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Brickstorm Malware Used in Long-Term Espionage Against U.S. Organizations

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4 unique sources, 6 articles

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The UNC5221 activity cluster, attributed to suspected Chinese hackers, has been using the BRICKSTORM malware in long-term espionage operations against U.S. organizations in the technology, legal, SaaS, and BPO sectors. The malware, a Go-based backdoor, has been active for over a year, with an average dwell time of 393 days. It has been used to steal data from various sectors, including SaaS providers and BPOs. The attackers exploit vulnerabilities in edge devices and use anti-forensics techniques to avoid detection. The malware serves multiple functions, including web server, file manipulation, dropper, SOCKS relay, and shell command execution. It targets appliances without EDR support, such as VMware vCenter/ESXi, and uses legitimate traffic to mask its C2 communications. The attackers aim to exfiltrate emails and maintain stealth through various tactics, including removing the malware post-operation to hinder forensic investigations. The attackers use a malicious Java Servlet Filter (BRICKSTEAL) on vCenter to capture credentials, and clone Windows Server VMs to extract secrets. The stolen credentials are used for lateral movement and persistence, including enabling SSH on ESXi and modifying startup scripts. The malware exfiltrates emails via Microsoft Entra ID Enterprise Apps, utilizing its SOCKS proxy to tunnel into internal systems and code repositories. UNC5221 focuses on developers, administrators, and individuals tied to China's economic and security interests. Mandiant has released a free scanner script to help defenders detect BRICKSTORM. The BRICKSTORM backdoor is under active development, with a variant featuring a delay timer for C2 communication. The attackers have exploited Ivanti Connect Secure zero-day vulnerabilities (CVE-2023-46805 and CVE-2024-21887) for initial access. The attackers have used a custom dropper to install a malicious Java Servlet filter (BRICKSTEAL) in memory, avoiding detection. The attackers have modified init.d, rc.local, or systemd files to ensure persistence on appliances. The attackers have targeted Windows environments in Europe since at least November 2022. The attackers have been linked to other related Chinese threat actors besides UNC5221. The campaign has been monitored by Mandiant since March 2025. The attackers have targeted downstream customers of compromised SaaS providers. The attackers are believed to be analyzing stolen source code to identify zero-day vulnerabilities in enterprise technologies. The attackers use a delay timer to lie dormant on infected systems until a hard-coded date. The malware employs Garble, an open-source tool, for code obfuscation to hide function names, structures, and logic. Brickstorm has been found on VMware vCenter and ESXi hosts, often deployed prior to pivoting to these systems. The attackers use legitimate cloud services like Cloudflare Workers or Heroku for C2 communications. The attackers use dynamic domains like sslip.io or nip.io that point directly to the C2 server’s IP. The attackers favor appliance and management-plane compromise, per-victim obfuscated Go binaries, delayed-start implants, and Web/DoH C2 to preserve stealth. The attackers harvest and use valid high-privilege credentials to appear as routine administrator tasks. The attackers deploy in-memory servlet filters, remove installer artifacts, and embed delayed-start logic to limit forensic traces. The attackers abuse virtualization management capabilities, such as cloning VMs to extract credential stores offline. The attackers deploy an in-memory Java Servlet filter on vCenter to intercept and decode web authentication to harvest high-privilege credentials. The attackers use a SOCKS proxy on compromised appliances to tunnel into internal networks for interactive access and file retrieval. F5 disclosed that unidentified threat actors stole files containing BIG-IP's source code and information related to undisclosed vulnerabilities. The attackers used the BRICKSTORM malware, attributed to a China-nexus espionage group dubbed UNC5221. The attackers were in F5's network for at least 12 months before detection. GreyNoise observed elevated scanning activity targeting BIG-IP in three waves on September 23, October 14, and October 15, 2025. Censys identified over 680,000 F5 BIG-IP load balancers and application gateways visible on the public internet. The attackers used a delay timer to lie dormant on infected systems until a hard-coded date. The malware employs Garble, an open-source tool, for code obfuscation to hide function names, structures, and logic. Brickstorm has been found on VMware vCenter and ESXi hosts, often deployed prior to pivoting to these systems. The attackers use legitimate cloud services like Cloudflare Workers or Heroku for C2 communications. The attackers use dynamic domains like sslip.io or nip.io that point directly to the C2 server’s IP. The attackers favor appliance and management-plane compromise, per-victim obfuscated Go binaries, delayed-start implants, and Web/DoH C2 to preserve stealth. The attackers harvest and use valid high-privilege credentials to appear as routine administrator tasks. The attackers deploy in-memory servlet filters, remove installer artifacts, and embed delayed-start logic to limit forensic traces. The attackers abuse virtualization management capabilities, such as cloning VMs to extract credential stores offline. The attackers deploy an in-memory Java Servlet filter on vCenter to intercept and decode web authentication to harvest high-privilege credentials. The attackers use a SOCKS proxy on compromised appliances to tunnel into internal networks for interactive access and file retrieval.

