The vulnerability is due to the parsing of XML data in both Jabber XCP, including JabberNow appliances, and Cisco Unified Presence. These products are vulnerable to the Exponential Entity Expansion attack. This attack is better known as an XML Bomb, referring to an XML document that is considered to be valid by the rules of the XML schema, but causes the parser or underlying server to crash or hang. This attack is also more commonly referred to the Billion Laughs Attack, due to the use of XML parsers to expand the string lol or ha up to a billion times or until server resources were exhausted.
This attack combines certain properties of XML to create valid but malicious XML using an extreme level of nested substitutions. When the XML parser attempts to expand all the nested entities, it will exhaust all the server resources.
An unauthenticated, remote attacker could trigger high CPU and memory usage from the XML parsers in Jabber XCP, including JabberNow appliances, and Cisco Unified Presence. This will result in the exhaustion of resources, causing processes to crash. This attack affects both client-to-server connections as well as server-to-server (federation) links.