The Apache open source project patched its web server software this week to quash a bug that a denial-of-service tool has been exploiting.

Apache 2.2.20, released Tuesday, plugs the hole used by "Apache Killer," an attack tool that hackers have been using for more than a week to cripple web servers.

On Aug. 24, project developers had promised a fix within 48 hours, then revised the timetable two days later to 24 hours. The security advisory did not explain the delay. Earlier, the project had offered web server administrators ways to protect their systems until a patch was available.

"We consider this release to be the best version of Apache available, and encourage users of all prior versions to upgrade," Tuesday's advisory stated.

Although the DoS vulnerability also exists in the older Apache 1.3, the project no longer supports that edition.

According to an update to the original advisory that Apache published last week, the fix reduces the amount of memory used by HTTP requests, and "weeds out or simplifies requests deemed too unwieldy."

Although the update patched the bug that Apache Killer exploited, Apache acknowledged that part of the problem lies in the HTTP protocol itself. As it noted the problem's history, Apache said it was "an issue for (probably all) web servers and currently subject of an IETF discussion to change the protocol."

"The Apache team should be applauded for testing and releasing an important security fix so quickly," said Chet Wisniewski, a security researcher with Sophos.

Others, however, have pointed out that because Apple bundles Apache with Mac OS X and maintains the software via its operating system updates, users running a Mac-based server will have to wait for Apple to deliver a fix.

"It will be interesting to see how Apple rates the bug and how quickly they patch," said Andrew Storms, director of security operations at nCircle.

According to statistics kept by Netcraft, Apache powers 65.2% of all web servers currently in use.