SS7 Vulnerability Isn’t a Flaw — It Was Designed That Way

A television news magazine recently ran a segment showing how German Chaos Computer Club (CCC) members could use the telephone network to access the voice data of a mobile phone, find its location and collect other information. All involved professed shock that such a thing could happen, and Democratic Rep. Ted Lieu of California even called for a congressional investigation about it.

But the Signaling System 7 (SS7) vulnerability is a reality. Not only that, it’s an intentional loophole that’s existed for years.

It’s a Feature, Not a Flaw

While it is interesting to see the reactions to this story, experts have known for the last 30 years that SS7, the switching protocol used by over 800 global telecoms, is insecure. It has been that way since its inception; in fact, the CCC gave almost the same demonstration in December 2014.

Since it was developed in 1975, a comprehensive threat model was never one of the design parameters for SS7. Whether that was an oversight or an intentional part of the protocol is irrelevant. SS7 exists, it is almost universally used and it does not guarantee user security.

SS7 controls telephone calls, both wired and wireless, through the use of a control signal that is separate from the actual voice circuit. It allows phone networks to exchange the information needed for passing calls and text messages between each other and to ensure correct billing. It also allows users on one network to roam on another, which is often needed in a foreign country, The Guardian explained.

SS7 can also be a revenue generator for the telco companies. Banks may want to know where a user’s phone is physically located before approving a charge, for example.

Users want the seamless experience it offers. If that weren’t the case, it would not have been adopted so widely. But the security of SS7 is only as good as its weakest link.

Should cybercriminals gain access, they can redirect the control channel used for a telephone number via spoofing (since redirects via spoofing are allowed in the protocols) and use it to gain call details. If a telco has a roaming agreement with the target network, it can access the phone. Users won’t know it is happening since that control channel is separated from the voice channel.

Dealing With the SS7 Vulnerability

Keeping a voice conversation private can be aided by both parties using end-to-end encryption apps. Someone may be able monitor the voice, but they won’t understand it.

A bigger problem is two-factor authentication. One factor is usually sent via SMS to a user. Someone listening can intercept that SMS message and use it for fraud. One possible solution is having that second factor sent to an encrypted messaging account.

Dealing with location monitoring is a bit trickier. Staying off the cell network and on Wi-Fi might help with fraudsters, but more competent adversaries could track the Wi-Fi’s IP address.

The most important takeaway is to understand that the telephone network that your phone connects to is not secure and probably never will be. There may be some improvements in the future with regard to telcos scanning for unauthorized users, but that won’t stop a determined player.

Telephone networks were not designed to be secure. Understanding that and adjusting habits may help when security is needed.

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Larry Loeb

Principal, PBC Enterprises

Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek. He wrote for IBM's DeveloperWorks site for seven years and has written a book on the Secure Electronic Transaction Internet protocol. His latest book has the commercially obligatory title of Hack Proofing XML. He's been online since uucp "bang" addressing (where the world existed relative to !decvax), serving as editor of the Macintosh Exchange on BIX and the VARBusiness Exchange.