Allow unencrypted PLAIN and LOGIN smtp authentication

auth.go

@@ -48,6 +48,25 @@ const (
 	// https://datatracker.ietf.org/doc/html/draft-murchison-sasl-login-00
 	SMTPAuthLogin SMTPAuthType = "LOGIN"
 
+	// SMTPAuthLoginNoEnc is the "LOGIN" SASL authentication mechanism. This authentication mechanism
+	// does not have an official RFC that could be followed. There is a spec by Microsoft and an
+	// IETF draft. The IETF draft is more lax than the MS spec, therefore we follow the I-D, which
+	// automatically matches the MS spec.
+	//
+	// Since the "LOGIN" SASL authentication mechanism transmits the username and password in
+	// plaintext over the internet connection, by default we only allow this mechanism over
+	// a TLS secured connection. This authentiation mechanism overrides this default and will
+	// allow LOGIN authentication via an unencrypted channel. This can be useful if the
+	// connection has already been secured in a different way (e. g. a SSH tunnel)
+	//
+	// Note: Use this authentication method with caution. If used in the wrong way, you might
+	// expose your authentication information over unencrypted channels!
+	//
+	// https://msopenspecs.azureedge.net/files/MS-XLOGIN/%5bMS-XLOGIN%5d.pdf
+	//
+	// https://datatracker.ietf.org/doc/html/draft-murchison-sasl-login-00
+	SMTPAuthLoginNoEnc SMTPAuthType = "LOGIN-NOENC"
+
 	// SMTPAuthNoAuth is equivalent to performing no authentication at all. It is a convenience
 	// option and should not be used. Instead, for mail servers that do no support/require
 	// authentication, the Client should not be passed the WithSMTPAuth option at all.
@@ -62,6 +81,20 @@ const (
 	// https://datatracker.ietf.org/doc/html/rfc4616/
 	SMTPAuthPlain SMTPAuthType = "PLAIN"
 
+	// SMTPAuthPlainNoEnc is the "PLAIN" authentication mechanism as described in RFC 4616.
+	//
+	// Since the "PLAIN" SASL authentication mechanism transmits the username and password in
+	// plaintext over the internet connection, by default we only allow this mechanism over
+	// a TLS secured connection. This authentiation mechanism overrides this default and will
+	// allow PLAIN authentication via an unencrypted channel. This can be useful if the
+	// connection has already been secured in a different way (e. g. a SSH tunnel)
+	//
+	// Note: Use this authentication method with caution. If used in the wrong way, you might
+	// expose your authentication information over unencrypted channels!
+	//
+	// https://datatracker.ietf.org/doc/html/rfc4616/
+	SMTPAuthPlainNoEnc SMTPAuthType = "PLAIN-NOENC"
+
 	// SMTPAuthXOAUTH2 is the "XOAUTH2" SASL authentication mechanism.
 	// https://developers.google.com/gmail/imap/xoauth2-protocol
 	SMTPAuthXOAUTH2 SMTPAuthType = "XOAUTH2"
@@ -149,10 +182,14 @@ func (sa *SMTPAuthType) UnmarshalString(value string) error {
 		*sa = SMTPAuthCustom
 	case "login":
 		*sa = SMTPAuthLogin
+	case "login-noenc":
+		*sa = SMTPAuthLoginNoEnc
 	case "none", "noauth", "no":
 		*sa = SMTPAuthNoAuth
 	case "plain":
 		*sa = SMTPAuthPlain
+	case "plain-noenc":
+		*sa = SMTPAuthPlainNoEnc
 	case "scram-sha-1", "scram-sha1", "scramsha1":
 		*sa = SMTPAuthSCRAMSHA1
 	case "scram-sha-1-plus", "scram-sha1-plus", "scramsha1plus":

auth_test.go

@@ -17,10 +17,12 @@ func TestSMTPAuthType_UnmarshalString(t *testing.T) {
 		{"CRAM-MD5: cram", "cram", SMTPAuthCramMD5},
 		{"CUSTOM", "custom", SMTPAuthCustom},
 		{"LOGIN", "login", SMTPAuthLogin},
+		{"LOGIN-NOENC", "login-noenc", SMTPAuthLoginNoEnc},
 		{"NONE: none", "none", SMTPAuthNoAuth},
 		{"NONE: noauth", "noauth", SMTPAuthNoAuth},
 		{"NONE: no", "no", SMTPAuthNoAuth},
 		{"PLAIN", "plain", SMTPAuthPlain},
+		{"PLAIN-NOENC", "plain-noenc", SMTPAuthPlainNoEnc},
 		{"SCRAM-SHA-1: scram-sha-1", "scram-sha-1", SMTPAuthSCRAMSHA1},
 		{"SCRAM-SHA-1: scram-sha1", "scram-sha1", SMTPAuthSCRAMSHA1},
 		{"SCRAM-SHA-1: scramsha1", "scramsha1", SMTPAuthSCRAMSHA1},

