Skype has disabled the account password reset option on its website following reports that the feature can be abused to hijack Skype accounts if the attackers know the email addresses associated with them.
Instructions on how to exploit the security hole were first reported late Tuesday on a Russian-language forum. The information was later posted on Reddit and some blogs where users confirmed that the account hijacking method worked.
It's not clear whether the issue was the result of a design logic flaw or a bug in Skype's client or website. (Here's what you could have done to stay safe.)
According to the reports, the Skype website allowed a potential attacker to create a new account using an email address that's already associated with an existing one.
The attacker could have then logged into the Skype client with the new account, performed a series of refreshes on the home screen and then triggered a password reset for the account through the Skype website. This would have resulted in the website sending an email with a unique password reset link to the email address on record.
However it would have also triggered a "Password token" notification on the home screen in the Skype client where the attacker would have already been logged in. Clicking a "more info" button for this notification would have provided the attacker with the password reset link and unique password reset token directly in the client, without requiring access to the email account.
Visiting the password reset link in a browser would have led to a page on the Skype website that would have provided the option to change the password for any of the accounts associated with that email address, including the original account of the legitimate user.
"We have had reports of a new security vulnerability issue," Skype representative Sravanthi Agrawal said Wednesday via email. "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."
If having multiple accounts associated with the same email address is intended functionality, a simple solution would have been for Skype to send an account activation link to the provided email address every time a new account is created. This is common procedure for other services and would have blocked the attack from the start, since the attacker wouldn't have been able to activate the new account and perform the attack's other steps.
It's not clear if the password token notification displayed in the Skype client is actually a feature intended to alert users of rogue password reset attempts or the result of a bug.
Since this vulnerability was publicly disclosed and might have been known in some circles for some time, concerned users might want to change their passwords just to be on the safe side.
The Skype website allows logged in users to associate a second email address with their accounts and delete the old one. This feature can be used to change the email address to one that is not known by anyone and therefore reduce the chances of the account being attacked through password reset bugs in the future.