Hackers have posted 450,000 user names and passwords online, stolen from Yahoo Voice, according to the Guardian newspaper.
Similar attacks have taken place against other online services, including Android forums and Formspring. Users are being advised to change their passwords to protect the accounts as soon as possible.
The Yahoo attack is certainly the most concerning. Yahoo bought Associated Content for $100 million in May 2010 and set it up as Yahoo Voices, which allows user generated content to be posted online. The site claim to have more than 600,000 contributors.
Security sources have said that the passwords are not encrypted, meaning it was possible for hackers to steal the emails and use them immediately.
Kitguru says: Another weak security system exposed.