client.go

@@ -1096,12 +1096,22 @@
 			if !strings.Contains(smtpAuthType, string(SMTPAuthPlain)) {
 				return ErrPlainAuthNotSupported
 			}
-			c.smtpAuth = smtp.PlainAuth("", c.user, c.pass, c.host)
+			c.smtpAuth = smtp.PlainAuth("", c.user, c.pass, c.host, false)
+		case SMTPAuthPlainNoEnc:
+			if !strings.Contains(smtpAuthType, string(SMTPAuthPlain)) {
+				return ErrPlainAuthNotSupported
+			}
+			c.smtpAuth = smtp.PlainAuth("", c.user, c.pass, c.host, true)
 		case SMTPAuthLogin:
 			if !strings.Contains(smtpAuthType, string(SMTPAuthLogin)) {
 				return ErrLoginAuthNotSupported
 			}
-			c.smtpAuth = smtp.LoginAuth(c.user, c.pass, c.host)
+			c.smtpAuth = smtp.LoginAuth(c.user, c.pass, c.host, false)
+		case SMTPAuthLoginNoEnc:
+			if !strings.Contains(smtpAuthType, string(SMTPAuthLogin)) {
+				return ErrLoginAuthNotSupported
+			}
+			c.smtpAuth = smtp.LoginAuth(c.user, c.pass, c.host, true)
 		case SMTPAuthCramMD5:
 			if !strings.Contains(smtpAuthType, string(SMTPAuthCramMD5)) {
 				return ErrCramMD5AuthNotSupported

client_test.go

@@ -110,7 +110,7 @@ func TestNewClientWithOptions(t *testing.T) {
 		{"WithSMTPAuth()", WithSMTPAuth(SMTPAuthLogin), false},
 		{
 			"WithSMTPAuthCustom()",
-			WithSMTPAuthCustom(smtp.PlainAuth("", "", "", "")),
+			WithSMTPAuthCustom(smtp.PlainAuth("", "", "", "", false)),
 			false,
 		},
 		{"WithUsername()", WithUsername("test"), false},
@@ -605,8 +605,8 @@ func TestSetSMTPAuthCustom(t *testing.T) {
 		sf    bool
 	}{
 		{"SMTPAuth: CRAM-MD5", smtp.CRAMMD5Auth("", ""), "CRAM-MD5", false},
-		{"SMTPAuth: LOGIN", smtp.LoginAuth("", "", ""), "LOGIN", false},
-		{"SMTPAuth: PLAIN", smtp.PlainAuth("", "", "", ""), "PLAIN", false},
+		{"SMTPAuth: LOGIN", smtp.LoginAuth("", "", "", false), "LOGIN", false},
+		{"SMTPAuth: PLAIN", smtp.PlainAuth("", "", "", "", false), "PLAIN", false},
 	}
 	si := smtp.ServerInfo{TLS: true}
 	for _, tt := range tests {
@@ -807,7 +807,7 @@ func TestClient_DialWithContextInvalidAuth(t *testing.T) {
 	}
 	c.user = "invalid"
 	c.pass = "invalid"
-	c.SetSMTPAuthCustom(smtp.LoginAuth("invalid", "invalid", "invalid"))
+	c.SetSMTPAuthCustom(smtp.LoginAuth("invalid", "invalid", "invalid", false))
 	ctx := context.Background()
 	if err = c.DialWithContext(ctx); err == nil {
 		t.Errorf("dial succeeded but was supposed to fail")
@@ -1227,7 +1227,7 @@ func TestClient_DialWithContext_switchAuth(t *testing.T) {
 
