Security experts have made a breakthrough in their five-month battle against the Conficker worm, with the discovery that the malware leaves a fingerprint on infected machines that is easy to detect using a variety of off-the-shelf network scanners.

The finding means that, for the first time, administrators around the world have easy-to-use tools to positively identify machines on their networks that are contaminated by the worm. As of mid-Monday, signatures will be available for at least half a dozen network scanning programs, including the open-source Nmap, McAfee's Foundstone Enterprise and Nessus, made by Tenable Network Security.

Up to now there were only two ways to detect Conficker, and neither was easy. One was to monitor outbound connections for each computer on a network, an effort that had already proved difficult for organizations with machines that count into the hundreds of thousands or millions. With the advent of the Conficker C variant, traffic monitoring became a fruitless endeavour because the malware has been programmed to remain dormant until April 1.

The only other method for identifying Conficker-infected computers was to individually scan each one, another measure that placed onerous requirements on admins.

The discovery of Conficker's tell-tale heart two days before activation may prove to be an ace up the sleeve of the the white hat security world.

"This is an extraordinarily inexpensive, not-very-time-intensive way of finding machines on your network that are actually running malicious software," said Dan Kaminsky, one of the three researchers who discovered the Conficker fingerprint. "This is not something we get to do all the time. Most pieces of malicious software are not that easy to find."

The availability of the new Conficker definitions is the result of the sleuthing and quick response of an industry-wide cast of characters, said Kaminsky, who is director of penetration testing at security company IOActive.

The finding came Friday afternoon as Kaminsky pored over data that members of the Honeynet Project had collected on the worm. Along with Honeynet's Tillmann Werner and Felix Leder, Kaminsky soon noticed that Conficker changes the way a small piece of the Windows operating system acts. The behavior, located in pre-authentication routines before users enter file-sharing passwords, makes easy-to-identify changes to the way machines look on a network.

"Once I heard that Conficker had code running on the anonymous surface, I said 'Wait, we can fingerprint that,'" Kaminsky said. "If you can get packets to a box, you can find out fairly reliably whether it's infected with Conficker."

Kaminsky said he then turned to help from Securosis researcher Rich Mogull, who on Saturday began mobilizing providers of network scanning products to add the Conficker definitions as soon as possible.

"This is the fastest turn-around I've ever seen," Kaminsky said.

Products from Qualys and ncircle are also expected to add anti-Conficker detection signatures. Werner and Leder have developed their own proof-of-concept scanner, which is available here.

Since showing up a few days after Microsoft released an emergency patch for Windows in late October, Conficker has elicited a grudging admiration from security professionals, who can't help acknowledging the worm's sophistication. It attacked multiple vectors, was able to crack passwords and spread like wildfire, infecting more than ten million boxes in just a few months' time, by some estimates.

Conficker's profile has only grown larger in the past few weeks as the calendar slowly approaches April 1. That's the day that machines infected with Conficker C will be able to tap into a much larger pool of internet addresses to receive instructions - 50,000 instead of the previous 250.

But it would appear the evil geniuses who spawned the malware made a fatal error that until now had gone unnoticed. Its discovery just a few days before an important deadline could lead to its eradication - but only if network admins worldwide put down what they're doing and make use of the tools now.

"We have no idea what Conficker is going to do on April 1," Kaminsky said. "Certainly there is no reason anyone wants to find out on their network. My recommendation is that people run one of the vulnerability scanners on Monday or Tuesday." ®