Introduction
Enterprise VPN and management connectivity can be disrupted without warning, leaving users and administrators unable to establish new secure sessions. CVE-2025-20127 highlights a critical resource exhaustion flaw in Cisco Secure Firewall ASA and FTD software for Firepower 3100 and 4200 Series devices, directly impacting encrypted traffic and device manageability.
About the affected products: Cisco is a global leader in networking and security, with its Firepower and ASA product lines widely deployed in enterprise and critical infrastructure environments. The Firepower 3100 and 4200 Series are high-performance security appliances providing firewall, VPN, and threat defense capabilities to organizations worldwide.
Technical Information
CVE-2025-20127 is caused by improper resource management in the TLS 1.3 implementation for the TLS_CHACHA20_POLY1305_SHA256 cipher suite within Cisco Secure Firewall ASA and FTD software running on Firepower 3100 and 4200 Series devices. When an authenticated remote attacker initiates a large number of TLS 1.3 connections using this cipher, the device fails to properly release resources associated with these connections. This leads to a gradual depletion of system resources. Once exhausted, the device will refuse all new SSL/TLS and VPN connections, affecting both user data and management traffic. The only recovery is a device reload. The vulnerability is classified under CWE-404 (Improper Resource Shutdown or Release).
No public code snippets or proof of concept details are available. The flaw is specific to the handling of the TLS_CHACHA20_POLY1305_SHA256 cipher in TLS 1.3 sessions. Attackers must have valid authentication to exploit this issue, which somewhat limits exposure but does not eliminate it, especially in environments with many authorized users or exposed management interfaces.
Affected Systems and Versions
- Cisco Secure Firewall Adaptive Security Appliance (ASA) Software for Firepower 3100 and 4200 Series devices
- Cisco Secure Firewall Threat Defense (FTD) Software for Firepower 3100 and 4200 Series devices
- Only devices running software that supports TLS 1.3 with the TLS_CHACHA20_POLY1305_SHA256 cipher suite are affected
- The vulnerability affects both data and management traffic
- Exact affected version numbers are not specified in the available advisory
Vendor Security History
Cisco has a history of recurring vulnerabilities in its security appliance product lines, especially related to TLS and resource management. In April 2024, multiple high-severity vulnerabilities (CVE-2024-20353, CVE-2024-20359, CVE-2024-20358) were disclosed in ASA and FTD products, including denial of service and remote code execution issues. Cisco generally publishes advisories and patches promptly, but the frequency of such issues highlights persistent challenges in secure protocol implementation and resource management.
References
- Cisco Security Advisory for CVE-2025-20127
- NVD Entry for CVE-2025-20127
- CWE-404: Improper Resource Shutdown or Release
- CSA Singapore Alert: Cisco ASA and FTD Vulnerabilities
- Stack.watch: Cisco Firepower Threat Defense Vulnerabilities
- CyberMaxx: High Severity Cisco ASA and FTD Firewall Vulnerabilities