The SIP channel driver in Asterisk Open Source 1.4.22, 1.4.23, and 1.4.23.1; 1.6.0 before 1.6.0.6; 1.6.1 before 1.6.1.0-rc2; and Asterisk Business Edition C.2.3, with the pedantic option enabled, allows remote authenticated users to cause a denial of service (crash) via a SIP INVITE request without any headers, which triggers a NULL pointer dereference in the (1) sip_uri_headers_cmp and (2) sip_uri_params_cmp functions.
http://www.vupen.com/english/advisories/2009/0667
http://www.securitytracker.com/id?1021834
http://www.securityfocus.com/bid/34070
http://www.securityfocus.com/archive/1/501656/100/0/threaded
http://secunia.com/advisories/34229
http://downloads.digium.com/pub/security/AST-2009-002.html