The remote host is missing an update to serendipity announced via advisory DSA 1528-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%201528-1

Peter Hüwe and Hanno Böck discovered that Serendipity, a weblog manager, did not properly sanitise input to several scripts which allowed for cross site scripting. For the stable distribution (etch), this problem has been fixed in version 1.0.4-1+etch1. The old stable distribution (sarge) does not contain a serendipity package. For the unstable distribution (sid), this problem has been fixed in version 1.3-1. We recommend that you upgrade your serendipity package.