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, discovered during the resolution of a Cisco TAC support case, was disclosed on August 14, 2025, and received a CVSS base score of 7.7 (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 has been assigned a CVSS score of 7.7 with the vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H/E:X/RL:X/RC:X ([Cisco Advisory](https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-31004200_tlsdos-2yNSCd54)).

A successful exploitation of this vulnerability results in a denial of service (DoS) condition where the device stops accepting any new SSL/TLS or VPN requests. The device consumes resources associated with incoming TLS 1.3 connections, affecting both data traffic and user-management traffic. Once the device enters the vulnerable state, it requires a reboot to restore connectivity (Cisco Advisory).

Cisco has released software updates that address this vulnerability. As a workaround, administrators can use the 'no ssl cipher tlsv1.3 custom' CLI command to remove the vulnerable cipher. The workaround has been tested and proven successful in test environments, though customers should evaluate its applicability and effectiveness in their specific environments (Cisco Advisory).