The remote host is missing an update to vnc4 announced via advisory DSA 1716-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%201716-1

It was discovered that xvnc4viewer, a virtual network computing client software for X, is prone to an integer overflow via a malicious encoding value that could lead to arbitrary code execution. For the stable distribution (etch) this problem has been fixed in version 4.1.1+X4.3.0-21+etch1. For the unstable (sid) distribution this problem has been fixed in version 4.1.1+X4.3.0-31. For the testing (lenny) distribution this problem will be fixed soon. We recommend that you upgrade your vnc4 packages.