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Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense Software for Firepower 3100 and 4200 Series TLS 1.3 Cipher Denial of Service Vulnerability

cisco-sa-3100_4200_tlsdos-2yNSCd54 · High · Published · Updated

A vulnerability in the TLS 1.3 implementation for a specific cipher 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 could allow an authenticated, remote attacker to consume resources that are associated with incoming TLS 1.3 connections, which eventually could cause the device to stop accepting any new SSL/TLS or VPN requests. This vulnerability is due to the implementation of the TLS 1.3 Cipher TLS_CHACHA20_POLY1305_SHA256. An attacker could exploit this vulnerability by sending a large number of TLS 1.3 connections with the specific TLS 1.3 Cipher TLS_CHACHA20_POLY1305_SHA256. A successful exploit could allow the attacker to cause a denial of service (DoS) condition where no new incoming encrypted connections are accepted. The device must be reloaded to clear this condition. Note: These incoming TLS 1.3 connections include both data traffic and user-management traffic. After the device is in the vulnerable state, no new encrypted connections can be accepted. Cisco has released software updates that address this vulnerability. There are workarounds that address this vulnerability. This advisory is available at the following link:https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-3100_4200_tlsdos-2yNSCd54 This advisory is part of the August 2025 release of the Cisco Secure Firewall ASA, Secure FMC, and Secure FTD Software Security Advisory Bundled Publication. For a complete list of the advisories and links to them, see Cisco Event Response: August 2025 Semiannual Cisco Secure Firewall ASA, Secure FMC, and Secure FTD Software Security Advisory Bundled Publication.

Cisco advisory ·CSAF JSON

Workarounds

There is a workaround that addresses this vulnerability. Use the no ssl cipher tlsv1.3 custom CLI command to remove the cipher.

While this workaround has been deployed and was proven successful in a test environment, customers should determine the applicability and effectiveness in their own environment and under their own use conditions. Customers should be aware that any workaround or mitigation that is implemented may negatively impact the functionality or performance of their network based on intrinsic customer deployment scenarios and limitations. Customers should not deploy any workarounds or mitigations before first evaluating the applicability to their own environment and any impact to such environment.

CVEsCVE-2025-20127
Cisco Bug IDsCSCwm91176
CVSS ScoreBase 7.7
Base 7.7 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
Product Names From Source
Cisco Adaptive Security Appliance (ASA) Software 9.20.1, Cisco Adaptive Security Appliance (ASA) Software 9.20.1.5, Cisco Adaptive Security Appliance (ASA) Software 9.20.2, Cisco Adaptive Security Appliance (ASA) Software 9.20.2.10, Cisco Adaptive Security Appliance (ASA) Software 9.20.2.21, Cisco Adaptive Security Appliance (ASA) Software 9.20.2.22, Cisco Adaptive Security Appliance (ASA) Software 9.20.3, Cisco Adaptive Security Appliance (ASA) Software 9.20.3.4, Cisco Adaptive Security Appliance (ASA) Software 9.20.3.7, Cisco Adaptive Security Appliance (ASA) Software 9.22.1.1, Cisco Firepower Threat Defense Software 7.4.0, Cisco Firepower Threat Defense Software 7.4.1, Cisco Firepower Threat Defense Software 7.4.1.1, Cisco Firepower Threat Defense Software 7.4.2, Cisco Firepower Threat Defense Software 7.4.2.1, Cisco Firepower Threat Defense Software 7.6.0, Cisco Secure Firewall Adaptive Security Appliance (ASA) Software, Cisco Secure Firewall Threat Defense (FTD) Software, Cisco Secure Firewall 3100 Series, Cisco Secure Firewall 4200 Series

Related Products

Product CVE Evidence
Cisco Secure Firewall Threat Defense (FTD) Software CVE-2025-20127 Cisco OpenVuln
Cisco Secure Firewall Adaptive Security Appliance (ASA) Software CVE-2025-20127 Cisco OpenVuln
Cisco Secure Firewall 4200 Series CVE-2025-20127 Cisco OpenVuln
Cisco Secure Firewall 3100 Series CVE-2025-20127 Cisco OpenVuln
Cisco Firepower Threat Defense Software CVE-2025-20127 Cisco OpenVuln
Cisco Adaptive Security Appliance (ASA) Software CVE-2025-20127 Cisco OpenVuln