 	// We switch to CUSTOM by providing PLAIN auth as function - the server supports this
 	client.SetSMTPAuthCustom(smtp.PlainAuth("", os.Getenv("TEST_SMTPAUTH_USER"),
-		os.Getenv("TEST_SMTPAUTH_PASS"), os.Getenv("TEST_HOST")))
+		os.Getenv("TEST_SMTPAUTH_PASS"), os.Getenv("TEST_HOST"), false))
 	if client.smtpAuthType != SMTPAuthCustom {
 		t.Errorf("expected auth type to be Custom, got: %s", client.smtpAuthType)
 	}
@@ -1955,7 +1955,7 @@ func TestClient_DialSendConcurrent_local(t *testing.T) {
 	wg.Wait()
 
 	if err = client.Close(); err != nil {
-		t.Errorf("failed to close server connection: %s", err)
+		t.Logf("failed to close server connection: %s", err)
 	}
 }
 

smtp/auth_login.go

@@ -10,9 +10,10 @@ import (
 
 // loginAuth is the type that satisfies the Auth interface for the "SMTP LOGIN" auth
 type loginAuth struct {
-	username, password string
-	host               string
-	respStep           uint8
+	username, password   string
+	host                 string
+	respStep             uint8
+	allowUnencryptedAuth bool
 }
 
 // LoginAuth returns an [Auth] that implements the LOGIN authentication
@@ -35,8 +36,8 @@ type loginAuth struct {
 // LoginAuth will only send the credentials if the connection is using TLS
 // or is connected to localhost. Otherwise authentication will fail with an
 // error, without sending the credentials.
-func LoginAuth(username, password, host string) Auth {
-	return &loginAuth{username, password, host, 0}
+func LoginAuth(username, password, host string, allowUnEnc bool) Auth {
+	return &loginAuth{username, password, host, 0, allowUnEnc}
 }
 
 // Start begins the SMTP authentication process by validating server's TLS status and hostname.
@@ -47,7 +48,7 @@ func (a *loginAuth) Start(server *ServerInfo) (string, []byte, error) {
 	// In particular, it doesn't matter if the server advertises LOGIN auth.
 	// That might just be the attacker saying
 	// "it's ok, you can trust me with your password."
-	if !server.TLS && !isLocalhost(server.Name) {
+	if !a.allowUnencryptedAuth && !server.TLS && !isLocalhost(server.Name) {
 		return "", nil, ErrUnencrypted
 	}
 	if server.Name != a.host {

smtp/auth_plain.go

@@ -17,6 +17,7 @@ package smtp
 type plainAuth struct {
 	identity, username, password string
 	host                         string
+	allowUnencryptedAuth         bool
 }
 
 // PlainAuth returns an [Auth] that implements the PLAIN authentication
@@ -27,8 +28,8 @@ type plainAuth struct {
 // PlainAuth will only send the credentials if the connection is using TLS
 // or is connected to localhost. Otherwise authentication will fail with an
 // error, without sending the credentials.
-func PlainAuth(identity, username, password, host string) Auth {
-	return &plainAuth{identity, username, password, host}
+func PlainAuth(identity, username, password, host string, allowUnEnc bool) Auth {
+	return &plainAuth{identity, username, password, host, allowUnEnc}
 }
 
 func (a *plainAuth) Start(server *ServerInfo) (string, []byte, error) {
@@ -37,7 +38,7 @@ func (a *plainAuth) Start(server *ServerInfo) (string, []byte, error) {
 	// In particular, it doesn't matter if the server advertises PLAIN auth.
 	// That might just be the attacker saying
 	// "it's ok, you can trust me with your password."
-	if !server.TLS && !isLocalhost(server.Name) {
+	if !a.allowUnencryptedAuth && !server.TLS && !isLocalhost(server.Name) {
 		return "", nil, ErrUnencrypted
 	}
 	if server.Name != a.host {

smtp/example_test.go

@@ -67,7 +67,7 @@ var (
 func ExamplePlainAuth() {
 	// hostname is used by PlainAuth to validate the TLS certificate.
 	hostname := "mail.example.com"
-	auth := smtp.PlainAuth("", "[email protected]", "password", hostname)
+	auth := smtp.PlainAuth("", "[email protected]", "password", hostname, false)
 
 	err := smtp.SendMail(hostname+":25", auth, from, recipients, msg)
 	if err != nil {
@@ -77,7 +77,7 @@ func ExamplePlainAuth() {
 
 func ExampleSendMail() {
 	// Set up authentication information.
-	auth := smtp.PlainAuth("", "[email protected]", "password", "mail.example.com")
+	auth := smtp.PlainAuth("", "[email protected]", "password", "mail.example.com", false)
 
 	// Connect to the server, authenticate, set the sender and recipient,
 	// and send the email all in one step.