SMS Firewall

An SMS firewall is the screening function for short-message traffic entering an operator's SMSC, SMS Router or SMS-GMSC. It inspects both legitimate channels (A2P bulk via SMPP and P2P MAP-based delivery from interconnect) and abuse channels (grey-route A2P that enters via SS7 to bypass commercial SMS termination, and SS7-borne SMS interception or location disclosure). The firewall typically pairs an SS7/SIGTRAN inspection plane with an SMPP application plane, and integrates with SMS home routing so the home network sees and controls every mobile-terminated message destined for its subscribers.

The reference controls are GSMA FS.21 and the related A2P guidelines. They cover sender-ID and originating-address validation (anti-spoofing of brand or short-code senders), content-based filters for smishing and bulk spam, anti-fraud rules for revenue-sensitive flows like OTP delivery, plausibility checks on SRI-SM (so the home network does not leak IMSI or the serving MSC to an outsider), and policy that closes grey routes by forcing all A2P through commercially-terminated paths. SMS home routing is the structural control underneath all of this: the SMSC of the home network is interposed on every mobile-terminated SMS, so the firewall can apply policy in one place instead of trying to police every visited network.

Done well, an SMS firewall contains four classes of problem at once: revenue leakage from grey-route A2P, smishing and brand impersonation, SS7-borne SMS interception or rerouting that targets two-factor authentication codes, and IMSI/location disclosure through abuse of SMS-related MAP messages. It is part of the same perimeter as the SS7 firewall and the Diameter firewall; the unified view is in the Signaling Firewall guide.