Because of a vulnerability in Apache web server, VMware ESX Server (from 2.0.2 to 3.0.1 version) is prone to a cross-site scripting attack:
A bug was found in Apache where an invalid Expect header sent to the server was returned to the user in an unescaped error message. This could allow an attacker to perform a cross-site scripting attack if a victim was tricked into connecting to a site and sending a carefully crafted Expect header.
While a web browser cannot be forced to send an arbitrary Expect header by a third-party attacker, it was recently discovered that certain versions of the Flash plugin can manipulate request headers. If users running such versions can be persuaded to load a web page with a malicious Flash applet, a cross-site scripting attack against the server may be possible.
Read the whole security advisor at source.
At the moment of writing there is no patch available from VMware to address this vulnerability.