The U.K. National Infrastructure Security Co-ordination Centre has discovered a number of vulnerabilities in various implementations of the telephony protocol H.323. Voice over Internet Protocol (VoIP) and video conferencing equipment and software can use these protocols to communicate over a variety of computer networks.
Products from Cisco, Nortel, Microsoft, Radvision and others are affected.
Full details can be found at http://www.uniras.gov.uk/vuls/2004/006489/h323.htm