In today’s evolving telecom threat landscape, fuzzing stands out as one of the most effective methods for discovering previously unknown vulnerabilities in protocol implementations. It is particularly relevant in mobile network environments where complex, proprietary, and often poorly documented protocols create an ideal breeding ground for hidden bugs and security flaws. In this blog post, we’ll explore practical telecom security use cases involving fuzzing techniques applied to mobile network protocols across 2G, 3G, 4G, and 5G.
What is Fuzzing in Telecom?
Fuzzing (or fuzz testing) is a dynamic application security testing method that involves injecting malformed, unexpected, or random inputs into a target system to observe how it behaves. In telecom environments, fuzzing is primarily used to test the robustness and security of protocol stacks in network components like base stations, core network elements, IMS nodes, and user equipment.
Unlike traditional vulnerability scanning, fuzzing doesn’t rely on known vulnerability signatures—it uncovers 0-days by triggering unexpected behavior such as crashes, memory leaks, or logic flaws. This makes it particularly suitable for telecom protocols like SS7, Diameter, GTP, SIP, SCTP, RANAP, NGAP, and NAS.
Use Case #1: Fuzzing GTP in LTE Networks
GTP (GPRS Tunneling Protocol) is a key protocol for mobility and session management in 4G LTE networks. Misimplementations can allow attackers to:
- Cause denial of service (DoS) to Serving or PDN Gateways.
- Manipulate billing records by interfering with charging-related AVPs.
- Leak subscriber identifiers through malformed Create Session Request messages.
By deploying GTP fuzzers targeting Create Session, Delete Session, and Update Bearer procedures, security teams can test for robustness and detect unknown vulnerabilities before attackers do.
Use Case #2: Fuzzing SS7 for Legacy Core Network Security
SS7 is still used globally in interconnect and roaming scenarios. Its openness and lack of authentication make it a goldmine for attackers—and fuzzing can expose protocol handling flaws in MAP, CAP, and TCAP stacks that would otherwise go unnoticed.
Practical fuzzing campaigns can:
- Reveal logic errors in handling of SendRoutingInfo and ProvideSubscriberInfo requests.
- Trigger unintended behaviors in HLR or MSC nodes.
- Crash under-tested or poorly segmented signaling routes.
This is particularly useful for MNOs migrating to Diameter or securing legacy interconnects.
Use Case #3: Fuzzing SIP and Diameter in IMS/VoLTE
IMS is the foundation for VoLTE, VoWiFi, and 5G voice services. SIP and Diameter are core to registration, session establishment, and QoS management. A malformed SIP INVITE or CER message can:
- Crash an IMS proxy (P-CSCF, S-CSCF).
- Cause inconsistent policy enforcement by the PCRF.
- Disrupt voice services by corrupting dialog state machines.
Fuzzing SIP and Diameter interfaces allows security researchers to simulate malformed signaling sequences and detect edge-case handling issues that traditional QA tests miss.
Use Case #4: Fuzzing NAS and RRC in 5G Standalone
5G introduces entirely new control plane interfaces and states, particularly in standalone (SA) mode. The NAS and RRC layers are responsible for mobility management, session management, and radio configuration.
Fuzzing NAS RegistrationRequest, ServiceRequest, and PDU Session Establishment procedures can:
- Expose misimplementations in AMF nodes.
- Cause undefined state transitions in UE simulators.
- Reveal unexpected behavior in connection setup timing or rejection handling.
This is especially important for operators deploying open or vendor-diverse 5G core and RAN infrastructure.
Why Fuzzing Is Critical for Telecom Security
- Unknown Vulnerabilities: Fuzzing helps discover zero-day flaws that are invisible to static or signature-based scans.
- Protocol Complexity: Telecom protocols are stateful, layered, and often proprietary—fuzzing uncovers edge cases that QA tests don’t.
- High Impact: A single flaw in GTP or NAS could lead to network-wide denial of service, subscriber impersonation, or billing fraud.
- Regulatory Compliance: Fuzzing supports proactive security hardening required by GSMA, ENISA, NIS2, and other standards.
How P1 Security Leverages Fuzzing
At P1 Security, fuzzing is integral to our offensive security assessments and vulnerability research services. Our custom-built fuzzers and telecom-aware fuzzing engines allow us to:
- Simulate real-world signaling attacks at protocol level.
- Identify and responsibly disclose 0-days to vendors.
- Help mobile operators test the resilience of their infrastructure under malformed traffic scenarios.
Whether targeting SS7 interconnects, 4G GTP tunnels, or 5G registration flows, fuzzing is a crucial tool in the mobile security arsenal.
