The stealthy Python-based backdoor framework deploys a persistent Windows implant likely designed for espionage.

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The malware provides remote access and control of infected devices and maintains post-patching persistence.

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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 device receives a termination signal or enters a reboot, the malware copies itself to a secondary location and rewrites the mount list to restore and relaunch itself after the system comes back online. 

Critically, a standard software reboot does not remove the implant. Only a hard reboot — physically disconnecting the device from its power supply — is sufficient to clear the persistence mechanism from memory, according to both CISA and Cisco.

From there, the malware injects malicious shellcode into LINA, the core networking and firewalling code of Cisco’s Adaptive Security Appliance and Firepower Threat Defense software. Once embedded, the malware intercepts a specific type of network request normally used for VPN authentication. When a request arrives containing a hidden trigger sequence, it executes code supplied by the attackers, giving them a backdoor into the device.

Ties to ongoing campaign

Cisco Talos noted that Firestarter shares significant technical similarities with a previously documented implant called RayInitiator, suggesting the tools share a common origin or development history within UAT-4356’s arsenal.

In the federal agency incident analyzed by CISA, the attackers first deployed a separate implant, called Line Viper, to gain access to device configurations, credentials, and encryption keys. Firestarter was installed shortly after, prior to Cisco’s September 2025 patches being applied to those specific devices. When the agency patched its systems, Firestarter stayed on the devices, and the actors used it to then redeploy Line Viper in March, nearly six months after the initial breach.

Cisco and CISA did not attribute the espionage attacks to a specific nation state, but Censys researchers previously said it found compelling evidence indicating a threat group based in China was behind the ArcaneDoor campaign. Censys noted it found evidence of multiple major Chinese networks and Chinese-developed anti-censorship software during its investigation into the early 2024 attacks.

The persistence vulnerability affects a broad range of Cisco hardware, including the Firepower 1000, 2100, 4100, and 9300 series, as well as the Secure Firewall 1200, 3100, and 4200 series.

Cisco has released updated software to address the persistence mechanism, though the company strongly recommends reimaging affected devices rather than relying solely on software updates where compromise is suspected.

The incident reflects a pattern increasingly seen among state-linked hackers: targeting the network edge devices that organizations rely on to enforce security boundaries. Because these appliances sit at the perimeter of enterprise and government networks, compromising them can expose internal traffic and give attackers a position to intercept credentials and communications.

CISA acknowledged active exploitation of the underlying vulnerabilities was ongoing at the time of publication.

A Cisco spokesperson told CyberScoop that customers needing assistance should contact Cisco Technical Assistance for support. CISA did not respond to a request for comment. 

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Published through five accounts, the extensions appear part of a coordinated campaign based on shared C&C infrastructure.

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The state-sponsored threat actor deployed kernel implants and passive backdoors enabling long-term, high-level espionage.

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Researchers uncovered more worrying details about a long-running cyber espionage campaign suspected to be backed by the Chinese government, exemplifying how such attacks often go undetected until they’ve already caused significant damage.

Google Threat Intelligence Group and Mandiant said the Chinese threat group UNC6201 has been exploiting a zero-day vulnerability in Dell RecoverPoint for Virtual Machines since at least mid-2024. The group overlaps with UNC5221, also known as Silk Typhoon, which has been burrowing into critical infrastructure and government agency networks undetected since at least 2022.

The zero-day exploitation marks an escalation from this particular cluster of actors.  State-sponsored attackers spent years implanting Brickstorm malware into networks before the campaign was finally detected last summer. By September, however, the attackers had replaced Brickstorm with Grimbolt, a more advanced malware that’s harder to detect, Google security researchers said Tuesday.

The zero-day vulnerability — CVE-2026-22769 — hinges on a hardcoded administrator password in Dell RecoverPoint for Virtual Machines that was pulled from Apache Tomcat. It carries a 10/10 CVSS rating. The Chinese threat group has been using the hardcoded password, which triggers the vulnerability and allows unauthenticated remote attackers to gain full system access with root-level persistence for at least 18 months, Google said. 

Dell Technologies disclosed and released a patch for the vulnerability Tuesday. A company spokesperson urged customers to follow guidance in its security advisory.

“We are aware of less than a dozen impacted organizations, but because the full scale of this campaign is unknown we recommend that organizations previously targeted by Brickstorm look out for Grimbolt in their environments,” Austin Larsen, principal analyst at GTIG, told CyberScoop.

When the Cybersecurity and Infrastructure Security Agency unveiled new details about the campaign in December, Google said dozens of U.S. organizations, not including downstream victims, had already been impacted by Brickstorm. 

“The actor is likely still active in unpatched and remediated environments, and because exploitation has been occurring since mid-2024, they have had significant time to establish persistence and carry out long-term espionage,” Larsen added.

The campaign — one of many concurrent efforts by China state-sponsored groups to embed themselves into networks for long-term access, disruptions and potential sabotage — remains a top area of concern for national security.

CISA, the National Security Agency and Canadian Centre for Cyber Security released new analysis on Brickstorm last week to share indicators and compromise that could help potential victims detect malicious activity on their networks.

Yet, the China-linked groups involved in this campaign have already moved on to Grimbolt, in some cases replacing older Brickstorm binaries with the new backdoor that’s more difficult to reverse engineer, according to Google.

Marci McCarthy, director of public affairs at CISA, told CyberScoop the agency will share further information on Wednesday.

Google’s fresh research on the China state-sponsored campaign demonstrates how the threat group’s tenacity, and ability to dwell undetected in networks longer than 400 days, keeps defenders and cyber authorities at a disadvantage.

The threat groups typically target edge applications and devices running on systems without endpoint detection and response, but researchers don’t know how attackers broke into the networks of the most recently discovered victims. 

Researchers only have a narrow view of the threat groups’ activities at large. 

“We suspect a significant portion of UNC5221 and UNC6201’s activity likely remains unknown, and there is a strong probability that they are developing or using undiscovered zero-days and malware,” Larsen said. “The most concerning aspect of this campaign is that additional organizations were likely compromised as part of this campaign and do not know it yet.”

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Lawrence Hoffman // As I previously mentioned I’m on vacation this week and next. As I like to go for long cross-country drives I’ve not had much time to keep […]

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