CVE-2025-20127: 
Cisco Adaptive Security Appliance (ASA) vulnerability analysis and mitigation

A vulnerability (CVE-2025-20127) has been identified in the TLS 1.3 implementation for Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software for Cisco Firepower 3100 and 4200 Series devices. The vulnerability was discovered during the resolution of a Cisco TAC support case and was disclosed on August 14, 2025. This vulnerability has been assigned a CVSS base score of 7.7, indicating a high severity rating (Cisco Advisory).

The vulnerability specifically affects the implementation of the TLS 1.3 Cipher TLSCHACHA20POLY1305SHA256. The issue manifests when the device is configured to allow this specific TLS 1.3 cipher, which is not the default configuration. The vulnerability is tracked as CWE-404 and can be identified using the show asp table socket | include SSL command to check for SSL listen sockets and show running-config all ssl | include TLSCHACHA20POLY1305SHA256 to verify the vulnerable cipher configuration (Cisco Advisory).

A successful exploitation of this vulnerability could allow an authenticated, remote attacker to consume resources associated with incoming TLS 1.3 connections, eventually causing the device to stop accepting any new SSL/TLS or VPN requests. When the device enters this failed state, no new encrypted connections can be accepted, and a device reboot is required to restore connectivity (Cisco Advisory).

Cisco has released software updates that address this vulnerability. A workaround is available by using the 'no ssl cipher tlsv1.3 custom' CLI command to remove the vulnerable cipher. This workaround has been tested and proven successful in test environments, though customers should evaluate its applicability and potential impact in their specific environments before implementation (Cisco Advisory).