So... there is vulnerabilities in ClamAV... Is Zimbra planning on releasing and updated version of Zimbra with a patched ClamAV?

From Secunia:

A vulnerability has been reported in ClamAV, which can be exploited by malicious people to cause a DoS (Denial of Service) and potentially to compromise a vulnerable system.

The vulnerability is caused due to a boundary error within the HTTP client in the Freshclam command line utility. This can be exploited to cause a stack-based buffer overflow when the HTTP headers received from a web server exceeds 8KB.

Successful exploitation requires that Freshclam is used to download virus signature updates from a malicious mirror web server e.g. via DNS poisoning.

The vulnerability has been reported in version 0.80 through 0.88.1.