Add support for unsecured SMTP LOGIN auth

Implemented an option to allow SMTP LOGIN authentication over unencrypted channels by introducing a new `SMTPAuthLoginNoEnc` type. Updated relevant functions and tests to handle the new parameter for unsecured 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.
@@ -65,7 +84,7 @@ const (
 	// 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, bye default we only allow this mechanism over
+	// 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)

client.go

@@ -1106,7 +1106,12 @@ func (c *Client) auth() error {
 			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

@@ -605,7 +605,7 @@ func TestSetSMTPAuthCustom(t *testing.T) {
 		sf    bool
 	}{
 		{"SMTPAuth: CRAM-MD5", smtp.CRAMMD5Auth("", ""), "CRAM-MD5", false},
-		{"SMTPAuth: LOGIN", smtp.LoginAuth("", "", ""), "LOGIN", false},
+		{"SMTPAuth: LOGIN", smtp.LoginAuth("", "", "", false), "LOGIN", false},
 		{"SMTPAuth: PLAIN", smtp.PlainAuth("", "", "", "", false), "PLAIN", false},
 	}
 	si := smtp.ServerInfo{TLS: true}
@@ -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")

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
+	username, password   string
-	host               string
+	host                 string
-	respStep           uint8
+	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 {
+func LoginAuth(username, password, host string, allowUnEnc bool) Auth {
-	return &loginAuth{username, password, host, 0}
+	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 {