Title: Malformed WebDAV Request Can Cause IIS
to Exhaust CPU Resources
Date: 08 March 2001
Software: IIS 5.0
Impact: Denial of Service
Bulletin: MS01-016

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS01-016.asp.

Issue:

WebDAV is an extension to the HTTP protocol that allows remote
authoring and management of web content. In the Windows 2000
implementation of the protocol, IIS 5.0 performs initial processing
of all WebDAV requests, then forwards the appropriate commands to the
WebDAV process. However, a flaw exists in the way WebDAV handles a
particular type of malformed request. If a stream of such requests
were directed at an affected server, it would consume all CPU
availability on the server.

Because the discoverer of this vulnerability has chosen to publish
code to exploit this vulnerability before a patch could be developed,
Microsoft has developed a workaround that can be used to defend
against attack. Knowledge Base article Q241520 provides step-by-step
instructions for changing the permissions on the .DLL that provides
WebDAV services in order to effectively disable it on the machine.
When a patch is available, we will re-release this bulletin and
provide updated information.

Microsoft recommends that customers consider applying the workaround
to any servers running IIS 5.0. Although this obviously includes web
servers, other services, notably Exchange 2000, may also require that
IIS 5.0 be enabled.

Mitigating Factors:

• The effect of an attack via this vulnerability would be temporary.
  The
  server would automatically resume normal service as soon as the
  malformed
  requests stopped arriving.

• The vulnerability does not provide an attacker with any capability
  to
  carry out WebDAV requests.

• The vulnerability does not provide any capability to compromise
  data on
  the server or gain administrative control over it.

Patch Availability:

• A patch is currently under development and will be released
  shortly. In
  the meantime, Knowledge Base article Q241520
  (http://www.microsoft.com/technet/support/kb.asp?ID=241520)
  provides a workaround that can be used to protect against this
  vulnerability.

Please read the Security Bulletin
http://www.microsoft.com/technet/security/bulletin/ms01-016.asp
for more information on this vulnerability.

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