Quoting from Secunia:
A vulnerability has been reported in VMware ESX Server, which has an unknown impact.
The vulnerability is caused due to an unspecified error in the VMware Management Interface. This can be exploited for code execution in the web browser.
The vulnerability has been reported in version 2.0.1, 2.1.2, and 2.5.2.
VMware already released patches for all these versions. I published the one for ESX Server 2.5.2 here.
Patch your version as soon as possible even if the vulnerability is actually considered less critical.