Denial Of Service attack handling by MS 4.54.6
mailscanner at ecs.soton.ac.uk
Wed Jul 26 18:15:44 IST 2006
Please can you put a copy of the message on a web URL somewhere and mail
me (off-list!) the URL so I can go and get it?
What happens (or should happen) is this:
Message Batch is scanned at one go
Scanner times out
Message Batch is scanned one message at a time, looking for the nasty
Scanner times out on the nasty message
DoS attack detected.
Message should be handled as a normal virus infection, not a silent one.
I could really do with a copy of the message if at all possible.
I certainly wouldn't advise setting the Virus Scanner Timeout any higher
than the default (5 minutes). You could easily reduce it to 2 minutes on
most systems, my default is very conservative.
It will always take at least double that figure to process the batch,
but I'm a bit worried by the factor of 4 you are seeing. Hence the need
to test it.
Jim Holland wrote:
> Hi Julian
> I have only just installed the above version of MailScanner and was
> interested to see how it dealt with the problem of DOS attacks affecting
> the virus scanner. I came across an example this morning:
> The message was 650 KB in size, with a PowerPoint attachment
> 10 minutes after scanning of the batch started, MailScanner reported
> "Commercial scanner clamav timed out!" (this corresponds with
> the time configured for Virus Scanner Timeout) followed by
> "Denial Of Service attack detected!"
> 10 minutes later there was another maillog entry reporting
> "Commercial scanner clamav timed out!" again followed by details
> of which message caused the denial of service attack.
> 22 minutes after the above, the message was finally quarantined.
> I presume that the above actions are as now intended. However there are
> still some associated problems:
> The complete batch of 16 messages totalling 805877 bytes took a
> full 45 minutes to be processed before the uninfected messages
> were delivered, in spite of my having set Virus Scanner Timeout to
> 10 minutes per batch.
> The message was treated as containing a silent virus, so there
> was no notification to the recipient.
> Are the following changes possible and if so, agreeable to you:
> Stop virus scanning any individual message once it exceeds a
> reasonable time - eg if "Virus Scanner Timeout = 300", then stop
> scanning a message after say (300/no of msgs in batch)*10 secs
> or the full virus scanner timeout time, whichever is the smaller.
> Flag the message as being infected with a Denial of Service attack
> (as it does now).
> Remove "Denial Of Service attack" from the list of silent viruses,
> so that such messages are delivered with a notice to say that the
> attachment has been removed.
> I presume that the final option could alternatively be handled by simply
> Virus: /Denial.of.Service/ yes
> which is what I have now done. However I think that in general such cases
> are far more likely to be genuine files than deliberately crafted bombs.
> Jim Holland
> System Administrator
> MANGO - Zimbabwe's non-profit e-mail service
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