Security: Mobile Core - GTP Interface Deep-Dive

🔗 Critical GTP Interfaces

1. S1-U (eNB ↔ S-GW)

• Purpose: User-plane traffic (GTP-U) between the base station and the serving gateway in 4G.
• Security Objective: Prevent user-plane interception and data injection.
• Tactical Vulnerability: GTP-U Hijacking. Absence of encryption on S1-U allows an attacker with access to the backhaul to intercept or modify subscriber data packets.

2. S5/S8 (S-GW ↔ P-GW)

• Purpose: Tunnelling user data and control signaling (GTP-C) between serving and packet gateways (S8 is for roaming).
• Security Objective: Protect session management signaling and roaming subscriber data.
• Tactical Vulnerability: Session Deletion. Injecting a spoofed Delete Session Request (GTPv2-C) on the S8 interface can lead to immediate subscriber disconnect.

3. N3 (gNB ↔ UPF)

• Purpose: 5G user-plane interface carrying GTP-U tunnels.
• Security Objective: Isolation of 5G user-plane traffic.
• Tactical Vulnerability: TEID-based Information Leakage. Monitoring N3 for active TEIDs (Tunnel Endpoint Identifiers) allows attackers to identify and target specific subscriber tunnels.

4. Gn/Gp (SGSN ↔ GGSN)

• Purpose: Legacy 2G/3G GTP interfaces (GTPv1).
• Security Objective: Secure legacy packet core interconnects.
• Tactical Vulnerability: Fragment Overlap Attacks. GTPv1 implementation flaws in packet fragmentation can be used to bypass shallow firewalls or crash the GGSN.

5. N4 (SMF ↔ UPF)

• Purpose: Control/User plane separation (CUPS) in 5G using PFCP (Packet Forwarding Control Protocol) to manage GTP-U tunnels.
• Security Objective: Authenticated control of user-plane forwarding rules.
• Tactical Vulnerability: PFCP Injection. Unauthorized PFCP messages to the UPF can redirect GTP-U tunnels to an external interception node.

🧭 Strategic Mapping (itu-t)

!TIPDefensive Strategy: Implement GTP Firewalls at the S5/S8 and N32 (5G) boundaries to perform stateful inspection of GTP-C sequence numbers and TEIDs, matching them against active subscriber contexts.