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RE: Microsoft ISA Server Fragmented Udp Flood Vulnerability
From: "Microsoft Security Response Center" <secure () microsoft com>
Date: Mon, 5 Nov 2001 10:48:14 -0800

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Hi all,

Wanted to take a moment and clarify this issue that's been posted.

We investigated the issue when it was initially brought to us at
secure () microsoft com, but this is strictly a flooding attack.  The
script simply sends a huge number of fragmented packets to the
server, and recombining the packets takes the server some finite
amount of work.  Send enough of them,quickly enough, and you can
monopolize the server.  But of course this is true for any server,
not just for ISA.  The attack requires a very high bandwidth between
the attack and the server, and normal processing resumes as soon as
the flooding stops.

ISA can be configured to drop fragmented packets and, if this is
done, it significantly helps protect the system against flooding
attacks like this.  However, even so, it's not a cure-all.  Even
inspecting and dropping packets takes some finite amount of work, and
once again if the attacker has sufficient bandwidth, he may be able
to flood the server.  Again, though, there isn't a flaw in ISA server
- - - -- it's strictly a flooding attack.

Regards,
secure () microsoft com

- - - -----Original Message-----
Subject: Microsoft ISA Server Fragmented Udp Flood Vulnerability

- - - - ----[ Summary
A fragmented Udp attack through the microsoft isa server makes the
system hampered by using the cpu at 100%. Meanwhile server uses
processor power too much and therefore packet process ratio
decreases.

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