Spamhaus, the leading anti-spam organisation, has restored its site and services to normal after a distributed denial of service attack rendered its site temporarily inaccessible for several hours on Monday.

The attack, launched from a botnet of compromised Windows PCs, witnessed a huge surge in spurious traffic to Spamhaus's web servers from 0900 BST until around 1400 BST.

Steve Linford, director of Spamhaus Project, explained that the site was restored to normal operation after technicians were able to filter (null route) attack traffic. It's not immediately clear who launched the attack, which is far from unprecedented. For example, in September 2003 spammers used PCs infected by the then prevalent SoBig worm to launch an attack against Spamhaus and other anti-spam sites. The attack forced a number of smaller anti-spam sites to close down. Other, less high-profile, denial of service attacks are commonplace and can be considered something of an occupational hazard.

Through its relationship with ISPs and enterprises, over 600m users make use of Spamhaus's block lists, according to Linford. "We're nowadays blocking approx 10bn spams per day. Spamhaus is about to release a new blocklist service which will pretty much put an end to most of the Trojan/virus spamming, so the spammers are getting desperate and trying everything possible to get us out of the way." ®