Interlock Ransomware - Exploited Cisco Firewall Zero-Day

What Happened

The Interlock ransomware gang has made headlines again by exploiting a zero-day vulnerability in Cisco's firewall software. This critical vulnerability, identified as CVE-2026-20131, was disclosed on March 4, 2026, but the gang started using it on January 26, weeks before the public was aware. CJ Moses, CISO of Amazon Integrated Security, detailed this alarming situation in a recent report.

The exploitation of this vulnerability allowed Interlock to compromise organizations without any immediate defenses in place. As Moses pointed out, this situation underscores a significant challenge in cybersecurity: zero-day exploits can give attackers a head start to infiltrate systems before defenders even know to look for threats.

Who's Being Targeted

Interlock has a history of targeting vulnerable sectors, particularly local governments and educational institutions. Their previous attacks have caused significant operational disruptions, as seen in the case of the St. Paul government, which required National Guard assistance to recover from a ransomware incident. The education sector has been particularly hard hit, with multiple K-12 schools listed on Interlock's leak site over the past six months.

The gang's strategy often involves leveraging the fear of regulatory fines to pressure victims into compliance, which adds another layer of complexity to the threat landscape. Organizations in critical infrastructure and healthcare sectors are also on their radar, making the potential impact of their attacks widespread.

Signs of Infection

Identifying signs of infection from Interlock's activities can be challenging. However, researchers discovered a misconfigured infrastructure server that served as a staging area for the gang's operations. This server contained a wealth of custom malware, reconnaissance scripts, and evasion techniques. The presence of legitimate security tools during their attacks, such as ConnectWise ScreenConnect and Volatility, complicates detection efforts.

Organizations should be vigilant for unusual network activity, especially if they operate in sectors frequently targeted by ransomware groups. The ransom note found during the investigation also serves as a reminder of the psychological tactics employed by these attackers.

How to Protect Yourself

To safeguard against such zero-day exploits, organizations must adopt a proactive security posture. Regularly updating and patching systems is crucial, but it’s equally important to monitor for any unusual behavior that could indicate an ongoing attack.

Implementing a robust incident response plan can help organizations react swiftly to potential breaches. Additionally, training employees to recognize phishing attempts and suspicious activity can significantly reduce the chances of falling victim to ransomware attacks. Understanding the tactics used by groups like Interlock will empower organizations to better defend themselves against future threats.