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If users have put off patching their Apache Web servers against the vulnerability discovered Monday, they should wait no longer, as an exploit to attack the security hole is now circulating on the Internet, according to Oliver Friedrichs, director of engineering at SecurityFocus.

The exploit--a tool that makes attacking a vulnerability easier--was posted to the Bugtraq security e-mail list on Wednesday, Friedrichs said.

The existence of an exploit "makes the possibility of a worm that targets these [systems] more likely," he said.

The vulnerability was announced Monday by Internet Security Systems, and then expanded upon by the Apache Software Foundation. The organizations say the bug could allow an attacker to take control of an affected Web server. Because of a flaw in the way Apache handles uploads, an attacker could send a specially formed request to the server and either cause it to deny service to legitimate users or take the system over, both groups said.

More than 60 percent of the Web servers on the Internet use Apache, according to data from Web server monitoring firm Netcraft.

Lull Before Attacks

The Computer Emergency Response Team/Coordination Center (CERT/CC), a federally funded computer security body located at Pittsburgh's Carnegie Mellon University, and Internet Security Systems both updated their advisories on the vulnerability after the release of the exploit, urging users to patch their systems.

Despite the presence of an exploit, SecurityFocus hasn't seen increased attack activity focused at Apache systems, Friedrichs said. SecurityFocus, which is based in San Mateo, California, monitors the networks of over 9000 companies in over 145 countries for security data and then aggregates it to create a picture of global, regional, and industry-specific Internet security.

The dearth of attacks isn't surprising to Friedrichs, as there is usually a period of one to two weeks between vulnerability announcements and attacks, he said.

Though the exploit released Wednesday attacks only Apache installations running on the OpenBSD operating system, "it's not a monumental task ... for someone to modify it [to work with other operating systems]," he said.

Users should patch their systems immediately and check with their vendors for more information, Friedrichs said.

"People ... should be making the patching of their Apache servers a high priority," he said.

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