
+	tokenOptions := authenticator.PKCETokenOptions(oauth2.GenerateVerifier())
+	if len(tokenOptions) != 0 {
+		t.Fatalf("expected no token options, got %d", len(tokenOptions))
+	}
+}

internal/app/auth/auth_oidc.go

@@ -30,6 +30,8 @@ type OidcAuthenticator struct {
 	allowedUserGroups    []string
 	endSessionEndpoint   string
 	logoutIdpSession     bool
+	usePKCE              bool
+	pkceMethod           string
 }
 
 func newOidcAuthenticator(
@@ -67,6 +69,14 @@ func newOidcAuthenticator(
 	provider.allowedDomains = cfg.AllowedDomains
 	provider.allowedUserGroups = cfg.AllowedUserGroups
 	provider.logoutIdpSession = cfg.LogoutIdpSession == nil || *cfg.LogoutIdpSession
+	provider.usePKCE = cfg.UsePKCE == nil || *cfg.UsePKCE
+	provider.pkceMethod = cfg.PKCEMethod
+	if provider.pkceMethod == "" {
+		provider.pkceMethod = pkceMethodS256
+	}
+	if provider.usePKCE && provider.pkceMethod != pkceMethodS256 && provider.pkceMethod != pkceMethodPlain {
+		return nil, fmt.Errorf("unsupported PKCE method %q, allowed: S256, plain", provider.pkceMethod)
+	}
 
 	var providerMetadata struct {
 		EndSessionEndpoint string `json:"end_session_endpoint"`
@@ -121,6 +131,32 @@ func (o OidcAuthenticator) GetLogoutUrl(idTokenHint, postLogoutRedirectUri strin
 	return logoutUrl.String(), true
 }
 
+// PKCEAuthCodeOptions returns PKCE options for the authorization request and the verifier for the token exchange.
+func (o OidcAuthenticator) PKCEAuthCodeOptions() ([]oauth2.AuthCodeOption, string) {
+	if !o.usePKCE {
+		return nil, ""
+	}
+
+	verifier := oauth2.GenerateVerifier()
+	if o.pkceMethod == pkceMethodPlain {
+		return []oauth2.AuthCodeOption{
+			oauth2.SetAuthURLParam("code_challenge", verifier),
+			oauth2.SetAuthURLParam("code_challenge_method", pkceMethodPlain),
+		}, verifier
+	}
+
+	return []oauth2.AuthCodeOption{oauth2.S256ChallengeOption(verifier)}, verifier
+}
+
+// PKCETokenOptions returns PKCE options for the token exchange.
+func (o OidcAuthenticator) PKCETokenOptions(verifier string) []oauth2.AuthCodeOption {
+	if !o.usePKCE || verifier == "" {
+		return nil
+	}
+
+	return []oauth2.AuthCodeOption{oauth2.VerifierOption(verifier)}
+}
+
 // RegistrationEnabled returns whether registration is enabled for this authenticator.
 func (o OidcAuthenticator) RegistrationEnabled() bool {
 	return o.registrationEnabled

internal/app/auth/auth_oidc_test.go

@@ -0,0 +1,79 @@
+package auth
+
+import (
+	"net/url"
+	"testing"
+
+	"golang.org/x/oauth2"
+)
+
+func authCodeValues(t *testing.T, options []oauth2.AuthCodeOption) url.Values {
+	t.Helper()
+
+	config := oauth2.Config{
+		ClientID:    "client-id",
+		Endpoint:    oauth2.Endpoint{AuthURL: "https://example.com/auth"},
+		RedirectURL: "https://wg.example.com/callback",
+	}
+	authCodeURL, err := url.Parse(config.AuthCodeURL("state", options...))
+	if err != nil {
+		t.Fatalf("failed to parse auth code URL: %v", err)
+	}
+
+	return authCodeURL.Query()
+}
+
+func TestOidcAuthenticatorPKCES256Options(t *testing.T) {
+	authenticator := OidcAuthenticator{usePKCE: true, pkceMethod: "S256"}
+
+	options, verifier := authenticator.PKCEAuthCodeOptions()
+	if verifier == "" {
+		t.Fatal("expected verifier")
+	}
+
+	values := authCodeValues(t, options)
+
+	if values.Get("code_challenge") == "" {
+		t.Fatal("expected code_challenge")
+	}
+	if values.Get("code_challenge_method") != "S256" {
+		t.Fatalf("expected S256 challenge method, got %q", values.Get("code_challenge_method"))
+	}
+
+	tokenOptions := authenticator.PKCETokenOptions(verifier)
+	if len(tokenOptions) != 1 {
+		t.Fatalf("expected one token option, got %d", len(tokenOptions))
+	}
+
+}
+
+func TestOidcAuthenticatorPKCEPlainOptions(t *testing.T) {
+	authenticator := OidcAuthenticator{usePKCE: true, pkceMethod: "plain"}
+
+	options, verifier := authenticator.PKCEAuthCodeOptions()
+	values := authCodeValues(t, options)
+
+	if values.Get("code_challenge") != verifier {
+		t.Fatalf("expected plain challenge %q, got %q", verifier, values.Get("code_challenge"))
+	}
+	if values.Get("code_challenge_method") != "plain" {
+		t.Fatalf("expected plain challenge method, got %q", values.Get("code_challenge_method"))
+	}
+}
+
+func TestOidcAuthenticatorPKCEDisabled(t *testing.T) {
+	authenticator := OidcAuthenticator{usePKCE: false, pkceMethod: "S256"}
+
+	options, verifier := authenticator.PKCEAuthCodeOptions()
+	if len(options) != 0 {
+		t.Fatalf("expected no auth code options, got %d", len(options))
+	}
+	if verifier != "" {
+		t.Fatalf("expected empty verifier, got %q", verifier)
+	}
+
+	tokenOptions := authenticator.PKCETokenOptions(oauth2.GenerateVerifier())
+	if len(tokenOptions) != 0 {
+		t.Fatalf("expected no token options, got %d", len(tokenOptions))
+	}
+}