The Asterisk Extension Language (AEL) in pbx/pbx_ael.c in Asterisk does not properly generate extensions, which allows remote attackers to execute arbitrary extensions and have an unknown impact by specifying an invalid extension in a certain form.
http://www.vupen.com/english/advisories/2007/1123
http://www.securityfocus.com/bid/23155
http://www.novell.com/linux/security/advisories/2007_34_asterisk.html
http://svn.digium.com/view/asterisk?rev=59073&view=rev
http://secunia.com/advisories/25582