LDAP: Add mTLS / client certificate authentication support

internal/bootstrap/service_bootstrap.go

@@ -37,6 +37,8 @@ func (app *BootstrapApp) initServices() (Services, error) {
 		BaseDN:       app.config.Ldap.BaseDN,
 		Insecure:     app.config.Ldap.Insecure,
 		SearchFilter: app.config.Ldap.SearchFilter,
+		AuthCert:     app.config.Ldap.AuthCert,
+		AuthKey:      app.config.Ldap.AuthKey,
 	})
 
 	err = ldapService.Init()

internal/config/config.go

@@ -66,6 +66,8 @@ type LdapConfig struct {
 	BaseDN       string `description:"Base DN for LDAP searches." yaml:"baseDn"`
 	Insecure     bool   `description:"Allow insecure LDAP connections." yaml:"insecure"`
 	SearchFilter string `description:"LDAP search filter." yaml:"searchFilter"`
+	AuthCert     string `description:"Certificate for mTLS authentication." yaml:"authCert"`
+	AuthKey      string `description:"Certificate key for mTLS authentication." yaml:"authKey"`
 }
 
 type ExperimentalConfig struct {

internal/service/auth_service.go

@@ -101,7 +101,7 @@ func (auth *AuthService) VerifyUser(search config.UserSearch, password string) b
 				return false
 			}
 
-			err = auth.ldap.Bind(auth.ldap.Config.BindDN, auth.ldap.Config.BindPassword)
+			err = auth.ldap.BindService(true)
 			if err != nil {
 				log.Error().Err(err).Msg("Failed to rebind with service account after user authentication")
 				return false

internal/service/ldap_service.go

@@ -19,21 +19,33 @@ type LdapServiceConfig struct {
 	BaseDN       string
 	Insecure     bool
 	SearchFilter string
+	AuthCert     string
+	AuthKey      string
 }
 
 type LdapService struct {
-	Config LdapServiceConfig // exported so as the auth service can use it
+	config LdapServiceConfig
 	conn   *ldapgo.Conn
 	mutex  sync.RWMutex
+	cert   *tls.Certificate
 }
 
 func NewLdapService(config LdapServiceConfig) *LdapService {
 	return &LdapService{
-		Config: config,
+		config: config,
 	}
 }
 
 func (ldap *LdapService) Init() error {
+	// Check whether authentication with client certificate is possible
+	if ldap.config.AuthCert != "" && ldap.config.AuthKey != "" {
+		cert, err := tls.LoadX509KeyPair(ldap.config.AuthCert, ldap.config.AuthKey)
+		if err != nil {
+			return fmt.Errorf("failed to initialize LDAP with mTLS authentication: %w", err)
+		}
+		ldap.cert = &cert
+		log.Info().Msg("Using LDAP with mTLS authentication")
+	}
 	_, err := ldap.connect()
 	if err != nil {
 		return fmt.Errorf("failed to connect to LDAP server: %w", err)
@@ -60,31 +72,46 @@ func (ldap *LdapService) connect() (*ldapgo.Conn, error) {
 	ldap.mutex.Lock()
 	defer ldap.mutex.Unlock()
 
-	conn, err := ldapgo.DialURL(ldap.Config.Address, ldapgo.DialWithTLSConfig(&tls.Config{
-		InsecureSkipVerify: ldap.Config.Insecure,
-		MinVersion:         tls.VersionTLS12,
-	}))
+	var conn *ldapgo.Conn
+	var err error
+
+	// TODO: There's also STARTTLS (or SASL)-based mTLS authentication
+	// scenarion, where we first connect to plain text port (389) and
+	// continue with a STARTTLS negotiation:
+	// 1. conn = ldap.DialURL("ldap://ldap.example.com:389")
+	// 2. conn.StartTLS(tlsConfig)
+	// 3. conn.externalBind()
+	if ldap.cert != nil {
+		conn, err = ldapgo.DialURL(ldap.config.Address, ldapgo.DialWithTLSConfig(&tls.Config{
+			MinVersion:   tls.VersionTLS12,
+			Certificates: []tls.Certificate{*ldap.cert},
+		}))
+	} else {
+		conn, err = ldapgo.DialURL(ldap.config.Address, ldapgo.DialWithTLSConfig(&tls.Config{
+			InsecureSkipVerify: ldap.config.Insecure,
+			MinVersion:         tls.VersionTLS12,
+		}))
+	}
 	if err != nil {
 		return nil, err
 	}
 
-	err = conn.Bind(ldap.Config.BindDN, ldap.Config.BindPassword)
+	ldap.conn = conn
+
+	err = ldap.BindService(false)
 	if err != nil {
 		return nil, err
 	}
-
-	// Set and return the connection
-	ldap.conn = conn
-	return conn, nil
+	return ldap.conn, nil
 }
 
 func (ldap *LdapService) Search(username string) (string, error) {
 	// Escape the username to prevent LDAP injection
 	escapedUsername := ldapgo.EscapeFilter(username)
-	filter := fmt.Sprintf(ldap.Config.SearchFilter, escapedUsername)
+	filter := fmt.Sprintf(ldap.config.SearchFilter, escapedUsername)
 
 	searchRequest := ldapgo.NewSearchRequest(
-		ldap.Config.BaseDN,
+		ldap.config.BaseDN,
 		ldapgo.ScopeWholeSubtree, ldapgo.NeverDerefAliases, 0, 0, false,
 		filter,
 		[]string{"dn"},
@@ -107,6 +134,19 @@ func (ldap *LdapService) Search(username string) (string, error) {
 	return userDN, nil
 }
 
+func (ldap *LdapService) BindService(rebind bool) error {
+	// Locks must not be used for initial binding attempt
+	if rebind {
+		ldap.mutex.Lock()
+		defer ldap.mutex.Unlock()
+	}
+
+	if ldap.cert != nil {
+		return ldap.conn.ExternalBind()
+	}
+	return ldap.conn.Bind(ldap.config.BindDN, ldap.config.BindPassword)
+}
+
 func (ldap *LdapService) Bind(userDN string, password string) error {
 	ldap.mutex.Lock()
 	defer ldap.mutex.Unlock()