chan_sip.c in the SIP channel driver in Asterisk Open Source 1.8.x before 1.8.7.1 and 10.x before 10.0.0-rc1 does not properly initialize variables during request parsing, which allows remote authenticated users to cause a denial of service (daemon crash) via a malformed request.
http://downloads.digium.com/pub/security/AST-2011-012.html
http://secunia.com/advisories/46420
http://securityreason.com/securityalert/8478
http://www.securityfocus.com/archive/1/520141/100/0/threaded
http://www.securityfocus.com/bid/50177