Plain-text passwords and account names linked to five million Gmail accounts have been leaked onto several Russian forums.
Security experts had already confirmed the data seemed legit, albeit approximately three years old, before Google put up its blog post on the subject.
The leak, to a variety of forums, not all of which are cybercrime related, likely originates from various sources, Peter Kruse, a partner and eCrime specialist at CSIS Security Group told El Reg.
The 50MB zipped file thrown up by the leak appears to come from a source other than Google itself, according to a preliminary analysis by CSIS.
"Our analysis reveals that there is substantial evidence that this data is up to three years old and primarily [linked to the] geographical catchment area [of] the United States and England, but [we've] also confirmed [a] dozen Danish Gmail accounts," CSIS reports, according to a Google Translate version of a Danish language advisory. "We base this on an overall correlation with other data leaks we've seen in recent years."
The purported Gmail leak comes days after a similar leak of login data for millions of email accounts held with Russian webmail service mail.ru. CSIS reckon both leaks come from the same source.
Anyone with concerns that their webmail account might have been compromised have been advised to change their passwords, as a precaution.
Yiannis Chrysanthou, security researcher in KPMG’s cyber security team, argued that simply changing passwords is no longer enough and a move towards multi-factor authentication is required.
“Businesses need to introduce multi-factor authentication instead of blaming weak passwords,” Chrysanthou said.
“Catalogues of previously leaked credentials serve as a database for password crackers. This then makes future hacks even easier and quicker; with many passwords cracked in zero time."
He added: "Password cracking research is moving towards intelligent, efficient and content aware attack techniques designed to crack the bulk of passwords fast. Every large scale credential leak makes cracking passwords easier for the next one, and organisations adding password complexity to their policies only slightly delays this process instead of stopping it."
El Reg put out a query to Google but we've yet to hear back. We'll update this story as and when we hear more. ®