A deleteriously-clever new bit of malware is invading the United Kingdom, and — in a feat not seen since 1066 — appears to be unstoppable.

The exploit, which began appearing late in 2007, is infecting large numbers of sites — some estimates are as high as 10,000 — by an unusual and incredibly difficult to remove method. It is being described by security researchers as an "on-the-fly" Trojan, because it does not install malware on the affected server and then wait for unsuspecting visitors to arrive and be infected — instead, it only installs itself when a visitor arrives, meaning it can't be scanned for by traditional methods. Similarly, most of the currently-available anti-virus and security software is unable to detect the attacks and prevent them on user's computers.

For now, researchers believe the attacks are limited to systems running Windows — another good reason to prefer Linux — but given it's unpredictable nature, they're unsure what to expect next.

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