eEye Digital Security
was doing some testing that apparently Microsoft hadn't done on its own webserver (IIS 5.0) running on its latest OS (Windows 2000, all versions). "Within a matter of minutes," they say, "a debugger kicked in on inetinfo.exe because of a 'buffer overflow error'" -- and two weeks later, we got simultaneous announcements from Microsoft and eEye. This is a remote SYSTEM-level exploit in a popular webserver, in the wild, i.e., Danger Will Robinson. eEye says about a million servers will need to be patched;
it may be more
Microsoft's writeup and patch
eEye's droll and informative writeup
which, now that an exploit is confirmed to be in the wild today, has added some source code.