Vulnerable Systems:
* 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
Immune Systems:
* Open Source Asterisk 1.2.40
* Switchvox
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.
Workaround:
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.
