Re: New vulnerability in IIS4.0/5.0

From: Dave Ahmad (
Date: 09/19/01

Date: Wed, 19 Sep 2001 13:50:02 -0600 (MDT)
From: Dave Ahmad <>
To: ALife // BERG <>
Subject: Re: New vulnerability in IIS4.0/5.0
Message-ID: <Pine.GSO.4.30.0109191127570.19628-100000@mail>

This seems to be just be another way to exploit the double decode
vulnerability (Bugtraq ID 2708). There is a possibility that it may be a
new issue due to the use of '%u' method of encoding. It does not look
that way to us.

On our test machines (and at eEye), systems do not seem to be vulnerable
after applying the MS01-026 hotfix (or the MS01-044 patch).

Ryan Permeh of eEye Digital Security provided a breakdown of an encoded
attack string:

The attack string used successfully against an IIS server (Win2K, SP2):


first decode sequence(it replaces %u0025 with %)

second decode sequence (it replaces %u005c with /)

The double decode vulnerability is fixed in MS01-026. I believe the fix
is included in the cumulative patch released with MS01-044.

It doesn't look like a new vulnerability, but we are awaiting confirmation
from Microsoft.

Has anyone managed to exploit a patched system?

Thanks Ryan & eEye.


Dave Ahmad
Security Focus

On Wed, 19 Sep 2001, ALife // BERG wrote:

> -----[ Bright Eyes Research Group | Advisory # be00001e ]-----------------
> Remote users can execute any command on several
> IIS 4.0 and 5.0 systems by using UTF codes
> -------------------------------------[ ]--------------
> Topic: Remote users can execute any command on several
> IIS 4.0 and 5.0 systems by using UTF codes
> Announced: 2001-09-19
> Credits: ALife <>
> Affects: Microsoft IIS 4.0/5.0
> --------------------------------------------------------------------------
> ---[ Description
> For example, target has a virtual executable directory (e.g.
> "scripts") that is located on the same driver of Windows system.
> Submit request like this:
> http://target/scripts/..%u005c..%u005cwinnt/system32/cmd.exe?/c+dir+c:\
> Directory list of C:\ will be revealed.
> Of course, same effect can be achieved by this kind of processing
> to '/' and '.'. For example: "..%u002f", ".%u002e/", "..%u00255c",
> "..%u0025%u005c" ...
> Note: Attacker can run commands of IUSR_machinename account privilege
> only.

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