IIS 5.0 contains a flaw affecting the way that an URL is handled if it has a specific construction and its length is within a very narrow range of values. If such an URL were repeatedly sent to an affected system, a confluence of events could cause a memory allocation error that would result in the failure of the IIS service.
Exchange 2000 is affected by the same vulnerability. To support Web-based mail clients, it 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.
Because the flaw occurs in two different code modules, one of which installs as part of IIS 5.0 and both of which install as part of Exchange 2000, it is important for Exchange 2000 administrators to install both this IIS patch, as well as the Exchange patch.