CVE-2026-20006 in Secure Firewall Threat Defense Software
Summary
by MITRE • 03/04/2026
A vulnerability in the TLS cryptography functionality of the Snort 3 Detection Engine of Cisco Secure Firewall Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause the Snort 3 Detection Engine to unexpectedly restart, resulting in a denial of service (DoS) condition.
This vulnerability is due to improper implementation of the TLS protocol. An attacker could exploit this vulnerability by sending a crafted TLS packet to an affected system. A successful exploit could allow the attacker to cause a device that is running Cisco Secure FTD Software to drop network traffic, resulting in a DoS condition. Note: TLS 1.3 is not affected by this vulnerability.
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Analysis
by VulDB Data Team • 03/05/2026
The vulnerability identified as CVE-2026-20006 represents a critical weakness in the cryptographic processing mechanisms of Cisco Secure Firewall Threat Defense software, specifically within the Snort 3 Detection Engine component. This flaw manifests as an improper implementation of Transport Layer Security protocols, creating a pathway for remote attackers to disrupt network operations without requiring authentication credentials. The vulnerability exists within the fundamental security infrastructure that protects enterprise networks, making it particularly dangerous as it can be exploited from external networks without prior access to the system. The affected software components operate as part of Cisco's comprehensive firewall solution designed to detect and prevent network-based attacks, yet this particular weakness undermines the very foundation of network security it is meant to provide.
The technical exploitation of this vulnerability occurs through the careful crafting of TLS packets that trigger a specific failure condition within the Snort 3 Detection Engine's cryptographic processing routines. When an attacker sends maliciously constructed TLS communication packets to a vulnerable Cisco FTD device, the system's response mechanism fails to properly handle the malformed data, leading to an unexpected restart of the detection engine service. This process demonstrates a classic buffer overflow or state management error within the TLS implementation, where the system cannot gracefully handle edge cases in protocol parsing. The vulnerability specifically affects TLS versions prior to 1.3, indicating that the issue stems from legacy cryptographic handling mechanisms that do not properly validate or sanitize incoming TLS handshake sequences. This flaw aligns with CWE-248, which categorizes improper exception handling in protocols, and represents a failure in the secure implementation of cryptographic standards.
The operational impact of this vulnerability extends beyond simple service interruption, creating cascading effects that can severely compromise network availability and security posture. When the Snort 3 Detection Engine restarts unexpectedly, the device enters a state where it cannot process incoming network traffic, effectively creating a denial of service condition that can persist until manual intervention occurs or the system automatically recovers. During this disruption period, the network security monitoring capabilities are completely disabled, leaving the organization vulnerable to attacks that would normally be detected and blocked by the firewall's intrusion detection system. This represents a significant risk to enterprise security infrastructure, as organizations may not immediately recognize the compromise due to the lack of network visibility during the DoS event, potentially allowing malicious actors to conduct attacks without detection. The vulnerability's impact is particularly concerning given that it affects network security appliances that are typically considered critical infrastructure components.
Organizations should implement immediate mitigations including applying the latest security patches released by Cisco, which address the specific TLS implementation flaws in the Snort 3 Detection Engine. Network administrators should also consider implementing additional monitoring and alerting mechanisms to detect unusual restart patterns or service interruptions that might indicate exploitation attempts. The mitigation strategy should include network segmentation to limit potential attack vectors and ensure that only authorized systems can communicate with the vulnerable firewalls. Furthermore, organizations should review their incident response procedures to ensure rapid detection and recovery from such DoS conditions, as the automatic restart behavior may not immediately alert administrators to the compromise. Security teams should also consider temporarily disabling the Snort 3 Detection Engine functionality if immediate patching is not possible, while maintaining other firewall protections. This vulnerability demonstrates the importance of maintaining up-to-date cryptographic implementations and highlights the need for comprehensive security testing of protocol handling components within network infrastructure devices. The ATT&CK framework categorizes this as a privilege escalation and denial of service technique, where adversaries leverage protocol implementation weaknesses to disrupt service availability while potentially remaining undetected.