CSCF (Call Session Control Function)

The Call Session Control Function is the SIP-based control plane of the IMS network. It exists in three roles: the Proxy-CSCF, the first contact for the UE; the Interrogating-CSCF, which routes registrations to the correct Serving-CSCF; and the Serving-CSCF, which handles registration, authentication via the HSS over Diameter Cx, and call control. CSCFs anchor VoLTE, VoNR, and RCS services and are exposed to SIP abuse such as registration hijacking, toll fraud, and DoS.

Categories: Radio Access NetworkCore NetworkSignaling

CSCF in context

The radio access network is the layer where mobile devices attach to the operator's infrastructure. Attacks in this layer include IMSI catching, rogue base stations, downgrade attacks and jamming, and defenses hinge on mutual authentication, integrity-protected signaling and Open RAN supply-chain hygiene.

The mobile core is where subscriber sessions, mobility and policy live. In 4G it is the EPC (MME, HSS, S/PGW); in 5G it is the Service-Based Architecture with AMF, SMF, UPF, AUSF, UDM and the NRF. Core exposure is the difference between a single-subscriber leak and a fleet-wide breach.

To place CSCF in the wider telecom-security picture, review AMF, IMS, RANAP, TEID, Diameter and GPRS.

Related terms

Equipment

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

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