Asterisk Project Security Advisory – There is a possibility to remotely crash an Asterisk server if the server is configured to use realtime IAX2 users. The issue occurs if either an unknown user attempts to authenticate or if a user that uses hostname matching attempts to authenticate.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/72906/AST-2008-012.txt
Source: https://packetstormsecurity.com/files/72906/Asterisk-Project-Security-Advisory-AST-2008-012.html