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Date: Sat, 14 Sep 2013 09:05:01 +0200
From: Agostino Sarubbo <ago@...too.org>
To: oss-security@...ts.openwall.com
Subject: CVE request: proftpd: mod_sftp/mod_sftp_pam invalid pool allocation during kbdint authentication
Hello,
From: http://kingcope.wordpress.com/2013/09/11/proftpd-mod_sftpmod_sftp_pam-invalid-pool-allocation-in-kbdint-authentication/
ProFTPd installs with mod_sftp and mod_sftp_pam activated contain the
vulnerability described in this post.
The current stable release of ProFTPd is 1.3.4d and the current release
candidate is 1.3.5rc3.
First I have to note that this vulnerability is unlikely to be exploited.
There is a way to control $rip instruction pointer
on 64 bit systems, for example on the Ubuntu 64Bit platform but I believe that
it is not possible to get full code execution with this bug.
The bug is useful to trigger a large heap allocation and exhaust all available
system memory of the underlying operating system.
Inside the file located at proftpd-1.3.5rc2/contrib/mod_sftp/kbdint.c ProFTPd
handles the SSH keyboard interactive authentication procedure, in this case it
will use pam as an authentication library therefore mod_sftp_pam has to be
active for an installation to be vulnerable.
Source code file and line kbdint.c:300 reads:
[1] resp_count = sftp_msg_read_int(pkt->pool, &buf, &buflen);
[2] list = make_array(p, resp_count, sizeof(char *));
for (i = 0; i < resp_count; i++) {
char *resp;
resp = sftp_msg_read_string(pkt->pool, &buf, &buflen);
*((char **) push_array(list)) = pstrdup(p, sftp_utf8_decode_str(p, resp));
}
Line 1 will read the kbdint response count which is an unsigned integer with a
size of 32 bits from the client during an SSH kbdint userauth info response
client request.
This value is used to allocate a buffer with the size
user_supplied_uint32_value multiplied by the size of a char pointer being
32bits or 64bits depending on the platform.
There is no size check before the request is sent to the pool allocator that
is called by make_array at Line 2.
The pool allocator can be tricked to handle negative allocation sizes if
resp_count is large enough.
There is a size check of the response count value but it’s done after this
function returns.
The DoS condition can be triggered by sending an int32 value for resp_count
that is slightly below the available memory of the target system and repeating
the request.
Noteably OpenSSH vulnerability CVE-2002-0640 is very similar to this ProFTPd
vulnerability. It has the very same code path.
Here is a reference to the OpenSSH Challenge-Response Authentication bug that
was exploited by GOBBLES Security in their year 2002 sshutuptheo.tgz exploit:
http://lwn.net/Articles/3531/.
Usage of keyboard interactive authentication in ProFTPd mod_sftp is rare as it
is not activated by default.
--
Agostino Sarubbo
Gentoo Linux Developer
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