Re: Nachi false positives

[Mark Nipper <nipsy () tamu edu>]

On 29 Oct 2003, Martin Jr., D. Michael wrote:
> I have been using the rule I've seen out there for detecting the
> Nachi/Welchi virus for some time with excellent results.  Lately, for
> ------------
> Nachi/Welchi rule:
>
> # Nachi Worm
> alert icmp $HOME_NET any -> $EXTERNAL_NET any (msg: "ALERT!!! NACHI
> Infection!!"; content: "|aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa|"; dsize:64;
> itype: 8; icode: 0; classtype:trojan-activity; sid: 10000008; rev: 1;)

        I've been using the same rule here at TAMU and have had
reports of false positives with some really old version of Yahoo!
pager as well.  But it seemingly does not work as an application
anymore anyway, so we've just been telling people to uninstall it
and download the latest software from Yahoo! if they want to
chitty-chat with people.  So, as an additional aye,aye, I think
there are some false positives, but the benefit out weighs the
drawback(s) in my opinion of this particular rule.

        Incidentally, it also seemed like the qhost worm was
actually making use of the fact that Yahoo! pager was trying to
use a specific host name, and qhost (since it modifies DNS
settings) was giving Yahoo! pager a bogus IP address where the
client would receive a notification of updates via Yahoo! pager,
which would automatically download and install, what else but,
more worms!  :)  This was second-hand information but didn't
surprise me too much.  :)

-- 
Mark Nipper                                                e-contacts:
Computing and Information Services                      nipsy () tamu edu
Texas A&M University                        http://ops.tamu.edu/nipsy/
College Station, TX 77843-3142     AIM/Yahoo: texasnipsy ICQ: 66971617
(979)575-3193                                      MSN: nipsy () tamu edu

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