Valve has informed users of its Steam online game distribution platform that hackers have probably downloaded encrypted credit card transaction data from a backup database during an intrusion last year.

In November 2011, Valve announced that hackers gained unauthorized access to Steam's user database, but said that there was no evidence to suggest a leak of encrypted credit card details at that time.

However, that has since changed. "Recently we learned that it is probable that the intruders obtained a copy of a backup file with information about Steam transactions between 2004 and 2008," said Gabe Newell, Valve's co-founder and managing director, in an email sent Friday to Steam users.

According to Newell, the backup file contained Steam user names, email addresses, encrypted credit card details and encrypted billing addresses, but no account passwords.

Valve doesn't have reasons to believe that the sensitive transaction data was decrypted, Newell said. However, this possibility should not be excluded.

"As I said in November it's a good idea to watch your credit card activity and statements," Newell said. "And of course keeping Steam Guard [a Steam account security system] on is a good idea as well."

Full story: Computerworld