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Facebook denies malware risk from message bug

Fixes security bypass flaw anyway

Internet Security Threat Report 2014

Facebook has fixed a flaw that may have allowed users to attach executables to messages sent to other punters on the social network.

Security blogger Nathan Power notified Facebook after finding the bug, which involved fiddling with messages sent between users who were not necessarily contacts. Facebook normally blocks the exchange of executable files - but Power discovered that by adding an extra space to the end of a filename, it was possible to get around these restrictions, as he explains in a blog post here.

These executable files could very well be laced with malware. Users can get infected if they are tricked into opening the malicious program on a vulnerable machine, something the bug (by itself) doesn't do. Emails with malicious attachments have proved to be an incredibly successful attack vector for years so seeing this getting replayed in the venue of social networking gives El Reg the fear.

Facebook said last week that a fix was unnecessary before deciding to modify its systems this week after successfully reproducing the "undesirable behaviour" reported by Power in late September.

In a statement, Facebook said: "We were originally unsuccessful in reproducing the behaviour described by the reporter, as this behaviour does not occur for all browser and operating system combinations. Upon further investigation, we have determined which scenarios were behaving undesirably and have pushed a fix to ensure a consistent attachment experience across browsers."

Despite the tweak, Facebook continues to insist that its users were never at risk. It said the social network has multiple layers of security to protect people from malicious files. "Facebook Messages does not rely solely on searching the plain-text of the file name to detect potentially malicious files but has antivirus protection that scans every message for malware and malicious links," it said.

Facebook uses anti-malware technology from Websense to carry out this screening of binaries. ®

Internet Security Threat Report 2014

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