Home    |    View Topics    |    Search    |    Contact Us    |   



Category:   Application (VoIP)  >   Asterisk Vendors:   Digium (Linux Support Services)
Asterisk Integer Overflow in Skinny Channel Driver Lets Remote Users Execute Arbitrary Code
SecurityTracker Alert ID:  1017089
SecurityTracker URL:
CVE Reference:   CVE-2006-5444   (Links to External Site)
Updated:  Jun 3 2008
Original Entry Date:  Oct 19 2006
Impact:   Execution of arbitrary code via network, Root access via network

Version(s): 1.0 -, 1.2 -
Description:   A vulnerability was reported in Asterisk. A remote user can execute arbitrary code on the target system.

A remote user can send specially crafted data to trigger an integer overflow in the Asterisk Skinny channel driver for Cisco SCCP phones ( and execute arbitrary code on the target system. The code will run with root level privileges.

The vulnerability resides in the get_input() function in 'chan_skinny.c'.

1.4.0.x versions are not affected.

Adam Boileau of discovered this vulnerability.

Impact:   A remote user can execute arbitrary code on the target system.
Solution:   The vendor has issued fixed versions (1.0.12, 1.2.13).
Vendor URL: (Links to External Site)
Cause:   Boundary error
Underlying OS:  Linux (Any), UNIX (Any)

Message History:   None.

 Source Message Contents

Subject:  [Full-disclosure] Advisory: Asterisk remote

= Asterisk - chan_skinny Remote Unauthenticated Heap Overflow
= Vendor Website:
= Affected Version:
=  All 1.2-branch releases prior to and including
=  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
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- 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

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 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
[2] "Exploiting Freelist[0] On Windows XP Service Pack 2" by Brett Moore

== About == is Australasia's leading team of Information
Security consultants specialising in providing high quality Information
Security services to clients throughout the Asia Pacific region. Our
clients include some of the largest globally recognised companies in
areas such as finance, telecommunications, broadcasting, legal and
government. Our aim is to provide the very best independent advice and
a high level of technical expertise while creating long and lasting
professional relationships with our clients. is committed to security research and
development, and its team continues to identify and responsibly publish
vulnerabilities in public and private software vendor's products.
Members of the R&D team are globally recognised
through their release of whitepapers and presentations related to new
security research. is an Endorsed Commonwealth Government of
Australia supplier and sits on the Australian Government
Attorney-General's Department Critical Infrastructure Project panel.
We are certified by both Visa and MasterCard under their Payment
Card Industry Data Security Standard Programs.

Full-Disclosure - We believe in it.
Hosted and sponsored by Secunia -


Go to the Top of This SecurityTracker Archive Page

Home   |    View Topics   |    Search   |    Contact Us

This web site uses cookies for web analytics. Learn More

Copyright 2021, LLC