The folks over at the Digium security team today released security bulletin AST-2011-012 for a remote crash vulnerability in the SIP channel drive. For info about the attack, they state only:
A remote authenticated user can cause a crash with a malformed request due to an uninitialized variable.
An assumption from this statement would be that an UNauthenticated user could not carry out this attack… but I admit to not personally knowing the SIP channel driver of Asterisk enough to be able to stand behind this conclusion.
Regardless, updates have been released in the form of new versions 18.104.22.168 and 10.0.0-rc1.