Flaws can be fixed easily and affordably, say authors of paper

Anonymity, privacy -- these are things we have come to expect when it comes to our cell phones.  The last thing people anticipate is for unknown -- possibly malicious -- third parties to able to quickly track our positions every time we place a phone call.

I. Exploiting the 3G Protocol to Track

But that's precisely what security researchers at the University of Birmingham (located in the central UK) are preparing to show off at the ACM Conference on Computer and Communications Security conference in Raleigh, N.C. next week.  The demonstrated proof-of-concept attack will no doubt add to the aging 3G communication standard's security woes.

The researchers simulated an attacking using affordable off-the-shelf components -- a rooted femtocell (which broadcast 3G signals) and other affordable components.

The attackers used an off-the-shelf femtocell. [Image Source:]

They then conducted two attacks geared at tracking the victim's position.  The attacks were conducted in Europe on a number of real-world networks, across various carriers.

The researchers used a so-called "paging attack" -- a denial-of-service (DOS) type attack that involves tricking basestations or mobile devices into an always "ready" state.  By sending a TMSI (Temporary Mobile Subscriber Identity) which appeared to contain a static IMSI (International Mobile Subscriber Identity), the attacker tricked the victim device into giving up its real IMSI.

That in turn allowed the target to be continuously tracked within a monitored region.

A second route to monitoring was also demonstrated, which used an Authentication and Key Agreement (AKA) protocol attack.  The target device returns a Mac error, while the rest of the devices would respond with a different error -- a synchronization error.

The authors write, "The captured authentication request can now be replayed by the adversary each time he wants to check the presence of [a device] in a particular area. In fact, thanks to the error messages, the adversary can distinguish any mobile station from the one the authentication request was originally sent to."

The caveat here is that the attackers first had to indentify example authentication requests by calling the victim's device.  But they argue that the flaw could still be abused in certain scenarios, such as if a boss wanted to track employees in a large office building.

The researchers elaborate, "[The employer] would first use the femtocell to sniff a valid authentication request. This could happen in a different area than the monitored one. Then the employer would position the device near the entrance of the building. Movements inside the building could be tracked as well by placing additional devices to cover different areas of the building.  If devices with wider area coverage than a femtocell are used, the adversary should use triangulation to obtain finer position data."

II. Fixing the Flaws

So what does all of this mean??  3G networks -- any 3G network, according to the authors -- are vulnerable to tampering which allows their users to be tracked, due to protocol weaknesses.  

The IMSI paging attack flaw seems to be the more dangerous attack as it can be used to track anonymous victims.

Locking the door
Researchers say the flaw can easily be fixed. [Image Source: North Miami Beach FL]

Fortunately, there's a fix to both problems.  The fix is to both modify the error messages, and adopt certain protocol changes.  Those changes would involve introducing a so-called "unlikability" session key to weed out malicious AKA requests, and to implement IMSI paging procedure fixes to prevent the DOS trickery.

The 3G mobile industry's security watchdog, 3GPP, is investigating the proof-of-concept attacks and is considering the proposed fixes, which the authors argue would have a "low... computational and economical cost".  Those fixes could (in theory) be rolled out in coming months to prevent attackers from exploiting "in the wild" the soon-to-be-published flaw.

Source: SC Magazine

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