Timeline

  1. 24.09.2025 17:00 6 articles · 26d ago

    Brickstorm Malware Used in Long-Term Espionage Against U.S. Organizations

    F5 disclosed that unidentified threat actors, attributed to the UNC5221 activity cluster, stole files containing BIG-IP's source code and information related to undisclosed vulnerabilities. The attackers were in F5's network for at least 12 months before detection. GreyNoise observed elevated scanning activity targeting BIG-IP in three waves on September 23, October 14, and October 15, 2025. Censys identified over 680,000 F5 BIG-IP load balancers and application gateways visible on the public internet. The attackers used a delay timer to lie dormant on infected systems until a hard-coded date. The malware employs Garble, an open-source tool, for code obfuscation to hide function names, structures, and logic. Brickstorm has been found on VMware vCenter and ESXi hosts, often deployed prior to pivoting to these systems. The attackers use legitimate cloud services like Cloudflare Workers or Heroku for C2 communications. The attackers use dynamic domains like sslip.io or nip.io that point directly to the C2 server’s IP. The attackers favor appliance and management-plane compromise, per-victim obfuscated Go binaries, delayed-start implants, and Web/DoH C2 to preserve stealth. The attackers harvest and use valid high-privilege credentials to appear as routine administrator tasks. The attackers deploy in-memory servlet filters, remove installer artifacts, and embed delayed-start logic to limit forensic traces. The attackers abuse virtualization management capabilities, such as cloning VMs to extract credential stores offline. The attackers deploy an in-memory Java Servlet filter on vCenter to intercept and decode web authentication to harvest high-privilege credentials. The attackers use a SOCKS proxy on compromised appliances to tunnel into internal networks for interactive access and file retrieval.

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F5 BIG-IP Source Code and Vulnerability Information Stolen in Cyberattack

Over 266,000 F5 BIG-IP instances are exposed online, potentially vulnerable to remote attacks following a breach disclosed by F5. The company has released security updates to address 44 vulnerabilities, including those stolen in the breach. F5 has not found evidence that the stolen information has been used in actual attacks or disclosed publicly. The breach was attributed to a highly sophisticated nation-state threat actor, and F5 has taken extensive actions to contain the threat. F5's BIG-IP is a critical product used in application delivery networking and traffic management by many large enterprises. The company has 23,000 customers in 170 countries, including 48 of the Fortune 50 entities. The breach did not compromise F5's software supply chain or result in suspicious code modifications. The company has validated the safety of BIG-IP releases through multiple independent reviews by leading cybersecurity firms and has advised users to apply the latest updates for BIG-IP and related products. The breach involved a nation-state threat actor gaining persistent, long-term access to F5's product development environment and engineering knowledge management platforms. F5 disclosed the breach on October 15, 2025, confirming that the attack was detected in August 2025. The threat actor exfiltrated files containing BIG-IP source code and information regarding undisclosed vulnerabilities. F5 has not found evidence of access to or exfiltration of data tied to its CRM, financial, support case management, or iHealth systems, nor the NGINX source code or product development environment. F5 has identified no evidence of modification to its software supply chain, including source code, build pipeline, and release pipeline. F5 has worked with multiple incident response firms and law enforcement to mitigate the event and believes it has contained the threat. F5 has rotated credentials, strengthened access controls, deployed improved inventory and patch management automation, integrated better monitoring and detection tools, and implemented enhancements to network security infrastructure. F5 advises customers to apply the latest BIG-IP updates and has shared guidance for hardening customers' systems. On October 15, 2025, CISA directed federal civilian executive branch (FCEB) agencies to inventory F5 BIG-IP products and apply updates where necessary. The US government has urged federal agencies to take immediate action after F5 revealed it had been breached by a nation-state actor. The US Cybersecurity and Infrastructure Security Agency (CISA) issued an emergency directive demanding that federal agencies evaluate if the networked management interfaces are accessible from the public internet and apply updates from F5. CISA warned that the threat actor's access to the F5 development environment could enable it to conduct static and dynamic analysis to discover logical flaws, zero-day vulnerabilities, and targeted exploits. The Justice Department ordered a delay in public disclosure of the breach on September 12, 2025. F5 has improved internal security including access controls, inventory and patch management, network security, and monitoring of all software development platforms. Tom Kelermann, VP of cyber risk at Hitrust, argued that the F5 breach is likely to be the first stage in a supply chain campaign. Ilia Kolochenko, CEO of ImmuniWeb, agreed that the stolen IP could be used to craft zero-day exploits for subsequent APT campaigns.

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