Re: McAfee Anti Virus V4.5.1 SP1

["Robert Slade, Threat Response Manager" <rslade () fortinet com>]

> We have had 3 or 4 machines come up infected with Nachi today but the on
> access scanner didn't pick it up. Carrying out a full system scan did
> pick it up.

Not terribly surprising.

First of all, Nachi (and a great many others of its ilk) is a worm, acting
specifically by making an attack on a vulnerability in an application or
an operating system.  In this case, it is, as you note, making RPC calls. 
(Turning off DCOM with something like dcomcnfg will prevent the attack
from succeeding, and shouldn't create any problems unless you are using an
MS Exchange mail server.)

Nachi creates the files you note, but it does not necessarily read them. 
Generally on-access scanners shortcut scanning (in order to improve
performance) and therefore the scanner will probably never scan the files.
 The full scan, as you noted, does.  (In addition, on-access or other
"automatic" scanners are always much less effective and accurate at
detection in comparision to the base manual versions.)

> Anyway... I'm trying to figure out why McAfee on access scanner isn't
> picking these files up but the full system scan is. There is no
> difference in the setup we have between on access or full scan.

Hope this explains matters.

-- 
rslade () sprint ca  rslade () fortinet com  p1 () cheerful com rslade () vcn bc ca
victoria.tc.ca/techrev/secgloss.htm   sun.soci.niu.edu/~rslade/mnbk.htm
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