SS7 (Signaling System No. 7)

Signalling System No. 7 is the legacy out-of-band signalling protocol family used to set up calls, deliver SMS, query subscriber data, and support roaming across PSTN, 2G, and 3G networks. Defined by ITU-T from 1980 and still in operational use globally, SS7 was designed for a closed trust model between a small number of state monopolies and has no built-in authentication or encryption. As access to SS7 spread to thousands of operators and resellers, attackers gained the ability to track subscriber location, intercept SMS-based one-time passwords, redirect calls, and conduct fraud. Mitigations require SS7 firewalls, Category 1/2/3 message filtering per GSMA FS.11, and SMS Home Routing.

Categories: SignalingRoaming and InterconnectIdentity and Subscriber

SS7 in context

Signaling protocols carry the control-plane messages that set up calls, register subscribers and route SMS. SS7, Diameter, GTP-C and SIP are the four dominant families, and interconnect exposure of any of them is treated by GSMA as top-tier telecom risk.

Roaming and interconnect are where two operators exchange signaling and user-plane traffic. Trust boundaries here are the primary attack surface for location tracking, SMS interception and fraud, which is why GSMA now mandates SEPP with PRINS on 5G interconnect.

To place SS7 in the wider telecom-security picture, review Signaling Screening, Behavioral Signaling Detection, Roaming, TCAP Abuse, SS7 Message Categories and Roaming Interconnect Security: each entry cross-references back to this page so you can walk the topic in either direction.

Related terms

Related topic hubs

  • SS7 Firewall GSMA FS.11 category 1/2/3 SS7 message screening at the STP edge.
  • Signaling Firewall Unified SS7/Diameter/GTP/SMS signaling firewall framework.
  • Roaming Security IPX/GRX interconnect risk, home-routed vs local-breakout, SEPP.
  • SS7 Security SS7/MAP/CAP/SCCP attacks and defenses across 2G/3G interconnect.

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