A vulnerability in Skype allows anyone to hijack its users' accounts just by knowing or guessing a punter's registered email address.

The embarrassing security hole, which is trivial to abuse, was first discussed on a Russian underground forum three months ago. Last night a Russian blog publicised the bug, and details of the flaw circulated the internet. The hijack is triggered by signing up for a new Skype account using the email address of another registered user. No access to the victim's inbox is required; one just simply needs to know the address.

Creating an account this way generates a warning that the email address is already associated with another user, but crucially the voice-chat website does not prevent the opening of the new account. From there it's possible to log into the service and request a new password for the victim's account: a security token is sent to the attacker's Skype app allowing the login credential to be reset.

Armed with this token, it is also possible to download private chat logs for the compromised account while the genuine owner is locked out.

In a holding statement, the Microsoft-owned VoIP biz confirmed it has disabled the password reset mechanism as a temporary measure:

We have had reports of a new security vulnerability issue. As a precautionary step we have temporarily disabled password reset as we continue to investigate the issue further. We apologize for the inconvenience but user experience and safety is our first priority.

This is a good move because, as Rik Ferguson of Trend Micro warns, the bug makes account hijacking "child's play".

"In essence the procedure is so simple it could be carried out by even the most inexperienced of computer users," Ferguson explains. "All that was necessary was to create a new Skype ID, and associate it with the email address of your victim.

"Once this procedure is complete, a flaw in the password reset procedure allowed the attacker to assume control over the victim account by using the online password reset form. This would lock the victim out of their Skype account and allow the hacker to receive and respond to all messages destined for that victim until further notice. I tested the vulnerability and the entire process took only a matter of minutes."

Before Skype temporarily disabled password resets, the only way to mitigate against the vulnerability was to register a secret email address with one's Skype account. Costin Raiu, senior security researcher at Kaspersky Lab, reports that the Skype account of Russian opposition leader Alexey Navalny was hacked using the exploit. ®