The infamous Blaster worm infected far more PCs than previously thought - at least eight million, according to Microsoft.
The figure come from the number of times a special Blaster clean-up tool, released by Microsoft in January, has been applied. The tool, available through Windows Update, is cleans up infected machines and also patches PC against the vuln which Blaster exploits.
More than 16 million PCs submitted to Windows Update were infected by Blaster; but that figure is less reliable than the tool download stat because it could include systems counted more than once. Many of those who go to Windows Update may not go through the disinfection process immediately, if at all.
Estimates from security firms and the Internet Storm Centre at the time of Blaster first appearance last August suggested only 500,000 systems had been infected. This data was derived from scanning probes generated from infected PCs. The discrepancy between early data and Microsoft's latest figures could be explained by the containment of scanning behind corporate firewalls, although this theory remains unproven.
Stephen Toulouse, security program manager for Microsoft's security response centre, is confident that the new figures are accurate.
"The [clean-up] tool doesn't even get offered to users, unless they had patches installed and we detected the existence of Blaster on their computer," he told CNET. ®