Hackers blocked access to several websites owned by Microsoft, just one day after the software giant corrected an employee's error that shut consumers out of its sites.
The so-called "denial-of-service attack" occurred yesterday, intermittently keeping web surfers shut out from Microsoft internet properties such as Microsoft.com and MSN.com, the company said in a statement.
The company said all sites were now accessible.
A denial-of-service attack targets network equipment that directs Internet traffic. It occurs when hackers, using automation software, overwhelm a site with so much fake traffic that legitimate Internet users cannot get through. It's like hundreds of pranksters trying to call a single phone number at once.
Hackers do not have to break in to a site to launch such an attack, although in many cases they break into third-party computer systems and launch attacks from there to avoid tracing.
A Microsoft employee said Thursday's attack was first spotted by company technicians who were monitoring network activity.
"It's not the first time people have made attempts," the source said. "This one was a little bit more effective."
Last February, Yahoo, Amazon, CNN, eBay and other major internet sites were crippled for hours at a time by similar attacks.
Last week, a 16-year-old Montreal boy known as "Mafiaboy" pleaded guilty to 56 charges of mischief in connection with some of those attacks. Ten other charges were dropped.
Microsoft said it had notified the FBI of yesterday's attack.
AP