Yahoo! is urging users of its Mail service to change their passwords to something secure and unique to the web giant – after a security breach exposed account login details to theft.

The company said that it has reset the passwords on accounts connected to what it termed a "third-party database compromise" – that database contained records on some of Yahoo!'s users. Yahoo! said the nabbed sensitive information is already being used by crooks' automated attack programs to compromise accounts and their users.

In addition to resetting account information, which will force folks to think up a new password, Yahoo! said that it will be implementing two-factor authentication: users who have associated a mobile number with their accounts will be asked to enter in a code from an SMS message.

The web company said that it had yet to find any evidence that its own systems were compromised by attackers. The company did not name the third-party that had been hacked.

"In addition to adopting better password practices by changing your password regularly and using different variations of symbols and characters, users should never use the same password on multiple sites or services," said Yahoo! senior vice president of platforms and personalization products Jay Rossiter.

"Using the same password on multiple sites or services makes users particularly vulnerable to these types of attacks."

The disclosure adds Yahoo! to a growing list of major brands which have suffered attacks on user data recently. Retail giants Target and Neiman Marcus have both reported major failures in which point of sale (POS) systems were compromised in order to lift user accounts and payment card details.

Last year, Adobe had to deal with an breach around its systems which left millions of customer accounts vulnerable. ®