Security: SIP/IMS - Interface Deep-Dive

🔗 Critical Signaling Interfaces

1. Mw (P-CSCF ↔ S-CSCF)

• Purpose: SIP registration and session control within the IMS core.
• Key Messages: REGISTER, INVITE, SUBSCRIBE.
• Security Objective: Ensure authenticity of proxy-to-server communication.
• Tactical Vulnerability: Registration Hijacking. If the Mw interface is not protected by IPsec or TLS, an attacker can spoof 200 OK responses to intercept registrations.

2. Cx (CSCF ↔ HSS)

• Purpose: Subscriber authentication and location management using Diameter or HTTP/2.
• Key Messages: UAR/UAA (User-Authorization), MAR/MAA (Multimedia-Auth).
• Security Objective: Protect subscriber credentials and profile data.
• Tactical Vulnerability: Credential Leakage. Unauthorized MAR queries can expose authentication vectors (RAND/AUTN) to rogue nodes.

3. Sh (AS ↔ HSS)

• Purpose: Allows Application Servers (AS) to access subscriber transparent and non-transparent data.
• Key Messages: UDR/UDA (User-Data), PNR/PNA (Push-Notification).
• Security Objective: Enforce strict access control for third-party application servers.
• Tactical Vulnerability: Profile Mining. Malicious AS nodes can scrape private subscriber info (MSISDN, service settings) via the Sh interface.

4. Mg (MGCF ↔ I-CSCF)

• Purpose: Interworking between CS networks and IMS.
• Security Objective: Authenticate incoming calls from legacy gateways.
• Tactical Vulnerability: Toll Fraud Gateway. Spoofing signaling from a rogue MGCF to inject free international calls into the IMS network.

5. Gm (UE ↔ P-CSCF)

• Purpose: The primary entry point for terminal devices into IMS.
• Security Objective: Establish the security association (IPsec) between the UE and the network core.
• Tactical Vulnerability: Insecure AKA. Failure to enforce IPsec after the first UNAUTHORIZED challenge allows for cleartext SIP interception.

🧭 Strategic Mapping (itu-t)

!TIPDefensive Strategy: Always deploy Session Border Controllers (SBC) as the P-CSCF/IBCF to enforce RATE LIMITING on SIP methods and perform deep packet inspection for malformed headers.