RNC Security Considerations in Legacy Mobile Networks

While the spotlight has shifted to 5G security concerns—such as network slicing, cloud-native threats, and supply chain risk—there remains a significant blind spot in many telecom cybersecurity strategies: the security of legacy infrastructure.

The Radio Network Controller (RNC), the control engine of 3G UMTS networks, still governs the radio access layer in thousands of active deployments worldwide. But many of these systems were never designed with modern cybersecurity threats in mind.

This blog outlines the key threat vectors, vulnerabilities, and mitigation strategies telecom operators must consider to protect RNCs and the wider legacy mobile infrastructure.

Why RNC Security Still Matters

Even though 3G subscriber numbers are declining in many markets, RNCs continue to:

• Handle fallback voice and data traffic

• Support embedded devices and IoT systems

• Provide national roaming and emergency call coverage

• Connect rural and low-density zones not covered by LTE/5G

• Interface with core network elements still in production use

A breach of the RNC could enable attackers to:

• Disrupt service across large areas

• Intercept or tamper with unencrypted data

• Move laterally into the core network

• Exploit legacy systems as pivot points into modern domains

The attack surface may be narrowing—but it remains real.

Key Vulnerabilities in RNC-Based Architectures

1. Outdated Ciphering and Integrity Protocols

While 3G introduced stronger security protocols than 2G, many RNCs still rely on:

• KASUMI-based encryption (UEA1)

• Weak integrity algorithms (UIA1)

• Static key lengths and known cipher weaknesses

If left unpatched or misconfigured, these systems may be vulnerable to cipher cracking or replay attacks.

2. Plaintext Interfaces and Backhaul

In some deployments, the Iub (Node B), Iu-CS, and Iu-PS (core-facing) interfaces are still transported over ATM or IP without encryption, particularly in private or pseudo-isolated networks. This leaves room for:

• Packet sniffing

• Man-in-the-middle attacks

• Traffic manipulation

3. Insecure Management Interfaces

Many legacy RNCs are configured via:

• Telnet or FTP-based management shells

• Unpatched Java-based web interfaces

• SNMPv1 or v2c with community strings

These expose operators to unauthenticated access, remote code execution, or credential harvesting if not isolated and hardened.

4. Weak Access Controls and Role Separation

Legacy operational models often lack:

• Granular role-based access control (RBAC)

• Multi-factor authentication (MFA)

• Logging and auditing on config changes

Inadequate separation of duties makes internal compromise easier and undermines forensics.

5. Lack of Endpoint Hardening

RNCs often run on embedded or proprietary operating systems, which may:

• Still use default passwords

• Lack disk encryption

• Have unpatched kernel vulnerabilities

Some systems have not received updates in over a decade—making them soft targets for intrusion.

Threat Scenarios: What Could Go Wrong?

• Traffic hijack – Attackers intercept or reroute user data through compromised interfaces

• Service outage – Malicious reconfiguration leads to dropped calls or degraded handovers

• Core compromise – Lateral movement into SGSN, MSC, or OSS domains

• Surveillance or manipulation – Lawful interception systems spoofed or bypassed

• IoT abuse – Legacy M2M connections hijacked for botnets or denial-of-service attacks

These scenarios have moved from theoretical to real-world risks in recent years.

Best Practices for Securing RNC Infrastructure

1. Network Segmentation

RNCs should be isolated in their own VLANs or security zones, with:

• Strict firewall policies

• Traffic filtering by protocol and port

• Zero-trust default-deny posture

Access from outside the operator’s secure management domain should be prohibited.

2. Encryption of Interfaces

Even for legacy systems, backhaul traffic should be encrypted where feasible using:

• IPsec VPNs

• MPLS with MACsec

• Transport-layer tunnelling

This is especially important on Iub and Iu interfaces in shared or carrier-provided transport networks.

3. Management Interface Hardening

• Disable unused services (e.g. Telnet, TFTP)

• Replace with SSHv2 and SFTP

• Introduce jump servers with MFA

• Regularly rotate credentials and apply strong password policies

4. Patch and Lifecycle Management

• Maintain a current software baseline across all RNCs

• Engage vendors for available firmware or security patches

• Conduct security reviews before applying updates

• Establish rollback procedures for operational safety

5. Logging, Monitoring, and Response

• Enable full audit logs for config changes and user actions

• Send logs to a central SIEM for real-time analysis

• Define incident response workflows for suspicious activity

• Monitor for brute force attempts and unauthorised config pushes

6. Physical and Environmental Controls

For operators still running physical RNCs:

• Restrict physical access to racks and rooms

• Audit cable security and link redundancy

• Secure console access and out-of-band management paths

Virtual RNC Security Considerations

Operators migrating to vRNCs must also consider:

• Hypervisor security and OS hardening

• API exposure in orchestration platforms

• Container or VM patching cycles

• Access control to virtual management planes

• Shared resource risks in multitenant cloud environments

Virtualisation solves some legacy security issues—but introduces new ones.

Regulatory and Compliance Pressure

In many jurisdictions, telecom operators are subject to regulations on:

• Data confidentiality (e.g. GDPR, CCPA)

• Lawful interception capability and integrity

• National critical infrastructure protection

• Subscriber identity and call tracing retention

RNC vulnerabilities can put operators out of compliance—potentially triggering legal, financial, or reputational consequences.

Strategic Considerations for Telecom Executives

Telecom boards and CISOs should ask:

• Is our RNC architecture still aligned with modern security practices?

• Are we aware of the patch status of all RNC nodes and interfaces?

• Do we have network-wide visibility into RNC logs and alerts?

• Is the cost of securing the RNC higher than virtualising or retiring it?

• How does our RNC security posture compare to that of our 4G and 5G domains?

These questions drive decisions about risk tolerance, investment allocation, and transformation timelines.

Conclusion: Secure the Legacy Before It Becomes Liability

As mobile networks evolve, the RNC may no longer be top of mind. But ignoring its security posture creates an unmonitored gap in the telecom perimeter.

Operators that take a proactive, layered approach to RNC security—not just patching, but isolation, access control, encryption, and monitoring—can continue to operate these systems with confidence.

Legacy does not mean exempt. And modern networks are only as secure as their oldest component.