CVE-2026-20050: Cisco Secure Firewall DoS Vulnerability

CVE-2026-20050 Overview

A vulnerability in the Do Not Decrypt exclusion feature of the SSL decryption feature of Cisco Secure Firewall Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device. This vulnerability is due to improper memory management during the inspection of TLS 1.2 encrypted traffic. An attacker could exploit this vulnerability by sending crafted TLS 1.2 encrypted traffic through an affected device. A successful exploit could allow the attacker to cause a reload of an affected device.

Critical Impact

Unauthenticated remote attackers can cause affected Cisco FTD devices to reload by sending specially crafted TLS 1.2 traffic, resulting in network security disruption and potential loss of protection.

Affected Products

• Cisco Secure Firewall Threat Defense (FTD) Software with SSL decryption enabled
• Devices utilizing the Do Not Decrypt exclusion feature
• Systems processing TLS 1.2 encrypted traffic

Discovery Timeline

• 2026-03-04 - CVE CVE-2026-20050 published to NVD
• 2026-03-05 - Last updated in NVD database

Technical Details for CVE-2026-20050

Vulnerability Analysis

This vulnerability affects Cisco Secure Firewall Threat Defense (FTD) Software and is classified under CWE-404 (Improper Resource Shutdown or Release). The flaw specifically resides in the Do Not Decrypt exclusion feature within the SSL decryption functionality. When the affected device inspects TLS 1.2 encrypted traffic, improper memory management occurs during the inspection process. This memory handling issue can be triggered remotely by an unauthenticated attacker without any user interaction required.

The vulnerability has a changed scope, meaning successful exploitation can impact resources beyond the vulnerable component itself. While the attack complexity is high, the potential for complete availability impact makes this a significant concern for network security operations. Notably, this vulnerability only affects traffic encrypted by TLS 1.2—other versions of TLS are not affected by this issue.

Root Cause

The root cause of CVE-2026-20050 is improper memory management during the inspection of TLS 1.2 encrypted traffic within the Do Not Decrypt exclusion feature. This falls under CWE-404 (Improper Resource Shutdown or Release), indicating that the affected software does not properly release allocated memory or resources when processing certain TLS 1.2 traffic patterns. When specific crafted traffic is inspected, the memory management flaw leads to a condition that forces the device to reload.

Attack Vector

The attack vector is network-based, allowing a remote unauthenticated attacker to exploit this vulnerability. The attacker must send specially crafted TLS 1.2 encrypted traffic through an affected Cisco FTD device that has SSL decryption and the Do Not Decrypt exclusion feature enabled. No user interaction is required for successful exploitation. The attack complexity is considered high as specific conditions must be met for the vulnerability to be triggered successfully.

The vulnerability manifests when the FTD device processes malformed TLS 1.2 traffic through its SSL decryption inspection path. For detailed technical information, refer to the Cisco Security Advisory.

Detection Methods for CVE-2026-20050

Indicators of Compromise

• Unexpected device reloads or crashes on Cisco FTD appliances
• Memory exhaustion warnings or critical memory alerts in system logs
• Anomalous TLS 1.2 traffic patterns targeting SSL decryption inspection points
• Repeated failover events in high-availability FTD deployments

Detection Strategies

• Monitor Cisco FTD system logs for unexpected reload events with memory-related crash signatures
• Implement network traffic analysis to detect abnormal TLS 1.2 handshake patterns or malformed packets
• Configure SNMP traps or syslog alerts for device reload events on FTD appliances
• Review SSL decryption statistics for unusual traffic volumes or inspection failures

Monitoring Recommendations

• Enable verbose logging for SSL decryption events on affected Cisco FTD devices
• Establish baseline metrics for device memory utilization and reload frequency
• Deploy network monitoring solutions to track TLS 1.2 traffic anomalies at network perimeters
• Configure alerting for multiple consecutive device reloads within short time periods

How to Mitigate CVE-2026-20050

Immediate Actions Required

• Review the official Cisco Security Advisory for specific mitigation guidance
• Assess whether SSL decryption with Do Not Decrypt exclusion is enabled on FTD devices
• Evaluate the feasibility of temporarily disabling the Do Not Decrypt exclusion feature if not operationally critical
• Consider implementing traffic filtering to limit exposure to untrusted TLS 1.2 sources

Patch Information

Cisco has published security guidance for this vulnerability. Administrators should consult the Cisco Security Advisory for information on affected software versions and available patches. Organizations should prioritize applying vendor-recommended updates to affected Cisco Secure Firewall Threat Defense deployments.

Workarounds

• Review Cisco's official advisory for any documented workarounds specific to your deployment
• Consider configuring access control rules to limit which traffic sources can reach the SSL decryption inspection engine
• Evaluate temporarily restricting TLS 1.2 traffic inspection while awaiting patch deployment
• Implement redundancy and failover configurations to minimize service disruption from potential device reloads

# Example: Review FTD SSL policy configuration
# Consult Cisco documentation for your specific FTD version
show ssl-policy-config
show running-config ssl