When most people think about 4G, they picture streaming video, faster apps, and “more bars.” But for those of us in the mobile network security world, 4G represents something more foundational—and more fragile.
Despite the rise of 5G, 4G LTE remains the dominant mobile network technology in use globally. It’s not just the infrastructure behind your phone; it’s also the backbone of most current 5G Non-Standalone (NSA) deployments. That means any vulnerabilities within 4G continue to impact billions of devices and enterprises around the world.
In this post, we’ll break down the core 4G LTE architecture, examine its key interfaces and protocols, and explore the security assumptions (and misconceptions) that make it both revolutionary—and risky.
What is 4G LTE?
4G, or Long Term Evolution (LTE), represents a shift from circuit-switched mobile communications (used in 2G/3G) to fully packet-switched IP networks. The changeover introduced:
- All-IP architecture
- Higher throughput (up to 100 Mbps downlink, 50 Mbps uplink)
- Lower latency (<10 ms)
- Native support for IMS (IP Multimedia Subsystem) and VoLTE (Voice over LTE)
This new model streamlined operations for mobile network operators but also introduced new attack surfaces across the core network, signaling infrastructure, and subscriber interfaces.
Inside the 4G LTE Core: EPC Breakdown
The Evolved Packet Core (EPC) is the heart of the 4G system. It’s built on five main components:
1. MME (Mobility Management Entity)
- Handles signaling related to mobility, authentication, and bearer establishment.
- Interfaces: S1-MME, S6a, S10, S11.
- Attack surface: susceptible to signaling storms, brute-force IMSI paging, and malformed NAS messages.
2. SGW (Serving Gateway)
- Routes and forwards user data packets.
- Acts as the local mobility anchor during handovers.
- Can be attacked via malformed GTP-U packets or spoofed tunnel endpoints.
3. PGW (Packet Data Network Gateway)
- Interface to external packet data networks (i.e., the internet).
- Applies policy enforcement and packet filtering.
- Key target for DoS and IP spoofing attacks.
4. HSS (Home Subscriber Server)
- Central database for user profiles and authentication vectors.
- Speaks Diameter over S6a.
- At risk from Diameter-based enumeration and manipulation.
5. PCRF (Policy and Charging Rules Function)
- Manages QoS, throttling, and charging.
- Manipulable via unverified AVPs in Gx or Rx messages.
Key Interfaces and Protocols in 4G
While the shift to IP was meant to simplify network operations, it also meant many legacy security assumptions were thrown out the window. Let’s look at the most critical protocols powering LTE:
✔️ Diameter Protocol
- Replaces SS7 for signaling between core network elements.
- Used for roaming (S6a, S9), policy (Gx), and charging (Gy).
- Security concerns:
- Lack of encryption (TLS rarely implemented).
- Weak peer authentication.
- Susceptibility to replay attacks and AVP injection.
✔️ GTP (GPRS Tunneling Protocol)
- Used in both user (GTP-U) and control (GTP-C) planes over S1-U and S5/S8 interfaces.
- Exploitable by:
- Spoofed Create Session Requests (CSRs).
- Tunnel ID prediction.
- GTP-in-GTP reflection attacks.
✔️ NAS (Non-Access Stratum) Protocol
- Handles signaling between UE and MME.
- Security gaps:
- IMSI exposed in initial attach.
- Can be fuzzed to crash MME or manipulate bearer setup.
✔️ SIP over IMS
- Enables VoLTE, SMS over IP, and rich communication services.
- Risks:
- SIP registration flooding.
- Caller ID spoofing.
- Toll fraud.
Roaming in 4G: New World, Old Problems
Although Diameter replaced SS7, many of the same flaws persist:
- Location tracking via Update Location Requests.
- Denial of Service via malformed or unauthorized Diameter messages.
- Silent SMS or IMSI paging through Diameter probes.
- S9 interface hijacking to manipulate QoS or policy settings.
Roaming security remains particularly weak because international carriers often operate with different enforcement policies, inconsistent TLS deployments, and weak federation trust models.
VoLTE and IMS: The Double-Edged Sword
Voice over LTE (VoLTE) relies on the IMS subsystem, which is essentially a SIP-based VoIP network built into the operator’s core.
Security pain points include:
- Lack of mutual TLS in SIP signaling.
- Unauthenticated registration flooding.
- Codec downgrade attacks.
- SIP fuzzing leading to SBC or CSCF crashes.
Operators often leave legacy SIP implementations enabled, increasing the attack surface. SIP scanners like SIPVicious or custom fuzzers can identify exploitable servers within minutes.
LTE Security Features (on Paper)
4G introduces several native security mechanisms, including:
- AKA (Authentication and Key Agreement) for mutual authentication.
- IPSec tunnels between UE and ePDG for untrusted Wi-Fi access.
- Confidentiality & Integrity Protection using AES and Snow 3G.
However, their effectiveness depends on correct implementation—something we rarely see across every layer.
Real-World LTE Threats Observed
P1’s field experience across telecom security assessments and audits reveals recurring weak points in 4G deployments:
- Cleartext Diameter over IPX
- MME with debug ports open
- GTP tunneling bypassing firewall inspection
- VoLTE call injection via SIP spoofing
- Exposed PCRF nodes on public IPs
Penetration tests regularly uncover misconfigured devices, outdated firmware, and insufficient logging—critical gaps attackers can leverage silently.
5G Depends on 4G More Than You Think
Most current 5G networks are Non-Standalone (NSA)—meaning 5G RAN (gNodeB) is paired with a 4G EPC core. In this model:
- IMS still handles VoNR fallback.
- S1 and GTP interfaces remain active.
- Legacy vulnerabilities are just one misconfiguration away.
Even with Standalone 5G, many operators continue to route legacy fallback traffic over LTE, making strong 4G security non-negotiable.
Conclusion: 4G Isn’t Done—and Neither Are Its Threats
4G LTE was a leap forward in mobile technology, but it brought along a massive set of security challenges. Diameter, GTP, IMS, and NAS protocols continue to offer rich attack surfaces—especially when implemented inconsistently across geographies and vendors.
As the telecom industry pivots to 5G, it's easy to forget that 4G still underpins most mobile communications globally. Operators that neglect LTE security do so at their own risk—and at the risk of their subscribers.
The bottom line? Don’t bury 4G just yet. Harden it. Audit it. Or be prepared to debug it when attackers do.
