US Federal Agency’s Cisco Firewall Infected With ‘Firestarter’ Backdoor

Summary

A China-linked state-sponsored threat actor (UAT-4356) exploited two zero-day vulnerabilities in Cisco ASA and Secure Firewall devices to deploy the Firestarter backdoor, which provides persistent remote access and resists firmware patching. At least one US federal agency was compromised; CISA issued updated Emergency Directive 25-03 requiring agencies to verify infections via core dumps and hard-reset affected devices by April 30, 2026.

Full text

At least one US federal agency was infected with a backdoor as part of a widespread China-linked espionage campaign targeting Cisco firewalls. In May 2024, Cisco patched two vulnerabilities in its Adaptive Security Appliance (ASA) firewall platform that had been exploited as zero-days in a state-sponsored campaign tracked as ArcaneDoor. A year later, the company fixed two more zero-days linked to the same campaign, tracked as CVE-2025-20333 and CVE-2025-20362, and impacting the VPN web server of ASA and Secure Firewall Threat Defense (FTD) software. In September 2025, the US cybersecurity agency CISA issued Emergency Directive 25-03 (ED 25-03), urging federal agencies to patch vulnerable Cisco devices in their environments immediately. In November, CISA updated its guidance to recommend additional mitigation actions. On Thursday, the agency updated ED 25-03 again, warning that patching vulnerable Cisco firewall devices did not remove malware deployed on them. Per the updated directive, federal agencies should upload device core dumps to the Malware Next Gen portal to verify whether they have been compromised, and notify CISA immediately if they have been, or apply the available patches if needed.Advertisement. Scroll to continue reading. The requirement applies to Firepower 1000, 2100, 4100, 9300 series and Secure Firewall 200, 1200, 3100, 4200, and 6100 series devices. All checks and updates should be performed by 11:59 PM EST on April 24, 2026, and devices should be hard-reset by April 30, CISA’s directive mandates. CISA’s updated directive is accompanied by instructions on the core dumps and by a deep dive into the Firestarter backdoor, which was identified as the malware used in these attacks. According to CISA, at least one federal agency was infected with Firestarter through the exploitation of a Firepower device vulnerable to CVE-2025-20333 and CVE-2025-20362. The backdoor is not removed by firmware updates, and devices compromised before patching remain vulnerable, it warns. Firestarter was deployed before September 25, persisted through remediation, and provided the attackers with remote access and control of the vulnerable firewall. “Firestarter attempts to install a hook—a way to intercept and modify normal operations—within Lina, the device’s core engine for network processing and security functions. This hook enables the execution of arbitrary shell code provided by the APT actors, including the deployment of Line Viper,” CISA explains. The backdoor resembles the RayInitiator bootkit, a previously detailed component of the ArcaneDoor campaign, and achieves persistence by modifying the mount list for Cisco Service Platform (CSP), which allows programs to execute during boot, Cisco explains. After a reboot, Firestarter restores the original list and removes the trojanized copy, meaning that the implant can be removed through a hard reboot, which involves unplugging the device from power, the company says. Cisco has attributed the attacks to UAT-4356, a state-sponsored threat actor focused on espionage, and has published a fresh advisory on CVE-2025-20333 and CVE-2025-20362’s continuous exploitation. Related: Organizations Warned of Exploited Cisco, Kentico, Zimbra Vulnerabilities Related: Cisco Patches Critical Vulnerabilities in Webex, ISE Related: Most Serious Cyberattacks Against the UK Now From Russia, Iran and China, Cyber Chief Says Related: FBI Warns of Data Security Risks From China-Made Mobile Apps Written By Ionut Arghire Ionut Arghire is an international correspondent for SecurityWeek. 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Indicators of Compromise

• cve — CVE-2025-20333
• cve — CVE-2025-20362
• malware — Firestarter
• malware — Line Viper
• malware — RayInitiator

Entities