Reddit hack: Millions of users could have been hit by huge cyber attack that stole personal details

Anyone potentially affected is being urged to take action

A sign hangs on the door of an office at Reddit headquarters in San Francisco, California April 15, 2014 (REUTERS/Robert Galbraith)

Reddit users have had their details stolen after a major cyber attack on the firm.

A hacker has broken into a few of its systems and taken user data from them, it said. That included current email addresses and a 2007 database that contained old passwords.

"Although this was a serious attack, the attacker did not gain write access to Reddit systems; they gained read-only access to some systems that contained backup data, source code and other logs," Reddit's founding engineer Christopher Slowe wrote.

Reddit will automatically ensure that anyone thought to have been caught up in the attack has to change their passwords, to ensure accounts are kept secure.

But it also advised that people take care to ensure that password is not used on any other sites, since attackers often use stolen passwords to access different websites.

Slowe said Reddit learned on June 19 an attacker compromised a few of its employees' accounts between June 14 and June 18.

The breach was carried out by intercepting text messages that were meant to reach employees with one-time login codes, Reddit said, adding that it was notifying the affected users.

Slowe said the firm hired its first head of security nearly three months ago. "So far he hasn't quit," Slow wrote in the announcement, noting that he had "been put through his paces in his first few months".