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Portcullis Security Advisory 05-013 - VoIP - Asterisk Stack Overflow
Jun 22 2005 03:16PM
Wade Alcorn (wja portcullis-security com)
Portcullis Security Advisory
wja (at) portcullis-security (dot) com [email concealed] -
wade (at) bindshell (dot) net [email concealed] - www.bindshell.net/voip/advisory-05-013.txt
This vulnerability affects Asterisk 1.0.7 and the development Asterisk
branch (known as CVS HEAD).
Asterisk Manager Interface Overflow
Vulnerability discovery and development:
Wade Alcorn discovered this vulnerability whilst performing an
assessment of the Asterisk PABX software package. It was then verified
that the instruction pointer could be controlled and made to point at
arbitrary memory locations. This led to a remote shell proof-of-concept,
which provides access to the user (root by default) running Asterisk.
This will occur when the Asterisk Manager Interface is enabled and a
valid manager username and password, with command line permissions, is used.
The issue was initially found and verified on the Linux Operating
System. Research is currently being undertaken to determine the extent
to which other operating systems are vulnerable.
There is a programming error in the function that parses commands in the
Asterisk system. This is used by the manager interface if the user is
allowed to submit CLI commands. The coding error can result in the
overflow of one of the parameters of the calling function. That is, the
command parsing function will return without error. However, the calling
function will cause a segmentation fault.
If the command string is specifically crafted, is it possible to use
this stack overflow to execute arbitrary code on the Asterisk system.
The resulting execution is (typically) run with root privileges.
A command consisting of a recurring string of two double quotes followed
by a tab character will induce the segmentation fault within a Call
Under the default configuration the Asterisk server does not start the
Manager interface, so a default Asterisk installation will not be
vulnerable to this avenue of attack.
The impact of this issue is mitigated by the Asterisk default
configuration. Configuration is controlled by settings in manager.conf.
The following options need to be in place for this vector of attack to
enabled = yes
bindaddr = 127.0.0.1
secret = mysecret
permit = 127.0.0.1
write = command
The relevant option is 'write = command'; without it, even properly
authenticated Manager interface users will be unable to exploit this
The result of a successfully exploited Asterisk command parsing overflow
will result in a remote root shell.
The error in the function means that any Asterisk server with the
appropriate configuration using the Manager interface is vulnerable. It
is possible for an authenticated user to gain a remote root shell on the
Mark Spencer, the primary developer of the Asterisk project, was
contacted on Thursday 19th May 2005 via email. This email contained a
summary of the vulnerability and explained Wade Alcorn had developed an
exploit for the issue.
The Asterisk development team was prompt and responsive to the
vulnerability alert. Portcullis was provided with an alternate means of
contact additional to email, if it was to be required. Wade Alcorn
(Portcullis), Mark Spencer (Asterisk), Kevin Fleming (Asterisk)
cooperatively provided and verified a solution to the problem.
The Asterisk project is thankful for the efforts of Wade Alcorn and
Portcullis Computer Security to identify and diagnose the source of this
problem. Wade Alcorn was able to provide a proposed solution and a
simple means of reproducing the problem, which were instrumental in our
ability to quickly solve the issue.
1) For a temporary workaround, disable the setting in manager.conf
detailed in the impact section.
2) Upgrade to version 1.0.8, or for development branch users, the most
recent CVS version available.
Copyright (c) Portcullis Computer Security Limited 2005. All rights
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