S-CSCF (Serving Call Session Control Function)

Also known as: Serving CSCF, Serving Call Session Control Function

S-CSCF is the central SIP registrar and session controller in the IMS. It authenticates users during IMS-AKA registration using authentication vectors fetched from the HSS over Cx, applies initial Filter Criteria to trigger Application Servers over the ISC interface, and routes originating and terminating SIP sessions. Compromise or misconfiguration of the S-CSCF can enable identity spoofing, service abuse, and lawful interception evasion.

Categories: SignalingInternet and Routing

S-CSCF in context

Signaling protocols carry the control-plane instructions that set up calls, register subscribers, hand over sessions 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 FS.11 / FS.19 / FS.20 as a top-tier telecom risk.

Telecom networks depend on Internet-style routing (BGP, DNS, MPLS) for interconnect, transport and application delivery. Route hijacks and DNS abuse can degrade or intercept signaling as easily as any application-layer attack, which is why RPKI and DNSSEC are now standard hardening for carrier networks.

To place S-CSCF in the wider telecom-security picture, review Third-Party Registration, SMS, P-CSCF, CALEA, STIR/SHAKEN and CAP.

Related terms

Equipment

IMS core (CSCF, TAS, MRF, MGCF, BGCF)

Articles in the P1 mobile network security guide

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