CVE-2026-20050 is a vulnerability identified in Cisco Secure Firewall Threat Defense (FTD) software, specifically within the Do Not Decrypt exclusion feature of its SSL decryption functionality. The flaw stems from improper memory management during the inspection of TLS 1.2 encrypted traffic, which can lead to improper resource shutdown or release. An unauthenticated, remote attacker can exploit this vulnerability by sending crafted TLS 1.2 encrypted packets through an affected device. Successful exploitation causes the device to reload unexpectedly, resulting in a denial of service (DoS) condition. This vulnerability affects a wide range of Cisco FTD software versions from 7.0.0 through 7.7.10.1 and is limited to TLS 1.2 traffic; other TLS versions are not impacted. The CVSS v3.1 base score is 6.8, reflecting a medium severity with network attack vector, high attack complexity, no privileges required, no user interaction, and impact limited to availability. No known exploits have been reported in the wild to date. The vulnerability highlights the risks associated with deep packet inspection and SSL/TLS decryption features in network security appliances, where improper handling of encrypted traffic can lead to service disruption.

The primary impact of CVE-2026-20050 is a denial of service condition on Cisco Secure Firewall Threat Defense devices, which are widely deployed in enterprise and service provider networks for perimeter security and traffic inspection. A successful attack causes the device to reload, temporarily disrupting firewall and security services, potentially exposing networks to further attacks during downtime. This can affect network availability and business continuity, especially in environments relying heavily on Cisco FTD for secure traffic inspection. Since the vulnerability is exploitable remotely without authentication, attackers can launch DoS attacks from anywhere on the internet or internal networks. The impact is limited to availability; confidentiality and integrity are not directly affected. However, repeated exploitation could degrade trust in network security infrastructure and increase operational costs due to downtime and recovery efforts. Organizations with high reliance on TLS 1.2 traffic inspection are particularly at risk, as the vulnerability does not affect newer TLS versions.

To mitigate CVE-2026-20050, organizations should apply Cisco's security patches as soon as they become available for the affected FTD software versions. Until patches are deployed, administrators can consider disabling the Do Not Decrypt exclusion feature or limiting TLS 1.2 traffic inspection where feasible to reduce exposure. Network segmentation and strict access controls can help limit potential attack sources. Monitoring network traffic for unusual TLS 1.2 patterns or repeated device reloads can provide early warning of exploitation attempts. Additionally, upgrading to newer TLS versions (e.g., TLS 1.3) where possible can reduce the attack surface since the vulnerability does not affect these versions. Regularly reviewing and updating firewall policies to minimize unnecessary TLS decryption can also help. Finally, maintaining robust incident response plans to quickly recover from DoS events will reduce operational impact.