Router Hacks on Legacy Infrastructure: The Hidden Risk in Outdated Network Devices

By Edi Rimkus

In today’s fast-evolving cybersecurity landscape, threats aren’t limited to flashy ransomware or sophisticated supply chain attacks. One persistent risk that often flies under the radar is the exploitation of legacy routers and network infrastructure. Many organizations unknowingly expose themselves to attack because critical devices—often running outdated software or unsupported operating systems—are ripe targets for hackers.

The Problem with Legacy Infrastructure

Outdated Hardware & Software:
Routers and other network components that have reached the end-of-life or end-of-support stage tend to remain in service long after their security patches and updates are discontinued. Attackers know that these devices rarely receive the robust protection afforded to current systems. Their outdated firmware often contains vulnerabilities that are well known in the cybersecurity community, yet remain unpatched on legacy equipment.

Common Attack Vectors:

• Exploitation of Known Vulnerabilities: Cybercriminals routinely scan for devices with known IDs of vulnerabilities. For example, a recent campaign involving a China-linked group—UNC3886—targeted end-of-life Juniper Networks MX Series routers by exploiting vulnerabilities like CVE-2025-21590.

• Use of Legacy Protocols: Many legacy devices still use protocols such as SNMP (Simple Network Management Protocol) or outdated authentication mechanisms. Poorly configured or left with default community strings, these protocols become goldmines for attackers trying to harvest administrative credentials.

• Installation of Backdoors: Once a vulnerability is exploited, attackers can install custom firmware modifications or backdoors (for example, TinyShell-based backdoors) that disable logging or mask the true activity of the device. This persistence mechanism makes detection difficult and allows adversaries to maintain long-term covert access.

Real-World Examples and Tactics

Juniper Networks MX Series Exploit

A tailored attack by UNC3886 focused on legacy Juniper Networks routers, taking advantage of devices that were no longer receiving vendor updates. By exploiting CVE-2025-21590, the attackers deployed TinyShell-based backdoors. These backdoors effectively bypassed standard security controls, allowed unauthorized access, and disabled logging mechanisms—resulting in a situation where organizations were unaware of the ongoing breach.

Cisco Router Attacks

Legacy Cisco routers have also been high on the target list. In past campaigns, groups such as APT28 (Fancy Bear) exploited vulnerabilities—including those related to SNMP misconfigurations—to gain access to routers running outdated Cisco IOS or IOS XE software. Even after vulnerabilities like CVE-2017-6742 were patched, many organizations continued to run software versions that left them exposed to:

• Automated exfiltration: Attackers modified SPAN port configurations to duplicate traffic for adversary use.

• Firmware tampering: In some rare but impactful cases, attackers modified elements of the ROMMON (ROM Monitor) to ensure a persistence mechanism that injected malicious code directly into memory.

• Keylogging & Credential Harvesting: Modified software images sometimes incorporated keylogging features to capture administrator credentials, enabling further lateral movement.

Broader Trends

Beyond targeted groups and specific vulnerabilities, research from Cisco’s security community and advisories (such as those from CISA) stress that legacy routers are a consistent “low-hanging fruit.” Organizations frequently run network infrastructure software that’s years out of date—making these assets far easier to compromise. As attackers increasingly automate their exploitation techniques, they can scan entire networks for vulnerable routers, turning them effectively into permanent footholds within enterprise environments.

Implications for Organizations

The exploitation of legacy infrastructure isn’t merely an issue of outdated hardware—it signals a broader gap in cybersecurity posture:

• Exposure to Advanced Persistent Threats (APTs): Once network devices are compromised, adversaries can use these entry points to move laterally within the network, escalate privileges, and eventually compromise valuable data.

• Increased Attack Surface: Every unpatched legacy device expands the potential attack surface. With each device potentially vulnerable to known exploits, the collective risk multiplies.

• Compliance Risks: Operating unsupported hardware may violate cybersecurity best practices and regulatory requirements, exposing organizations to compliance violations and potential fines.

Recommendations for Mitigation

To protect your network against these persistent threats, consider taking the following measures:

1. Inventory and Update: Conduct a thorough review of all network devices. Identify and prioritize the replacement or upgrade of legacy routers and infrastructure.

2. Patch and Harden: Where upgrades are not immediately feasible, ensure that all available patches are applied. Additionally, harden the configurations by changing default credentials and disabling unnecessary protocols.

3. Segment Your Network: Isolate legacy devices from core network resources using strict segmentation and firewall controls. This limits lateral movement in the event of an exploit.

4. Deploy Advanced Monitoring: Implement robust logging, intrusion detection systems (IDS), and threat detection tools that can alert you to unusual authentication requests or configuration changes.

5. Plan for Migration: Develop a roadmap for migrating away from unsupported equipment. Proactive investment in modern, supported solutions will significantly reduce the risk of exploitation.

Conclusion

Router hacks on legacy infrastructure underscore a critical—and often overlooked—vulnerability in our digital ecosystems. Attacks that exploit known vulnerabilities like CVE-2025-21590 serve as a potent reminder that staying current with technology isn’t just about performance; it’s about safeguarding the integrity of your entire network. By understanding the risks and taking decisive action, organizations can significantly reduce their exposure to these persistent, evolving threats.

Stay vigilant, update regularly, and ensure that every device in your network is fortified against the inevitable challenges of tomorrow’s cyber threats.