========================================================================
= Asterisk - chan_skinny Remote Unauthenticated Heap Overflow
=
= Vendor Website:
= http://www.asterisk.org
=
= Affected Version:
=  All 1.2-branch releases prior to and including 1.2.12.1
=  All 1.0-branch releases prior to and including 1.0.12
=
= Not Affected:
=  All 1.4-branch beta releases (1.4.0-beta1, 1.4.0-beta2)
=
= Public disclosure on Oct 19, 2006
========================================================================

== Overview ==

Asterisk is "The Opensource PBX", a popular software telephony server.

The Asterisk Skinny channel driver for Cisco SCCP phones chan_skinny.so)
incorrectly validates a length value in the packet header. An integer
wrap-around leads to heap overwrite, and arbitrary remote code execution
as root.

== Details ==

The function 'static int get_input(struct skinnysession *s)' in
chan_skinny.c incorrectly validates a user supplied length in the packet
header. In the code below, four bytes of data are read from the socket,
cast to a signed integer, and assigned to dlen. If dlen is between -1
and -8 then (dlen + 8) will integer wrap to be greater than zero, but
less than sizeof(s->inbuf) for the purposes of this comparison.

Next, dlen + 4 is passed to read() as the maximum number of bytes to
write to s->inbuf+4. Read() takes an unsigned value, so dlen is
interpreted as a very large number. For example, a value of -6 is
interpreted as 0xfffffffa bytes. This instructs read() to write beyond
the allocated 1000 byte length of the buffer s->inbuf.

Code asterisk-1.2.12.1/channels/chan_skinny.c lines 2860-2870
----------------
res = read(s->fd, s->inbuf, 4);     // <- integer read from attacker
if (res != 4) {
  ast_log(LOG_WARNING, "Skinny Client sent less data than expected.\n");
  return -1;
}
dlen = letohl(*(int *)s->inbuf);        // <- input 0xfffffffa
                                        //  interpreted as signed
if (dlen+8 > sizeof(s->inbuf))          // <- integer wrap to +2

  dlen = sizeof(s->inbuf) - 8;          //  bypasses this check
}
*(int *)s->inbuf = htolel(dlen);        // casting just for amusement
res = read(s->fd, s->inbuf+4, dlen+4);  /* <- dlen now unsigned again
                                          *  permitting read() to write
                                          * up to 0xfffffffa bytes off
                                          * the end of s->inbuf
                                          */
----------------


== Exploitation ==

An attacker who can connect to the Asterisk server SCCP "Skinny" port
(by default 2000/tcp) can attack the vulnerable function prior to
registering as a configured Skinny phone, permitting pre-authentication
remote compromise.

Once the initial length header value in the packet performs an
integer-wraparound an attacker can overflow off the end of the
malloc()ed input buffer, and into heap space above it. Exploitation is
possible via standard heap-overflow malloc-unlink-macro technique[1] on
glibc versions prior to 2.3.5. On systems with newer glibc, a more
sophisticated exploitation method is necessary due to the improved
validation of malloc's internal heap management linked lists. Brett
Moore's work[2] on bypassing similar restrictions in WinXPSP2 is
instructive.

Our proof-of-concept exploit uses vanilla malloc-unlink() to overwrite a
GOT entry to point execution back into our buffer, and executes
Metasploit port-binding shellcode.

== Solutions ==

 - Disable the chan_skinny module if it is not required.
 - Firewall port 2000/tcp from untrusted networks.
 - Install the vendor supplied upgrades:
	1.0-branch: Upgrade to 1.0.12 or later
	1.2-branch: Upgrade to 1.2.13 or later

== Credit ==

Discovered and advised to Digium 17th October, 2006 by Adam Boileau of
Security-Assessment.com.

Security-Assessment.com commends Digium on their extremely rapid
response, releasing an updated version within two days of receiving our
vulnerability report.

== References ==

[1] "Advanced Doug Lea's Malloc Exploits" by jp
  http://doc.bughunter.net/buffer-overflow/advanced-malloc-exploits.html
[2] "Exploiting Freelist[0] On Windows XP Service Pack 2" by Brett Moore
  http://www.security-assessment.com/technical/

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