Microsoft Patches Dangerous Backdoor In Skype For Mac OS X

Microsoft has patched a backdoor in Skype for Mac OSX that would allow an attacker to log and record Skype call audio, retrieve user contact information, read the content of incoming messages, create chat sessions, modify messages, and carry out other malicious activity.

The backdoor provided nearly complete access without authentication to Skype on OS X, and appears to have been around since at least 2010, security vendor Trustwave said in an advisory this week.

The vulnerability seems to have been created by a developer at Skype prior to Microsoft’s takeover of the company, and likely exposed some 30 million Mac OS X users.

Trustwave described the backdoor as a locally exploitable issue in the Skype Desktop API for Mac OS-X that gave any program, including malware, unauthenticated access to the API.

The vulnerable Application Programming Interface has been discontinued and is being slowly phased out in versions of Skype across all platforms.

The backdoor appears to have been put in place to enable older versions of the Skype Dashboard Widget plugin to access the Desktop API without user interaction, says a researcher from the UK pen testing team at Trustwave’s SpiderLab, who requested anonymity. What makes this a very likely possibility is the fact that the Desktop API provides for a client named “Skype Dashbd Wdgt Plugin.”

The widget is a small program that runs on the OS-X Dashboard and is designed to let users make Skype calls from the Dashboard by entering a number.  

Interestingly, though, there is no indication that the widget itself actually used the backdoor to access the Desktop API, according to Trustwave. So it is possible the backdoor is the result of code being accidentally left behind when the widget was being implemented. "It is likely the backdoor was used at some time in the past but fell into disuse and just got left there," the researcher says.

The result was that local programs had unauthenticated access to the Desktop API as long as the programs used "Skype Dashbd Wdgt Plugin" as their client name identifier, the researcher says. 

Trustwave described the backdoor as easy to access and requiring little more than a simple change to the client application name to "Skype Dashbd Wdgt Plugin." Once this was done, the API made no attempt to determine what programs were accessing it - making it trivially easy for attackers to slip in malware.

"There was no means for the user to deny access via the backdoor," the researcher says. "In older versions of OS-X and Skype, the user was not informed at all if the backdoor was being used, in later versions, potentially sometime around OS-X 10.9-10, a notification was present, but the user still did not have the ability to deny access."

The Desktop API in previous versions of Skype permitted access to almost everything within the application so attackers could take complete control by abusing the backdoor.  So "[an] attacker could write a piece of code into their malware to siphon every piece of info you could think of from Skype" including contact lists, instant messages, and conversations, the researcher says.

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