The remote host is missing an update to pidgin announced via advisory DSA 1932-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%201932-1

It was discovered that incorrect pointer handling in the purple library, an internal component of the multi-protocol instant messaging client Pidgin, could lead to denial of service or the execution of arbitrary code through malformed contact requests. For the stable distribution (lenny), this problem has been fixed in version 2.4.3-4lenny5. For the unstable distribution (sid), this problem has been fixed in version 2.6.3-1. We recommend that you upgrade your pidgin package.