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No one knows the identity of Conficker's "patient zero" computer, or precisely when it was infected. It was probably a machine that the hackers already controlled. Once installed, the software set to work, surreptitiously scanning the internet for other vulnerable machines to send itself to.

The new worm soon ran into a listening device, a "network telescope", housed by the San Diego Supercomputing Center at the University of California. The telescope is a collection of millions of dummy internet addresses, all of which route to a single computer. It is a useful monitor of the online underground: because there is no reason for legitimate users to reach out to these addresses, mostly only suspicious software is likely to get in touch.

The telescope's logs show the worm spreading in a flash flood. For most of 20 November, about 3000 infected computers attempted to infiltrate the telescope's vulnerable ports every hour - only slightly above the background noise generated by older malicious code still at large. At 6 pm, the number began to rise. By 9 am the following day, it was 115,000 an hour. Conficker was already out of control.

That same day, the worm also appeared in "honeypots" - collections of computers connected to the internet and deliberately unprotected to attract criminal software for analysis. It was soon clear that this was an extremely sophisticated worm. After installing itself, for example, it placed its own patch over the vulnerable port so that other malicious code could not use it to sneak in. As Brandon Enright, a network security analyst at the University of California, San Diego, puts it, smart burglars close the window they enter by.

Conficker also had an ingenious way of communicating with its creators. Every day, the worm came up with 250 meaningless strings of letters and attached a top-level domain name - a .com, .net, .org, .info or .biz - to the end of each to create a series of internet addresses, or URLs. Then the worm contacted these URLs. The worm's creators knew what each day's URLs would be, so they could register any one of them as a website at any time and leave new instructions for the worm there.

It was a smart trick. The worm hunters would only ever spot the illicit address when the infected computers were making contact and the update was being downloaded - too late to do anything. For the next day's set of instructions, the creators would have a different list of 250 to work with. The security community had no way of keeping up.

No way, that is, until Phil Porras got involved. He and his computer security team at SRI International in Menlo Park, California, began to tease apart the Conficker code. It was slow going: the worm was hidden within two shells of encryption that defeated the tools that Porras usually applied. By about a week before Christmas, however, his team and others - including the Russian security firm Kaspersky Labs, based in Moscow - had exposed the worm's inner workings, and had found a list of all the URLs it would contact.

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