Merge pull request #344 from wneessen/feature/342_allow-bypass-plain-auth-tls-check

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 @@ func (c *Client) auth() error {
 			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("", "user@example.com", "password", hostname)
+	auth := smtp.PlainAuth("", "user@example.com", "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("", "user@example.com", "password", "mail.example.com")
+	auth := smtp.PlainAuth("", "user@example.com", "password", "mail.example.com", false)
 
 	// Connect to the server, authenticate, set the sender and recipient,
 	// and send the email all in one step.

smtp/smtp_test.go

@@ -50,7 +50,7 @@ type authTest struct {
 
 var authTests = []authTest{
 	{
-		PlainAuth("", "user", "pass", "testserver"),
+		PlainAuth("", "user", "pass", "testserver", false),
 		[]string{},
 		"PLAIN",
 		[]string{"\x00user\x00pass"},
@@ -58,7 +58,15 @@ var authTests = []authTest{
 		false,
 	},
 	{
-		PlainAuth("foo", "bar", "baz", "testserver"),
+		PlainAuth("", "user", "pass", "testserver", true),
+		[]string{},
+		"PLAIN",
+		[]string{"\x00user\x00pass"},
+		[]bool{false, false},
+		false,
+	},
+	{
+		PlainAuth("foo", "bar", "baz", "testserver", false),
 		[]string{},
 		"PLAIN",
 		[]string{"foo\x00bar\x00baz"},
@@ -66,7 +74,7 @@ var authTests = []authTest{
 		false,
 	},
 	{
-		PlainAuth("foo", "bar", "baz", "testserver"),
+		PlainAuth("foo", "bar", "baz", "testserver", false),
 		[]string{"foo"},
 		"PLAIN",
 		[]string{"foo\x00bar\x00baz", ""},
@@ -74,7 +82,7 @@ var authTests = []authTest{
 		false,
 	},
 	{
-		LoginAuth("user", "pass", "testserver"),
+		LoginAuth("user", "pass", "testserver", false),
 		[]string{"Username:", "Password:"},
 		"LOGIN",
 		[]string{"", "user", "pass"},
@@ -82,7 +90,15 @@ var authTests = []authTest{
 		false,
 	},
 	{
-		LoginAuth("user", "pass", "testserver"),
+		LoginAuth("user", "pass", "testserver", true),
+		[]string{"Username:", "Password:"},
+		"LOGIN",
+		[]string{"", "user", "pass"},
+		[]bool{false, false},
+		false,
+	},
+	{
+		LoginAuth("user", "pass", "testserver", false),
 		[]string{"User Name\x00", "Password\x00"},
 		"LOGIN",
 		[]string{"", "user", "pass"},
@@ -90,7 +106,7 @@ var authTests = []authTest{
 		false,
 	},
 	{
-		LoginAuth("user", "pass", "testserver"),
+		LoginAuth("user", "pass", "testserver", false),
 		[]string{"Invalid", "Invalid:"},
 		"LOGIN",
 		[]string{"", "user", "pass"},
@@ -98,7 +114,7 @@ var authTests = []authTest{
 		false,
 	},
 	{
-		LoginAuth("user", "pass", "testserver"),
+		LoginAuth("user", "pass", "testserver", false),
 		[]string{"Invalid", "Invalid:", "Too many"},
 		"LOGIN",
 		[]string{"", "user", "pass", ""},
@@ -237,7 +253,47 @@ func TestAuthPlain(t *testing.T) {
 		},
 	}
 	for i, tt := range tests {
-		auth := PlainAuth("foo", "bar", "baz", tt.authName)
+		auth := PlainAuth("foo", "bar", "baz", tt.authName, false)
+		_, _, err := auth.Start(tt.server)
+		got := ""
+		if err != nil {
+			got = err.Error()
+		}
+		if got != tt.err {
+			t.Errorf("%d. got error = %q; want %q", i, got, tt.err)
+		}
+	}
+}
+
+func TestAuthPlainNoEnc(t *testing.T) {
+	tests := []struct {
+		authName string
+		server   *ServerInfo
+		err      string
+	}{
+		{
+			authName: "servername",
+			server:   &ServerInfo{Name: "servername", TLS: true},
+		},
+		{
+			// OK to use PlainAuth on localhost without TLS
+			authName: "localhost",
+			server:   &ServerInfo{Name: "localhost", TLS: false},
+		},
+		{
+			// Also OK on non-TLS secured connections. The NoEnc mechanism is meant to allow
+			// non-encrypted connections.
+			authName: "servername",
+			server:   &ServerInfo{Name: "servername", Auth: []string{"PLAIN"}},
+		},
+		{
+			authName: "servername",
+			server:   &ServerInfo{Name: "attacker", TLS: true},
+			err:      "wrong host name",
+		},
+	}
+	for i, tt := range tests {
+		auth := PlainAuth("foo", "bar", "baz", tt.authName, true)
 		_, _, err := auth.Start(tt.server)
 		got := ""
 		if err != nil {
@@ -283,7 +339,51 @@ func TestAuthLogin(t *testing.T) {
 		},
 	}
 	for i, tt := range tests {
-		auth := LoginAuth("foo", "bar", tt.authName)
+		auth := LoginAuth("foo", "bar", tt.authName, false)
+		_, _, err := auth.Start(tt.server)
+		got := ""
+		if err != nil {
+			got = err.Error()
+		}
+		if got != tt.err {
+			t.Errorf("%d. got error = %q; want %q", i, got, tt.err)
+		}
+	}
+}
+
+func TestAuthLoginNoEnc(t *testing.T) {
+	tests := []struct {
+		authName string
+		server   *ServerInfo
+		err      string
+	}{
+		{
+			authName: "servername",
+			server:   &ServerInfo{Name: "servername", TLS: true},
+		},
+		{
+			// OK to use LoginAuth on localhost without TLS
+			authName: "localhost",
+			server:   &ServerInfo{Name: "localhost", TLS: false},
+		},
+		{
+			// Also OK on non-TLS secured connections. The NoEnc mechanism is meant to allow
+			// non-encrypted connections.
+			authName: "servername",
+			server:   &ServerInfo{Name: "servername", Auth: []string{"LOGIN"}},
+		},
+		{
+			authName: "servername",
+			server:   &ServerInfo{Name: "servername", Auth: []string{"CRAM-MD5"}},
+		},
+		{
+			authName: "servername",
+			server:   &ServerInfo{Name: "attacker", TLS: true},
+			err:      "wrong host name",
+		},
+	}
+	for i, tt := range tests {
+		auth := LoginAuth("foo", "bar", tt.authName, true)
 		_, _, err := auth.Start(tt.server)
 		got := ""
 		if err != nil {
@@ -317,7 +417,11 @@ func TestXOAuth2OK(t *testing.T) {
 	if err != nil {
 		t.Fatalf("NewClient: %v", err)
 	}
-	defer c.Close()
+	defer func() {
+		if err := c.Close(); err != nil {
+			t.Errorf("failed to close client: %s", err)
+		}
+	}()
 
