Sometimes I read configuration guides that advise installing anti-virus products on servers. Since I don't run Windows servers in production environments, I can usually ignore such advice. The proponents of the "anti-virus everywhere" mindset think that adding anti-virus is, at the very least, a "defense-in-depth" measure. This was debated last year, actually.
A lesson I learned from the excellent book Protect Your Windows Network is that "defense-in-depth" is not a cost-free justification for security measures. Every configuration and installation aspect of a system provides benefits as well as costs. Something implemented for "defense-in-depth" (whether truly believed to be helpful, or ignorantly applied) may turn out to harm a system.
Thanks to Harlan Carvey, I learned of another example of a defense-in-depth technique damaging security. This is the worst of all possible worlds -- adding a security measure that results in massive vulnerability. This upcoming eEye advisory warns:
A remotely exploitable vulnerability exists within the Symantec Antivirus program. This flaw does not require any end user interaction for exploitation and can compromise affected systems, allowing for the execution of malicious code with SYSTEM level access.
So you add anti-virus to a server, and BANG. 0wn3d.
Harlan focused on the following quote in the email he sent me:
"People shouldn't panic," [eEye's] Maiffret said. "There shouldn't be any exploits until a patch is produced."
This is a reference to the fact that once a patch is released, white, gray, and black hat security researchers race to analyze it to identify the vulnerable code fixed by the patch. Harlan wonders (accurately) if the underground (or others) already know about this vulnerability, and whether they are already exploiting it.
Keep this case in mind if you believe that "adding security" is a cost-free endeavor.