The remote host is missing an update to citadel announced via advisory DSA 2250-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%202250-1

Wouter Coekaerts discovered that the jabber server component of citadel, a complete and feature-rich groupware server, is vulnerable to the so-called billion laughs attack because it does not prevent entity expansion on received data. This allows an attacker to perform denial of service attacks against the service by sending specially crafted XML data to it. For the oldstable distribution (lenny), this problem has been fixed in version 7.37-8+lenny1. For the stable distribution (squeeze), this problem has been fixed in version 7.83-2squeeze2. For the testing (wheezy) and unstable(sid) distributions, this problem will be fixed soon. We recommend that you upgrade your citadel packages.