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Security

False Base Station or IMSI Catcher: What You Need to Know

Tao Wan
Principal Architect, Security

Oct 23, 2019

You might have heard of False Base Station (FBS), Rogue Base Station (RBS), International Mobile Subscriber Identifier (IMSI) Catcher or Stingray. All four of these terminologies refer to a tool consisting of hardware and software that allow for passive and active attacks against mobile subscribers over radio access networks (RANs). The attacking tool (referred to as FBS hereafter) exploits security weaknesses in mobile networks from 2G (second generation) to 3G, 4G and 5G. (Certain improvements have been made in 5G, which I’ll discuss later.)

In mobile networks of all generations, cellular base stations periodically broadcast information about the network. Mobile devices or user equipment (UE) listen to these broadcasting messages, select an appropriate cellular cell and connect to the cell and the mobile network. Because of practical challenges, broadcasting messages aren’t protected for confidentiality, authenticity or integrity. As a result, broadcasting messages are subject to spoofing or tampering. Some unicasting messages aren’t protected either, also allowing for spoofing. The lack of security protection of mobile broadcasting messages and certain unicasting messages makes FBS possible.

An FBS can take various forms, such as a single integrated device or multiple separated components. In the latter form [1], an FBS usually consists of a wireless transceiver, a laptop and a cellphone. The wireless transceiver broadcasts radio signals to impersonate legitimate base stations. The laptop connects to the transceiver (e.g., via an USB interface) and controls what to broadcast as well as the strength of the broadcasting signal. The cellphone is often used to capture broadcasting messages from legitimate base stations and feed into the laptop to simplify the configuration of the transceiver. In either form, an FBS can be made compact with a small footprint, allowing it to be left in a location unnoticeably (e.g., mounted to a street pole) or carried conveniently (e.g., inside a backpack).

An FBS often broadcasts the same network identifier as a legitimate network but with a stronger signal to lure users away. How much stronger does an FBS’s signal need to be to succeed? The answer to that question hasn’t been well understood until recently. According to the experiments in the study [2], an FBS’s signal must be more than 30db stronger than a legitimate signal to have any success. When the signal is 35db stronger, the success rate is about 80 percent. When it’s 40db stronger, the success rate increases to 100 percent. In these experiments, FBS broadcasts the same messages with the same frequency and band as the legitimate cell. Another strategy taken by an FBS is to broadcast the same network identifier but with a different tracking area code, tricking the UE into believing that it has entered a new tracking area, and then switch to the FBS. This strategy can make it easier to lure the UE to the FBS and should help reduce the signal strength required by the FBS to be successful. However, the exact signal strength requirement in this case wasn’t measured in the experiments.

Once camped at an FBS, a UE is subject to both passive and active attacks. In passive attacks, an adversary only listens to radio signals from both the UE and legitimate base stations without interfering with the communication (e.g., with signal injection). Consequences from passive attacks include—but are not limited to—identity theft and location tracking. In addition, eavesdropping often forms a stepping stone toward active attacks, in which an adversary also injects signals. An active attacker can be a man-in-the-middle (MITM) or man-on-the-side (MOTS) attacker.

In MITM attacks, the attacker is on the path of the communication between a UE and another entity and can do pretty much anything to the communication, such as reading, injecting, modifying and deleting messages. One such attack is to downgrade a UE to 2G with weak or null ciphers to allow for eavesdropping. Another example of an MITM attack is aLTEr [3], which only tampers with DNS requests in LTE networks, without any downgrading or tampering of control messages. Although user plane data is encrypted in LTE, it’s still subject to tampering if the encryption (e.g., AES counter mode) is malleable due to the lack of integrity protection.

In MOTS attacks, an attacker doesn’t have the same amount of control over communication as with an MITM attack. More often, the attacker injects messages to obtain information from the UE (e.g., stealing the IMSI by an identity request), send malicious messages to the UE (e.g., phishing SMS) or hijack services from a victim UE (e.g., answering a call on behalf of the UE [4]). A MOTS attacker, without luring a UE to connect to it, can still interfere with existing communication—for example, by injecting slightly stronger signals that are well timed to overwrite a selected part of a legitimate message [2].

FBS has been a security threat to all generations of mobile networks since 2G. The mitigation to FBS was studied by 3GPP in the past—however, without any success due to practical constraints such as deployment challenges in cryptographic key management and difficulty in timing synchronization. In 5G release 15 [5], network side detection of FBS is specified, which can help mitigate the risk, albeit fail to prevent FBS. 5G release 15 also introduces public key encryption of subscriber permanent identifier (SUPI) before it is sent out from the UE, which—if implemented—makes it difficult for FBS to steal SUPI. In 5G release 16 [6], FBS is being studied again. Various solutions have been proposed, including integrity protection of broadcasting, paging and unicasting messages. Other detection approaches have also been proposed.

Our view is that FBS arises mainly from the lack of integrity protection of broadcasting messages. Thus, a fundamental solution is to protect broadcasting messages with integrity (e.g., using public key based digital signatures). Although challenges remain with such a solution, we don’t believe those challenges are insurmountable. Other solutions are based on the signatures of attacks, which may help but can eventually be bypassed when attacks evolve to change their attacking techniques and behaviors. We look forward to agreement from 3GPP SA3 on a long-term solution that can fundamentally solve the problem of FBS in 5G.

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References

[1] Li, Zhenhua, Weiwei Wang, Christo Wilson, Jian Chen, Chen Qian, Taeho Jung, Lan Zhang, Kebin Liu, Xiangyang Li, and Yunhao Liu. “FBS-Radar: Uncovering Fake Base Stations at Scale in the Wild.” In Proceedings of ISOC Symposium on Network and Distributed Systems Security (NDSS), February 2017.

[2] Hojoon Yang, Sangwook Bae, Mincheol Son, Hongil Kim, Song Min Kim, and Yongdae Kim. “Hiding in Plain Signal: Physical Signal Overshadowing Attack on LTE.” In Proceedings of 28th USENIX Security Symposium (USENIX Security), August 2019.

[3] Rupprecht D, Kohls K, Holz T, and Popper C. “Breaking LTE on Layer Two.” In Proceedings of IEEE Symposium on Security & Privacy (S&P), May 2019.

[4] Golde N, Redon K, and Seifert JP. “Let Me Answer That for You: Exploiting Broadcast Information in Cellular Networks.” In Proceedings of the 22nd USENIX Security Symposium (USENIX Security), August 2013.

[5] 3GPP TS 33.501, “Security Architecture and Procedures for 5G System” (Release 15), v15.5.0, June 2019.

[6] 3GPP TR 33.809, “Study on 5G Security Enhancement against False Base Stations” (Release 16), v0.5.0, June 2019.

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