Over the past few weeks, I’ve been reinvigorating a SIM swap detection platform we originally designed and built at
@tagomisystems. The underlying concept was to safeguard customer accounts—especially those reliant on SMS-based MFA—by identifying whether a phone number had undergone a SIM swapping attack. This system was designed to be an early indicator of compromised accounts, even if users were using phishing-resistant MFA on our platform.
We worked closely with well known mobile network security researchers, mobile virtual network operators, and other industry intelligence sharing groups. Our goal was to ensure the solution propagated rapidly and comprehensively across the industry, given the seriousness of SIM swapping attacks.
SIM swapping remains a relatively cheap yet highly effective way to circumvent MFA, especially for high-value targets. While SMS-based MFA continues to be common for banks, investment accounts, and other critical financial platforms, it is also one of the most vulnerable methods of second-factor authentication.
What is a SIM swap? A SIM swap occurs when a mobile network operator (MNO) reassigns a phone number to a new IMSI (International Mobile Subscriber Identity), whether for legitimate reasons (changing carriers, upgrading devices) or malicious purposes (intercepting SMS messages).
Detection mechanism: By comparing the IMSI used during previous account activity with the current IMSI, we can identify a SIM swap event. At that point, service providers can apply stricter controls, such as restricting high-risk transactions or forcing more secure authentication flows.
Implementation Challenges: TMSIs (Temporary Mobile Subscriber Identities) are insufficient for detection due to their short-lived nature. Accessing IMSI information directly has become more difficult over time, largely due to expanded "privacy" concerns that limit how carriers share network-level data.
Industry Solutions: Twilio integrated this idea into a commercial API, partnering with carriers that support "SIM swap status checks". Other commercial providers like Vonage have launched similar services. These solutions are valuable, but not foolproof: If a phone number is transferred to a carrier that does not support these "SIM swap status checks", commercial API providers and service providers lose visibility. Additionally, carriers strictly control historical IMSI change logs for "privacy" reasons, preventing service providers from conducting deeper investigations or retrospective analysis.
While HLR (Home Location Register) and VLR (Visitor Location Register) lookups can still yield some actionable data, true SIM swap prevention/detection will require architecture improvements at the carrier level and SS7 routing attacks will require network level architecture improvements.