The threat involves the use of finger.exe, a legacy Windows executable implementing the finger protocol originally from UNIX systems, which communicates over TCP port 79. In the ClickFix attacks, adversaries exploit finger.exe to retrieve malicious scripts via the finger protocol. The finger protocol is fixed to TCP port 79 and cannot be changed, and finger.exe is not proxy-aware, meaning it does not route traffic through configured proxies. This characteristic allows network defenders to block or allow finger.exe communications based on proxy configurations and firewall rules. Specifically, in corporate environments with explicit proxies that block direct Internet connections except through the proxy, finger.exe cannot communicate externally. However, in networks with transparent proxies that allow TCP connections to port 79, finger.exe can successfully retrieve malicious payloads. The attack leverages a rarely used protocol and a legacy executable that is still present on Windows systems, making it a living-off-the-land binary (LOLBin) technique. This approach can evade detection by traditional security tools focused on more common protocols and executables. Although no known exploits are currently active in the wild, the medium severity rating reflects the potential for script retrieval and execution leading to further compromise. The threat highlights the importance of monitoring and controlling legacy protocols and binaries that may be exploited for malicious purposes.

For European organizations, the impact of this threat primarily lies in the potential for attackers to stealthily retrieve and execute malicious scripts using a legacy protocol and executable that may be overlooked by standard security controls. Successful exploitation could lead to initial footholds in networks, enabling further lateral movement or payload deployment. Organizations with legacy Windows systems and network environments that permit TCP port 79 traffic are particularly vulnerable. The use of finger.exe bypasses proxy-based filtering if transparent proxies allow port 79, potentially circumventing perimeter defenses. This could result in unauthorized data access, system compromise, or persistence mechanisms being established. However, the lack of known active exploits and the medium severity rating suggest that the threat is currently limited in scope and impact. Nonetheless, the presence of this attack vector increases the attack surface and requires attention to prevent exploitation. European organizations with critical infrastructure or sensitive data are at risk of targeted attacks leveraging this technique, especially if network segmentation and outbound traffic controls are insufficient.

1. Block outbound TCP traffic on port 79 at network firewalls and perimeter devices to prevent finger protocol communications. 2. Disable or restrict the use of finger.exe on Windows systems through application whitelisting or group policy to prevent its misuse. 3. Implement explicit proxy configurations that block direct Internet access except through the proxy, ensuring finger.exe cannot bypass filtering. 4. Monitor network traffic for any use of TCP port 79 and finger protocol communications, using intrusion detection systems or network monitoring tools to detect anomalous activity. 5. Educate security teams about the risks associated with legacy protocols and LOLBins like finger.exe to improve detection and response capabilities. 6. Conduct regular audits of legacy tools and protocols present in the environment and remove or restrict those not required. 7. Apply network segmentation to limit the ability of compromised hosts to communicate externally using uncommon protocols. 8. Incorporate detection rules in endpoint detection and response (EDR) solutions to flag execution of finger.exe retrieving external scripts. These targeted measures go beyond generic advice by focusing on controlling the specific protocol and executable involved in the ClickFix attack.