Posted by Nataliya Stanetsky and Roger Piqueras Jover, Android Security & Privacy Team

Cell-site simulators, also known as False Base Stations (FBS) or Stingrays, are radio devices that mimic real cell sites in order to lure mobile devices to connect to them. These devices are commonly used for security and privacy attacks, such as surveillance and interception of communications. In recent years, carriers have started reporting new types of abuse perpetrated with FBSs for the purposes of financial fraud.

In particular, there is increasingly more evidence of the exploitation of weaknesses in cellular communication standards leveraging cell-site simulators to inject SMS phishing messages directly into smartphones. This method to inject messages entirely bypasses the carrier network, thus bypassing all the sophisticated network-based anti-spam and anti-fraud filters. Instances of this new type of fraud, which carriers refer to as SMS Blaster fraud, have been reported in Vietnam, France, Norway, Thailand and multiple other countries.

GSMA’s Fraud and Security Group (FASG) has developed a briefing paper for GSMA members to raise awareness of SMS Blaster fraud and provide guidelines and mitigation recommendations for carriers, OEMs and other stakeholders. The briefing paper, available for GSMA members only, calls out some Android-specific recommendations and features that can help effectively protect our users from this new type of fraud.

What are SMS Blasters?

SMS Blaster is the term that global carriers use to refer to FBS and cell-site simulators operated unlawfully with the goal of disseminating (blast) SMS payloads. The most common use case is to leverage these devices to inject Smishing (SMS phishing) payloads into user devices. Fraudsters typically do this by driving around with portable FBS devices, and there have even been reports of fraudsters carrying these devices in their backpacks.

The method is straightforward and replicates known techniques to trick mobile devices to an attacker-controlled 2G network. SMS Blasters expose a fake LTE or 5G network which executes a single function: downgrading the user’s connection to a legacy 2G protocol. The same device also exposes a fake 2G network, which lures all the devices to connect to it. At this point, attackers abuse the well known lack of mutual authentication in 2G and force connections to be unencrypted, which enables a complete Person-in-the-Middle (PitM) position to inject SMS payloads.

SMS Blasters are sold on the internet and do not require deep technical expertise. They are simple to set up and ready to operate, and users can easily configure them to imitate a particular carrier or network using a mobile app. Users can also easily configure and customize the SMS payload as well as its metadata, including for example the sender number.

SMS Blasters are very appealing to fraudsters given their great return on investment. Spreading SMS phishing messages commonly yields a small return as it is very difficult to get these messages to fly undetected by sophisticated anti-spam filters. A very small subset of messages eventually reach a victim. In contrast, injecting messages with an SMS blaster entirely bypasses the carrier network and its anti-fraud and anti-spam filters, guaranteeing that all messages will reach a victim. Moreover, using an FBS the fraudster can control all fields of the message. One can make the message look like it is coming from the legitimate SMS aggregator of a bank, for example. In a recent attack that impacted hundreds of thousands of devices, the messages masqueraded as a health insurance notice.

Although the type of abuse carriers are uncovering recently is financial fraud, there is precedent for the use of rogue cellular base stations to disseminate malware, for example injecting phishing messages with a url to download the payload. It is important to note that users are still vulnerable to this type of fraud as long as mobile devices support 2G, regardless of the status of 2G in their local carrier.

Android protects users from phishing and fraud

There are a number of Android-only security features that can significantly mitigate, or in some cases fully block, the impact of this type of fraud.

Android 12 introduced a user option to disable 2G at the modem level, a feature first adopted by Pixel. This option, if used, completely mitigates the risk from SMS Blasters. This feature has been available since Android 12 and requires devices to conform to Radio HAL 1.6+.

Android also has an option to disable null ciphers as a key protection because it is strictly necessary for the 2G FBS to configure a null cipher (e.g. A5/0) in order to inject an SMS payload. This security feature launched with Android 14 requires devices that implement radio HAL 2.0 or above.

Android also provides effective protections that specifically tackles SMS spam and phishing, regardless of whether the delivery channel is an SMS Blaster. Android has built-in spam protection that helps to identify and block spam SMS messages. Additional protection is provided through RCS for Business, a feature that helps users identify legitimate SMS messages from businesses. RCS for Business messages are marked with a blue checkmark, which indicates that the message has been verified by Google.

We advocate leveraging a couple of important Google security features which are available on Android, namely Safe Browsing and Google Play Protect. As an additional layer of protection, Safe Browsing built-in on Android devices protects 5 billion devices globally and helps warn the users about potentially risky sites, downloads and extensions which could be phishing and malware-based.

Let’s say a user decides to download an app from the Play store but the app contains code that is malicious or harmful, users are protected by Google Play Protect which is a security feature that scans apps for malware and other threats. It also warns users about potentially harmful apps before they are installed.

Android’s commitment to security and privacy

Android is committed to providing users with a safe and secure mobile experience. We are constantly working to improve our security features and protect users from phishing, fraud, and other threats.

Working with global carriers and other OEMs through the GSMA to support the ecosystem in the development and adoption of further cellular security and privacy features is a priority area for Android. We look forward to partnering with ecosystem partners in further raising the security bar in this space to protect mobile users from threats like SMS blasters.

Thank you to all our colleagues who actively contribute to Android’s efforts in tackling fraud and FBS threats, and special thanks to those who contributed to this blog post: Yomna Nasser, Gil Cukierman, Il-Sung Lee, Eugene Liderman, Siddarth Pandit.