 	auth := XOAuth2Auth("user", "token")
 	err = c.Auth(auth)
@@ -355,7 +459,11 @@ func TestXOAuth2Error(t *testing.T) {
 	if err != nil {
 		t.Fatalf("NewClient: %v", err)
 	}
-	defer c.Close()
+	defer func() {
+		if err := c.Close(); err != nil {
+			t.Errorf("failed to close client: %s", err)
+		}
+	}()
 
 	auth := XOAuth2Auth("user", "token")
 	err = c.Auth(auth)
@@ -707,7 +815,7 @@ func TestBasic(t *testing.T) {
 	// fake TLS so authentication won't complain
 	c.tls = true
 	c.serverName = "smtp.google.com"
-	if err := c.Auth(PlainAuth("", "user", "pass", "smtp.google.com")); err != nil {
+	if err := c.Auth(PlainAuth("", "user", "pass", "smtp.google.com", false)); err != nil {
 		t.Fatalf("AUTH failed: %s", err)
 	}
 
@@ -1278,7 +1386,7 @@ func TestHello(t *testing.T) {
 		case 3:
 			c.tls = true
 			c.serverName = "smtp.google.com"
-			err = c.Auth(PlainAuth("", "user", "pass", "smtp.google.com"))
+			err = c.Auth(PlainAuth("", "user", "pass", "smtp.google.com", false))
 		case 4:
 			err = c.Mail("test@example.com")
 		case 5:
@@ -1523,7 +1631,7 @@ func TestSendMailWithAuth(t *testing.T) {
 		}
 	}()
 
-	err = SendMail(l.Addr().String(), PlainAuth("", "user", "pass", "smtp.google.com"), "test@example.com", []string{"other@example.com"}, []byte(strings.Replace(`From: test@example.com
+	err = SendMail(l.Addr().String(), PlainAuth("", "user", "pass", "smtp.google.com", false), "test@example.com", []string{"other@example.com"}, []byte(strings.Replace(`From: test@example.com
 To: other@example.com
 Subject: SendMail test
 
@@ -1531,6 +1639,7 @@ SendMail is working for me.
 `, "\n", "\r\n", -1)))
 	if err == nil {
 		t.Error("SendMail: Server doesn't support AUTH, expected to get an error, but got none ")
+		return
 	}
 	if err.Error() != "smtp: server doesn't support AUTH" {
 		t.Errorf("Expected: smtp: server doesn't support AUTH, got: %s", err)
@@ -1558,7 +1667,7 @@ func TestAuthFailed(t *testing.T) {
 
 	c.tls = true
 	c.serverName = "smtp.google.com"
-	err = c.Auth(PlainAuth("", "user", "pass", "smtp.google.com"))
+	err = c.Auth(PlainAuth("", "user", "pass", "smtp.google.com", false))
 
 	if err == nil {
 		t.Error("Auth: expected error; got none")