The remote host is missing an update to zsync announced via advisory DSA 797-2.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%20797-2

zsync, a file transfer program, includes a modified local copy of the zlib library, and is vulnerable to certain bugs fixed previously in the zlib package. There was a build error for the sarge i386 proftpd packages released in DSA 797-1. A new build, zsync_0.3.3-1.sarge.1.2, has been prepared to correct this error. The packages for other architectures are unaffected.