US, UK agencies warn hackers were hiding on Cisco firewalls long after patches were applied
US, UK agencies warn hackers were hiding on Cisco firewalls long after patches were applied
https://cyberscoop.com/cisco-firestarter-malware-cisa-warning/
Publish Date: 2026-04-23 16:28:00
Source Domain: cyberscoop.com
A state-sponsored hacking group has implanted a custom backdoor on Cisco network security devices that can survive firmware updates and standard reboots, U.S. and British cybersecurity authorities disclosed Thursday, marking a significant escalation in a campaign that has targeted government and critical infrastructure networks since at least late 2025.
The Cybersecurity and Infrastructure Security Agency and the United Kingdom’s National Cyber Security Centre jointly published a malware analysis report identifying the backdoor, code-named Firestarter. Cisco’s threat intelligence division, Talos, attributed the malware to a threat actor it tracks as UAT-4356. The company attributed the same group to a 2024 espionage campaign called ArcaneDoor, which focused on compromising network perimeter devices.
CISA confirmed it discovered Firestarter on a U.S. federal civilian agency’s Cisco Firepower device after identifying suspicious connections through continuous network monitoring. The finding prompted an updated emergency directive issued Thursday, requiring all federal civilian agencies to audit their Cisco firewall infrastructure and submit device memory snapshots for analysis by Friday.
A backdoor that outlasts patches
The central concern driving the updated directive is the attack group’s ability to persist on compromised devices, even after enterprises applied security patches Cisco released in September 2025. Those patches addressed two vulnerabilities — CVE-2025-20333, a remote code execution flaw in the VPN web server component, and CVE-2025-20362, an unauthorized access vulnerability — that UAT-4356 exploited to gain initial entry. According to CISA, devices compromised before patching may still harbor the implant.
Firestarter allows attackers to achieve persistence by manipulating the Cisco Service Platform mount list, a configuration file that governs which programs execute during the device’s boot sequence. When the…
