Asterisk Project Security Advisory – An attacker attempting to negotiate T.38 over SIP can remotely crash Asterisk by modifying the FaxMaxDatagram field of the SDP to contain either a negative or exceptionally large value. The same crash occurs when the FaxMaxDatagram field is omitted from the SDP as well.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/85890/AST-2010-001.txt
Source: https://packetstormsecurity.com/files/85890/Asterisk-Project-Security-Advisory-AST-2010-001.html