Femto Hacking in UMTS and LTE

Couple of weeks back, The Hacker’s Choice (THC) made available some documents about how the Vodafone's (UK) Femtocell (a.k.a. SureSignal) is unsecure and can be hacked. Everyone seemed to jump on this bandwagon with some news articles even sounding like the whole Vodafone network has been hacked and hackers may be sending messages and making calls via your phone number.

In the end it came to light that the problem was fixed over a year back when Vodafone was made aware of this problem. THC is still arguing that there is an architecture fault and the Femto can be compromised.

As a result I decided to think about what could happen if the Femtocell is hacked.

Lets take case of UMTS Femtocell. A simple network architecture with femtocell (oficially known as Home NodeB) is as follows:

As you can see, the signalling over the air interface is encrypted and integrity protected. If a hacker is able to get into the Femto and able to listen to all the packets using some tool like WireShark, he would be able to get hold of the Ciphering and Integrity Keys as they come in cleartext in the RANAP Security Mode Command message.

It wouldnt be difficult to have a device that can listen to the conversations once provided with this keys. In fact if the hacker is able to listen to the messages, there is no reason he cannot stick his own messages at the right interval (when a voice call is ongoing) to send SMS and would appear that the message actually went from the phone number. Note that this message would be inserted in the Home NodeB and would be a NAS message. The end user would generally never find out that a message has been sent on behalf of his phone.

One thing that should be remembered though is that the phone would have to be in the range of the Femtocell and connected successfully to the network (via the Femto). One question someone may have is that can I not reverse engineer the key so that I can clone the SIM card. Fortunately for us, this is not easily possible. There are multiple levels of protection and generally it would be difficult to get the algorithms for generating the key. Also it should be noted that the authentication algorithms are confidential and only the operators know the algorithm.

Now lets look at the LTE Femtocell (a.k.a. Home eNodeB) as shown below:

One of the differences you may notice is that the signalling from Femto to the Core Network over S1 is encrypted and Integrity Protected. In case of the LTE Femto, there are multiple keys and only the required key (Kenb) is provided to the Femto. See the key hierarchy below:

Source: RedYoda

This would sound like an ideal protection from the end user perspective but some of the problems still remain. If the hacker can get hold of the Kenb which is sent in cleartext over the S1 interface via Initial Context Setup Request message then he could easily use it to listen to the packets. Since there is no voice support as of yet in LTE, it would only be the packets that the hacker can listen to.

As you may notice, there is now an Integrity and Ciphering on the S1 interface for the UE messages, the hacker cannot get hold of the Kasme or the master keys K, CK and IK. This means that he cannot insert rouge messages that would for example send unsolicited SMS on behalf of the user as he would be able to do in case of UMTS.

There is a small caveat though. There are multiple Ciphering and Integrity algorithms defined in the standard. No ciphering is defined as eea0 algorithm. In Release-8 of LTE, there was no possibility to have Integrity switched off as there was no eia0 algorithm defined. In Release-9 though, the new eia0 has been defined which means that the network can set the Integrity to NULL. I am sure that the network would not want to do so as it makes absolutely no sense but the hacker can force it to do so.

When the Network requests the UE to send the capability information, the hacker can force it to say that it only supports eia0 and eea0 which would mean that the integrity and ciphering in the call would be off. To be honest, this is quite a difficult thing to do in real time and also the network would not accept a UE that does not support other Integrity and Ciphering algorithms.

3GPP has already forseen these kind of threats that could be affecting the networks in the future when they roll out the Femtocells. As a result they have produced 3GPP TR 33.820 that lists all the possible threats and the best practices that can help to minimise the chances of the network being compromised. If that document is too big and technical, you can go though this presentation as it summarises some of the problems.

Feel free to comment or correct any mistakes that you think I have made.