As many as 450,000 account details have allegedly been stolen from a Yahoo server by a new hacker group. The group, which goes by the name D33DS Company ("Leetspeak" for "deeds," which uses numbers in place of letters), claims to have hacked into a subdomain of Yahoo's website to gain access to unencrypted account information. The data is believed to belong to Yahoo's VoIP service, Yahoo Voices, which runs on the company's instant messenger.

Some logins for Google Inc, AOL Inc and Microsoft Corp services were among those compromised. The three companies said they required affected users to reset passwords for sites including Gmail, AOL, Hotmail, MSN and Live.com.

Yahoo issued a statement apologizing for the breach, the latest setback for a company that has lost two chief executives in a year and is struggling to revive stalled revenue growth. Chairman Alfred Amoroso acknowledged that Yahoo had experienced a "tumultuous" year at its annual shareholder meeting on Thursday morning. Interim CEO Ross Levinsohn told attendees he was optimistic about the company's progress.

The breach prompted criticism from security experts who said that a major Internet firm like Yahoo should do a better job at protecting user data.

"This points to some very lax security practices," said Rob D'Ovidio, associate professor of criminal justice at Drexel University.

As an example, he noted that the hackers were able to produce more than 400,000 cleartext passwords within a day. That indicates that Yahoo either did not encrypt them at all or used an encryption method that was easy to crack, he said.

The professional networking service LinkedIn recently came under similar criticism. Security experts chided the company for failing to use sophisticated encryption practices to secure its passwords, millions of which were released following a breach last month.