Software Engineering Institute

Microsoft IIS 5.0 contains a vulnerability that allows an intruder to cause a memory allocation error by repeatedly submitting a malicious URL, and thus cause IIS to fail. Additionally, Exchange 2000 is affected by a similar problem. According to Microsoft:

To support web-based mail clients, [Exchange] introduces the ability to address items on the store via URLs. This is done in part by using IIS 5.0, and in part via code that is specific to Exchange 2000. Both pieces of code contain the flaw, but the effect of exploiting the vulnerability via either would be the same -- it could be used to cause the IIS service to fail, but could not be used to attack the Exchange service itself. That is, successfully attacking an Exchange server via this vulnerability would disrupt web-based mail clients' use of the server, but not that of MAPI-based mail clients like Outlook.

By default, IIS 5.0 will automatically restart after a failure and resume normal operations.

Intruders can interrupt the normal operation of IIS 5.0. If the attack is against a web server, web services will be disrupted. If the attack is against an Exchange server, web-based mail service will be disrupted, but not MAPI-based mail service. In either case, IIS is the component that fails.

Apply a path as described in MS01-014. Note that this bulletin actually describes two patches, one for IIS and one for Exchange. It is necessary to install both patches to protect Exchange servers. Alternately, to protect a web server running IIS (but not Exchange) you can install the cumulative patch described in MS01-044.