Skype's password reset system is vulnerable to an attack that gives attackers full control over affected accounts. The only information attackers need to successfully compromise a Skype account is the victim's email address. Skype checks the email address that you enter when you create a new account. If it already exists in the database, it will give you the option to create a new Skype name using that email address and links both accounts internally.
The issue here is that Skype won't ask you to verify the email address that you have just entered during setup. Instead, you are automatically logged in to the account. While you can't see the contacts, chat history and other information of the original user just yet, the following method gets you full access to that username's account.
When you use Skype's password reset system you are asked to enter the email address associated with the account. Skype interestingly enough sends the password token to the associated email address and displays its in the Skype interface as well. You can use that token to reset the password of the current account or the original account. Skype displays all linked accounts here and once again fails to verify at any stage if you are really the account owner of the original account.
To paraphrase: Skype links accounts automatically when the same email address is entered during account creation. The password recovery system displays the token to change the password in Skype, and not only in the password recovery email. Since both accounts are linked users can reset the password of the original account to one of their liking to gain access to that account.
Skype has reacted to the vulnerability and disabled the service's password reset system for now. The only option to protect the account at the time of writing is to use an email address that no one knows.
It is likely that Skype is going to fix the system before it is re-enabled. It is easy enough to do so, for instance by requiring confirmation before accounts get linked, or by disabling the option to reset the password from within Skype without confirmation email.
Update: Skype has fixed the issue
Early this morning we were notified of user concerns surrounding the security of the password reset feature on our website. This issue affected some users where multiple Skype accounts were registered to the same email address. We suspended the password reset feature temporarily this morning as a precaution and have made updates to the password reset process today so that it is now working properly. We are reaching out to a small number of users who may have been impacted to assist as necessary. Skype is committed to providing a safe and secure communications experience to our users and we apologize for the inconvenience.
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