Hundreds of millions of hacked usernames and passwords for email accounts and other websites are being traded in Russia's criminal underworld, a security expert told Reuters. The discovery of 272.3 million stolen accounts included a majority of users of Mail.ru, Russia's most popular email service, and smaller fractions of Google, Yahoo and Microsoft email users, said Alex Holden, founder and chief information security officer of Hold Security. It is one of the biggest stashes of stolen credentials to be uncovered since cyber-attacks hit major US banks and retailers two years ago.

Holden was previously instrumental in uncovering some of the world's biggest known data breaches, affecting tens of millions of users at Adobe Systems, JPMorgan and Target and exposing them to subsequent cyber crimes.

The latest discovery came after Hold Security researchers found a young Russian hacker bragging in an online forum that he had collected and was ready to give away a far larger number of stolen credentials that ended up totalling 1.17 billion records.

After eliminating duplicates, Holden said, the cache contained nearly 57 million Mail.ru accounts - a big chunk of the 64 million monthly active email users Mail.ru said it had at the end of last year. It also included tens of millions of credentials for the world's three big email providers, Gmail, Microsoft and Yahoo, plus hundreds of thousands of accounts at German and Chinese email providers.

"This information is potent. It is floating around in the underground and this person has shown he's willing to give the data away to people who are nice to him," said Holden, the former chief security officer at US brokerage RW Baird. "These credentials can be abused multiple times," he said.

Less than $1

Mysteriously, the hacker asked just 50 roubles — less than $1 — for the entire trove, but gave up the dataset after Hold researchers agreed to post favourable comments about him in hacker forums, Holden said. He said his company's policy is to refuse to pay for stolen data.

Such large-scale data breaches can be used to engineer further break-ins or phishing attacks by reaching the universe of contacts tied to each compromised account, multiplying the risks of financial theft or reputational damage across the web.

Hackers know users cling to favourite passwords, resisting admonitions to change credentials regularly and make them more complex. It's why attackers reuse old passwords found on one account to try to break into other accounts of the same user.