A hacker group has posted online the details of 450,000 user accounts and passwords it claims to have stolen from a Yahoo server.

The passwords could pose a severe security risk to Yahoo users if they use the same password and email combination across other sites.

The information was posted by a previously unknown hacker group.

The Ars Technica technology news website reported that the group, which calls itself D33DS Company, hacked into an unidentified subdomain of Yahoo's website where they retrieved unencrypted account details.

A Yahoo spokesperson declined to comment.

The affected accounts appeared to belong to a voice-over-Internet-protocol, or VOIP, service called Yahoo Voices, which runs on Yahoo's instant messenger...