Yahoo Contributor Network is a platform that generates high-volume, low-cost content by letting writers photographers, and others share their work with Yahoo members and earn money based on the traffic their content generates. Users who contribute to the network are required to sign in using a Yahoo, Google or Facebook ID.

Associated Content, which was founded in 2005, was bought by Yahoo for just over $100 million in May 2010. Yahoo renamed the service in late 2011, when it also launched Yahoo Voices, a portal where users access content posted by the Yahoo Contributor Network.

According to Yahoo, only people who registered as providers with Associated Content before the 2010 acquisition were affected by the password theft. "[The] compromised file was a standalone file that was not used to grant access to Yahoo! systems and services," Yahoo maintained.

Just under a third of the stolen passwords were linked to accounts registered to a yahoo.com email address, security company Rapid7 said Thursday. Significant chunks of the file, however, were composed of Gmail (23.6% of all accounts) and Hotmail (12.2%) addresses.

All users with older Associated Content accounts, no matter the email address used, should immediately change the passwords for those email accounts as well as any identical or similar passwords used to secure other online services or websites, security experts have said.