Olle Johansson recently alerted us that there is a “dialstring injection” vulnerability in Asterisk. As Olle notes in his post about the vulnerability, this is similar to a SQL injection attack against a database where there is not enough filtering being done on strings that are being input to the system. Olle writes:

Many VoIP protocols, including IAX2 and SIP, have a very large allowed character set in the dialed extension, a character set that allows characters that are used as separators to the dial() and the queue() applications, as well as within the dialstring that these applications send to the channel drivers in Asterisk. A user can change the dial options and dial something we should not be able to dial in your system. This article describes the issue in more detail and gives you some help on how to avoid this causing trouble in your Asterisk server.

Olle goes on to explain the issue in more detail and explain about how input from VoIP channels should be filtered before being sent to the Asterisk ‘dialplan’ for processing. He includes a plea for assistance:

We need everyone involved to pump this information out in all the veins that runs through the Asterisk eco-system. Audit your dialplans, fix this issue. And do it now. Everyone that runs a web site with dialplan examples – audit your examples, fix them. Everyone that has published books – publish errata on your web site. Please help us – and do it now.

Olle’s article goes into much more detail and offers suggestions for what you can do to protect your system. If you are an Asterisk administrator, it’s definitely an issue you should investigate and act on.

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