An authentication bypass vulnerability affecting the Mac version of Skype could have been exploited by malware installed on the device to interact with the application without the user’s knowledge.
The flaw was discovered by researchers at Trustwave, who classified it as a backdoor. The weakness is related to the Desktop API, which allows third-party apps to communicate with Skype. The primary purpose of this API is to help developers manage calls from outside Skype, but Microsoft has been working on phasing it out.
The problem was that a local program could connect to the API and gain the ability to interact with Skype simply by identifying itself as “Skype Dashbd Wdgt Plugin.”
Old versions of the API enabled access to nearly all features of Skype, including incoming message notifications, message content, creating chat sessions, modifying messages, logging and recording audio calls, and fetching contacts. More recent versions of the API don’t allow access to text messages, but other features are still available.
The flaw could have been easily exploited by a piece of malware, as shown by a proof-of-concept (PoC) made available by Trustwave.
The vulnerability affects Skype 7.35 and earlier, and it was reported to Microsoft on September 13. The issue was patched nearly two weeks later in Skype 7.37, but the release notes for this version only mention “general fixes.”
Trustwave researchers pointed out that this may have been an intentional backdoor introduced by developers to allow the “Skype Dashbd Wdgt Plugin” to access the Desktop API without user interaction. This theory is backed up by the fact that “Skype Dashbd Wdgt Plugin” is an undocumented identifier.
On the other hand, it’s also possible that this was a coding error, considering that the plugin does not actually seem to use this channel to access the API. The flaw is believed to have existed for more than five years, possibly since before Microsoft acquired Skype.