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Defcom Labs Advisory def-2001-26

           IIS WebDav Lock Method Memory Leak DoS

Author: Peter Grьndl <[email protected]>
Release Date: 2001-05-17

------------------------=[Brief Description]=-------------------------
The WebDav extensions for Internet Information Server 5.0 contain a
flaw that could allow a malicious user to consume all available memory
on the server.

------------------------=[Affected Systems]=--------------------------

• httpext.dll versions prior to 0.9.3940.21 (Windows 2000 SP2)

----------------------=[Detailed Description]=------------------------
The lock method contains a memory leak that will trigger if you send
it continous requests for non-existing files. Eg.

LOCK /aaaaaaaaaaaaaaaaaaaaaaaaaa.htw HTTP/1.0

Eventually the server will run out of memory and run really slow, you
might argue that the server will then crash, reboot and return to
normal again, but there are a few things that can be done to determine
when you get close to filling up the servers memory, and then it is
just a matter of stopping, and the server won't free the memory. One
way is to combine the attack with asp executions, eg.

GET /iisstart.asp?uc=a HTTP/1.0

which ofcourse requires the presence of iisstart.asp (but this is
just an example). The script will return execution errors when it
runs out of temporary space on the server to execute the .asp script
and that's when the server is almost out of memory.

---------------------------=[Workaround]=-----------------------------
The problem has been corrected in httpext.dll v.0.9.3940.21, which is
packaged with Windows 2000 Service Pack 2 and according to Microsoft:

"it will ship with each IIS5 hotfix that we release going forward
(and will be available for SP0, SP1, and SP2+.)"

You can find Service Pack 2 on Microsofts webpage at:

www.microsoft.com/windows2000/downloads/servicepacks/sp2/default.asp

-------------------------=[Vendor Response]=--------------------------
This issue was brought to the vendor's attention on the 3rd of March,
2001, and the vendor released a patch on the 16th of May.

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This release was brought to you by Defcom Labs

          [email protected]             www.defcom.com

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