go-mail/smtp/auth_login.go

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// SPDX-FileCopyrightText: Copyright (c) 2022-2024 The go-mail Authors
//
// SPDX-License-Identifier: MIT
package smtp
import (
"fmt"
)
// loginAuth is the type that satisfies the Auth interface for the "SMTP LOGIN" auth
type loginAuth struct {
username, password string
host string
respStep uint8
}
// LoginAuth returns an [Auth] that implements the LOGIN authentication
// mechanism as it is used by MS Outlook. The Auth works similar to PLAIN
// but instead of sending all in one response, the login is handled within
// 3 steps:
// - Sending AUTH LOGIN (server might responds with "Username:")
// - Sending the username (server might responds with "Password:")
// - Sending the password (server authenticates)
// This is the common approach as specified by Microsoft in their MS-XLOGIN spec.
// See: https://msopenspecs.azureedge.net/files/MS-XLOGIN/%5bMS-XLOGIN%5d.pdf
// Yet, there is also an old IETF draft for SMTP AUTH LOGIN that states for clients:
// "The contents of both challenges SHOULD be ignored.".
// See: https://datatracker.ietf.org/doc/html/draft-murchison-sasl-login-00
// Since there is no official standard RFC and we've seen different implementations
// of this mechanism (sending "Username:", "Username", "username", "User name", etc.)
// we follow the IETF-Draft and ignore any server challange to allow compatiblity
// with most mail servers/providers.
//
// 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}
}
// Start begins the SMTP authentication process by validating server's TLS status and hostname.
// Returns "LOGIN" on success.
func (a *loginAuth) Start(server *ServerInfo) (string, []byte, error) {
// Must have TLS, or else localhost server.
// Note: If TLS is not true, then we can't trust ANYTHING in ServerInfo.
// 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) {
return "", nil, ErrUnencrypted
}
if server.Name != a.host {
return "", nil, ErrWrongHostname
}
a.respStep = 0
return "LOGIN", nil, nil
}
// Next processes responses from the server during the SMTP authentication exchange, sending the
// username and password.
func (a *loginAuth) Next(fromServer []byte, more bool) ([]byte, error) {
if more {
switch a.respStep {
case 0:
a.respStep++
return []byte(a.username), nil
case 1:
a.respStep++
return []byte(a.password), nil
default:
return nil, fmt.Errorf("%w: %s", ErrUnexpectedServerResponse, string(fromServer))
}
}
return nil, nil
}