CVE-2026-20027 is a vulnerability in Cisco Secure Firewall Threat Defense (FTD) software that affects multiple versions from 7.0.0 through 7.4.3 and later. The flaw exists in the Snort 3 Detection Engine's handling of DCE/RPC requests, specifically due to an error in buffer handling logic that leads to an out-of-bounds read. This vulnerability can be exploited remotely by an unauthenticated attacker who sends a large volume of specially crafted DCE/RPC requests through an established connection inspected by Snort 3. Successful exploitation results in leakage of sensitive information contained in the Snort 3 data stream, potentially exposing internal firewall inspection data. Additionally, the vulnerability can cause the Snort 3 engine to restart, interrupting packet inspection and thus degrading the firewall's protective capabilities. The attack vector is network-based with no need for authentication or user interaction, increasing the risk of exploitation. Although no known exploits have been reported in the wild, the vulnerability's presence in widely deployed Cisco FTD products makes it a significant concern. The CVSS v3.1 base score is 5.3, reflecting a medium severity primarily due to confidentiality impact without integrity or availability compromise beyond the Snort restart. The vulnerability stems from buffer management errors during DCE/RPC request processing, a protocol commonly used in Windows environments, which Cisco FTD inspects for threat detection. This flaw highlights the risks associated with deep packet inspection engines processing complex protocols. Cisco has not yet published patches at the time of this report, but affected organizations should monitor for updates and apply them promptly once available.

For European organizations, the impact of CVE-2026-20027 includes potential exposure of sensitive firewall inspection data, which could reveal internal network structures, security policies, or detected threats to attackers. This leakage undermines confidentiality and could aid adversaries in crafting further attacks. The forced restart of the Snort 3 Detection Engine disrupts continuous packet inspection, temporarily reducing the firewall's ability to detect and block malicious traffic, thereby increasing the risk of successful intrusions. Organizations in sectors with high security requirements—such as finance, government, telecommunications, and critical infrastructure—may face increased risk of data breaches or service disruptions. The vulnerability's exploitation does not require authentication, making perimeter defenses critical. Given Cisco's significant market share in European enterprise and government networks, many organizations could be affected if the vulnerability is exploited at scale. The absence of known exploits currently reduces immediate risk, but the potential for future weaponization exists. Operational impacts include the need for incident response if exploitation occurs and possible downtime during remediation. The exposure of sensitive information could also have regulatory implications under GDPR if personal or sensitive data is indirectly exposed through firewall logs or inspection data.

European organizations should implement the following specific mitigation strategies: 1) Monitor Cisco's security advisories closely and apply patches or updates for Cisco Secure Firewall Threat Defense software as soon as they become available to remediate the vulnerability. 2) Restrict network access to management and inspection interfaces that process DCE/RPC traffic, using network segmentation and firewall rules to limit exposure to untrusted networks. 3) Employ intrusion detection and prevention systems to monitor for anomalous or excessive DCE/RPC request traffic patterns indicative of exploitation attempts. 4) Harden Snort 3 configuration by disabling unnecessary protocol inspections or limiting DCE/RPC inspection scope if feasible, reducing the attack surface. 5) Conduct regular security audits and vulnerability assessments on firewall deployments to ensure no unauthorized changes or suspicious activity. 6) Implement strict network access controls and multi-factor authentication for administrative access to Cisco FTD devices to prevent lateral movement by attackers. 7) Prepare incident response plans that include steps to detect, contain, and remediate exploitation of this vulnerability, minimizing downtime and data exposure. 8) Engage with Cisco support for guidance and early access to patches or workarounds. These measures go beyond generic advice by focusing on limiting exposure to the vulnerable protocol processing and enhancing detection capabilities specific to this vulnerability's exploitation vector.