漏洞预警：LibOFX标签解析代码执行漏洞CVE-2017-2816

2017 年 9 月 14 日 E安全 E小编

E安全9月15日讯思科Talos团队的Cory Duplantis发现LibOFX 0.9.11中存在可利用的缓冲区溢出漏洞。特制OFX文件可能会导致写带外数据（ Out of Bound data）引起堆栈缓冲区溢出。攻击者可创建恶意OFX文件触发该漏洞——CVE-2017-2816。

带外(Out of Band，OOB)数据

端对端数据传输时，用于迅速通告对方本端发生的重要事件。带外数据比普通数据（也称带内数据）有更高的优先级，总是立即发送，而不论发送缓冲区是否有排队等待发送的普通数据。带外数据的传输可以使用一条独立的传输层连接，也可以映射到传输普通数据的连接中，即带外数据并不是通过两条套接字连接来实现的，这类数据也是通过已有的连接来传输。

Talos公司此次发现一项可被利用的缓冲区溢出漏洞，利用特定OFX文件，其可能出现写入操作边界溢出，并导致对方获得代码执行能力：

TALOS-2017-0317 (CVE-2017-2816) - LibOFX标签解析代码执行漏洞

此项漏泄目前尚未得到修复，而Talos方面在《供应商漏洞报告与披露政策》指定的期限之内亦未收到相关开发人员给出的回复。

漏洞预警：LibOFX标签解析代码执行漏洞CVE-2017-2816-E安全

讽刺的是，该漏洞位于清理功能当中的标签解析流程内。在此功能当中，标签名称会以本地方式存储在堆栈之内，而过长的标签名称则会造成堆栈溢出。

CVE-2017-2816漏洞详情

OFX是一个开放性财务数据交换协议，金融机构使用OFX与客户共享财务数据。

LibOFX是开放性金融交易（OFX）的开源解析库，金融机构使用OFX与客户共享金融数据。作为一套复杂的标准实现方案，这套库被GnuCash等财务软件所广泛使用。GnuCash能使用OFX的开源解析库libOFX导入OFX记录。

漏洞预警：LibOFX标签解析代码执行漏洞CVE-2017-2816-E安全

解析给定OFX记录的标签时， libOFX尝试去掉现有OFX的专有标签。

检查正确OFX标签

【代码】

lib/ofx_preproc.cpp:75
const unsigned int READ_BUFFER_SIZE = 1024;

lib/ofx_preproc.cpp:417
string sanitize_proprietary_tags(string input_string)
{
unsigned int i;
size_t input_string_size;
bool strip = false;
bool tag_open = false;
int tag_open_idx = 0; //Are we within < > ?
bool closing_tag_open = false; //Are we within ?
int orig_tag_open_idx = 0;
bool proprietary_tag = false; //Are we within a proprietary element?
bool proprietary_closing_tag = false;
int crop_end_idx = 0;
char buffer[READ_BUFFER_SIZE] = "";
char tagname[READ_BUFFER_SIZE] = "";
int tagname_idx = 0;
char close_tagname[READ_BUFFER_SIZE] = "";

for (i = 0; i < input_string_size; i++)

    if (input_string.c_str()[i] == '<')
    {
    tag_open = true;
    tag_open_idx = i;
    if (proprietary_tag == true && input_string.c_str()[i+1] == '/')
    {
        ...
    }
    else if (proprietary_tag == true)
    {
        //It is the start of a new tag, following a proprietary tag
        crop_end_idx = i - 1;
        strip = true;
    }
    }
    else if (input_string.c_str()[i] == '>')
    {
        ...
    }
    else if (tag_open == true && closing_tag_open == false)
    {
    if (input_string.c_str()[i] == '.')
    {
        if (proprietary_tag != true)
        {
        orig_tag_open_idx = tag_open_idx;
        proprietary_tag = true;
        }
    }
    tagname[tagname_idx] = input_string.c_str()[i]; [0]
    tagname_idx++;

由于input_string变量会造成循环，如果input_string大于READ_BUFFER_SIZE，这个堆栈变量tagname就造成缓冲区溢出，并可能潜在执行恶意代码。

【崩溃信息】

==6542==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7fffffbb4260 at pc 0x7fab9d30ccc1 bp 0x7fffffbb39b0 sp 0x7fffffbb39a8
WRITE of size 1 at 0x7fffffbb4260 thread T0
    #0 0x7fab9d30ccc0 (/home/vagrant/fuzzing/libofx-asan/libofx-0.9.11/lib/.libs/libofx.so.7+0x30cc0)
    #1 0x7fab9d30aba0 (/home/vagrant/fuzzing/libofx-asan/libofx-0.9.11/lib/.libs/libofx.so.7+0x2eba0)
    #2 0x7fab9d3057cb (/home/vagrant/fuzzing/libofx-asan/libofx-0.9.11/lib/.libs/libofx.so.7+0x297cb)
    #3 0x4f8ba2 (/home/vagrant/fuzzing/libofx-asan/libofx-0.9.11/ofxdump/.libs/lt-ofxdump+0x4f8ba2)
    #4 0x7fab9c06982f (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
    #5 0x419618 (/home/vagrant/fuzzing/libofx-asan/libofx-0.9.11/ofxdump/.libs/lt-ofxdump+0x419618)

Address 0x7fffffbb4260 is located in stack of thread T0 at offset 2208 in frame
    #0 0x7fab9d30c38f (/home/vagrant/fuzzing/libofx-asan/libofx-0.9.11/lib/.libs/libofx.so.7+0x3038f)

This frame has 9 object(s):
    [32, 1056) 'buffer'
    [1184, 2208) 'tagname' <== Memory access at offset 2208 overflows this variable
    [2336, 3360) 'close_tagname'
    [3488, 3520) ''
    [3552, 3584) ''
    [3616, 3617) ''
    [3632, 3664) ''
    [3696, 3728) ''
    [3760, 3761) ''
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
    (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow (/home/vagrant/fuzzing/libofx-asan/libofx-
    0.9.11/lib/.libs/libofx.so.7+0x30cc0)
Shadow bytes around the buggy address:
0x10007ff6e7f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10007ff6e800: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10007ff6e810: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10007ff6e820: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10007ff6e830: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x10007ff6e840: 00 00 00 00 00 00 00 00 00 00 00 00[f2]f2 f2 f2
0x10007ff6e850: f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 00 00 00 00
0x10007ff6e860: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10007ff6e870: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10007ff6e880: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10007ff6e890: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable:           00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone:       fa
Heap right redzone:      fb
Freed heap region:       fd
Stack left redzone:      f1
Stack mid redzone:       f2
Stack right redzone:     f3
Stack partial redzone:   f4
Stack after return:      f5
Stack use after scope:   f8
Global redzone:          f9
Global init order:       f6
Poisoned by user:        f7
Container overflow:      fc
Array cookie:            ac
Intra object redzone:    bb
ASan internal:           fe
Left alloca redzone:     ca
Right alloca redzone:    cb
==6542==ABORTING