ASA-2019-00422 – Asterisk: Remote crash in chan_sip channel driver

When T.38 faxing is done in Asterisk a T.38 reinvite may be sent to an endpoint to switch it to T.38. If the endpoint responds with an improperly formatted SDP answer including both a T.38 UDPTL stream and an audio or video stream containing only codecs not allowed on the SIP peer or user a crash will occur. The code incorrectly assumes that there will be at least one common codec when T.38 is also in the SDP answer. This requires Asterisk to initiate a T.38 reinvite which is only done when executing the ReceiveFax dialplan application or performing T.38 passthrough where a remote endpoint has requested T.38.