The remote host is missing an update to expat announced via advisory DSA 1921-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%201921-1

Peter Valchev discovered an error in expat, an XML parsing C library, when parsing certain UTF-8 sequences, which can be exploited to crash an application using the library. For the old stable distribution (etch), this problem has been fixed in version 1.95.8-3.4+etch1. For the stable distribution (lenny), this problem has been fixed in version 2.0.1-4+lenny1. For the testing distribution (squeeze) and the unstable distribution (sid), this problem will be fixed soon. We recommend that you upgrade your expat packages.