Cisco Firewall Vulnerability Exploited as Zero-Day in Interlock Ransomware Attacks

A vulnerability patched earlier this month by Cisco in its firewalls has been exploited as a zero-day since at least late January, according to Amazon’s threat intelligence team. The vulnerability is tracked as CVE-2026-20131 and it affects the Secure Firewall Management Center (FMC) software. The availability of patches was announced on March 4, when Cisco patched dozens of other vulnerabilities in its FMC, ASA, and Secure FTD products. CVE-2026-20131 impacts the web-based management interface of FMC software and it can be exploited by a remote, unauthenticated attacker to execute arbitrary Java code with root privileges.  Cisco noted at the time of disclosure that not exposing the FMC management interface to the internet reduces the vulnerability’s attack surface. [ Read: Only 4 Corporate Giants Still Silent on Oracle EBS Hack ] An investigation by Amazon researchers found evidence that the Interlock cybercrime group, known for several high-profile ransomware attacks, had exploited the vulnerability as a zero-day since at least January 26.  Cisco updated its advisory for CVE-2026-20131 on Wednesday to inform customers about in-the-wild exploitation.  The Amazon researchers came across a misconfigured infrastructure server used by the Interlock group, which enabled them to collect information on the threat actor’s attack chain, custom RATs, reconnaissance scripts, and evasion techniques.  “Interlock has historically targeted specific sectors where operational disruption creates maximum pressure for payment,” Amazon noted. “Education represents the largest share of their activity, followed by engineering, architecture, and construction firms, manufacturing and industrial organizations, healthcare providers, and government and public sector entities.” An analysis of timestamps from threat activity, server artifacts, and embedded metadata indicates that the threat actor most likely operates in the UTC+3 time zone. The observed pattern shows initial daily activity around 08:30, peak operations between 12:00 and 18:00, and a consistent low-activity window from approximately 00:30 to 08:30.  While Amazon has not named the country from where the cybercriminals appear to be operating, this temporal analysis suggests the hackers are likely based in Russia, with secondary possibilities in Belarus or select Middle Eastern countries. Amazon has shared indicators of compromise (IoCs) to help defenders detect and block Interlock ransomware attacks. Related: Cisco Patches High-Severity IOS XR Vulnerabilities Related: Recent Cisco Catalyst SD-WAN Vulnerability Now Widely Exploited Related: Cisco Warns of More Catalyst SD-WAN Flaws Exploited in the Wild WRITTEN BY Eduard Kovacs Eduard Kovacs (@EduardKovacs) is senior managing editor at SecurityWeek. He worked as a high school IT teacher before starting a career in journalism in 2011. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering. 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