Product

Asterisk

Summary

Dialplan injection vulnerability

Nature of Advisory

Data injection vulnerability

Susceptibility

Remote Unauthenticated Sessions

Severity

Critical

Exploits Known

Yes

Reported On

10/02/10

Reported By

Hans Petter Selasky

Posted On

16/02/10

Last Updated On

February 25, 2010

Advisory Contact

Leif Madsen < lmadsen AT digium DOT com >

CVE Name

CVE-2010-0685

Description

A common usage of the ${EXTEN} channel variable in a dialplan with wildcard pattern matches can lead to a possible string injection vulnerability. By having a wildcard match in a dialplan, it is possible to allow unintended calls to be executed, such as in this example:

exten => _X.,1,Dial(SIP/${EXTEN})

If you have a channel technology which can accept characters other than numbers and letters (such as SIP) it may be possible to craft an INVITE which sends data such as 300&Zap/g1/4165551212 which would create an additional outgoing channel leg that was not originally intentioned by the dialplan programmer.

Usage of the wildcard character is common in dialplans that require variable number length, such as European dial strings.

Please note that this is not limited to an specific protocol or the Dial() application.

The expansion of variables into programmatically-interpreted strings is a common behavior in many script or script-like languages, Asterisk included. The ability for a variable to directly replace components of a command is a feature, not a bug - that is the entire point of string expansion.

However, it is often the case due to expediency or design misunderstanding that a developer will not examine and filter string data from external sources before passing it into potentially harmful areas of their dialplan. With the flexibility of the design of Asterisk come these risks if the dialplan designer is not suitably  
cautious as to how foreign data is allowed to continue into the system.

This security release is intended to raise awareness of how it is possible to insert malicious strings into dialplans, and to advise developers to read the best practices documents so that they may easily avoid these dangers.

Resolution

One resolution is to wrap the ${EXTEN} channel variable with the FILTER() dialplan function to only accept characters which are expected by the dialplan programmer. The recommendation is for this to be the first priority in all contexts defined as incoming contexts in the channel driver configuration files.

Examples of this and other best practices can be found in the new README-SERIOUSLY.bestpractices.txt document in the top level folder of your Asterisk sources.

Asterisk 1.2.40 has also been released with a backport of the FILTER() dialplan function from 1.4 in order to provide the tools required to resolve this issue in your dialplan.

Affected Versions

Product

Release Series

Asterisk Open Source

1.2.x

All versions

Asterisk Open Source

1.4.x

All versions

Asterisk Open Source

1.6.x

All versions

Asterisk Business Edition

B.x.x

All versions

Asterisk Business Edition

C.x.x

All versions

Switchvox

None

No versions affected

Document

SVN URL

Branch

http://svn.asterisk.org/svn/asterisk/branches/1.2/README-SERIOUSLY.bestpractices.txt

v1.2

http://svn.asterisk.org/svn/asterisk/branches/1.4/README-SERIOUSLY.bestpractices.txt

v1.4

http://svn.asterisk.org/svn/asterisk/branches/1.6.0/README-SERIOUSLY.bestpractices.txt

v1.6.0

http://svn.asterisk.org/svn/asterisk/branches/1.6.1/README-SERIOUSLY.bestpractices.txt

v1.6.1

http://svn.asterisk.org/svn/asterisk/branches/1.6.2/README-SERIOUSLY.bestpractices.txt

v1.6.2

Corrected In

Product

Release

Open Source Asterisk

1.2.40

Links

https://issues.asterisk.org/view.php?id=16810

https://issues.asterisk.org/view.php?id=16808

Asterisk Project Security Advisories are posted at http://www.asterisk.org/security

This document may be superseded by later versions; if so, the latest version will be posted at http://downloads.digium.com/pub/security/AST-2010-002.pdf and http://downloads.digium.com/pub/security/AST-2010-002.html

Revision History

Date

Editor

Revisions Made

16/02/10

Leif Madsen

Initial release

25/02/10

Leif Madsen

Update CVE Name field