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Secure Firewall Threat DefenseApplication · Cisco

CVE-2025-20127

HIGH · 7.7 CVSS v3.1 Published 2025-08-14
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
83/100
Remediation priority · High
Remotely reachable Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

A vulnerability in the TLS 1.3 implementation for cipher TLS_CHACHA20_POLY1305_SHA256 in Cisco Secure Firewall ASA and FTD for Firepower 3100/4200 Series allows an authenticated remote attacker to cause resource exhaustion by sending a large number of TLS 1.3 connections using this specific cipher, resulting in the device being unable to accept new SSL/TLS or VPN connections.

MitigationDisable the TLS_CHACHA20_POLY1305_SHA256 cipher in the ASA/FTD configuration and apply available Cisco patches when released; if already compromised, reload the device to restore functionality.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Secure Firewall Threat DefenseApplication
Affected:= 7.4.0= 7.4.1= 7.4.1.1= 7.4.2= 7.4.2.1= 7.6.0
Adaptive Security Appliance SoftwareOperating system
Affected:= 9.20.1= 9.20.1.5= 9.20.2= 9.20.2.10= 9.20.2.21= 9.20.2.22= 9.20.3= 9.20.3.4= 9.20.3.7= 9.22.1.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Scope
Changed
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify device model and software type
    Determine if the device is a Cisco Firepower 3100/4200 Series running ASA or FTD software. On the device console or via 'show version', note the hardware model and whether it runs ASA or FTD.
    Affected if Device is a Firepower 3100/4200 series running ASA or FTD software from the affected version list.
  2. Check installed ASA software version
    Run 'show version' on the ASA CLI. Look for the software version number in the output.
    Affected if The ASA version matches any of: 9.20.1, 9.20.1.5, 9.20.2, 9.20.2.10, 9.20.2.21, 9.20.2.22, 9.20.3, 9.20.3.4, 9.20.3.7, 9.22.1.1.
  3. Check installed FTD software version
    Run 'show version' or check via FMC/FTD CLI. Look for the FTD version number.
    Affected if The FTD version matches any of: 7.4.0, 7.4.1, 7.4.1.1, 7.4.2, 7.4.2.1, 7.6.0.
  4. Verify TLS 1.3 is enabled
    Run 'show ssl' or 'show run ssl' to view SSL/TLS configuration. Look for 'protocol-version' settings that include TLS 1.3.
    Affected if TLS 1.3 is enabled in the SSL/TLS configuration.
  5. Check if TLS_CHACHA20_POLY1305_SHA256 cipher is configured
    Run 'show ssl cipher' or view the cipher suite configuration. Look specifically for TLS_CHACHA20_POLY1305_SHA256 in the active cipher list.
    Affected if The TLS_CHACHA20_POLY1305_SHA256 cipher is in the active cipher configuration.

You are affected if your device is a Firepower 3100/4200 running ASA or FTD with a matching version AND TLS 1.3 AND the TLS_CHACHA20_POLY1305_SHA256 cipher are both enabled.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Disable the TLS_CHACHA20_POLY1305_SHA256 cipher in the ASA/FTD configuration and apply available Cisco patches when released; if already compromised, reload the device to restore functionality.

Recommended fix Moderate confidence

Fixed FTD release (7.4.x > 7.4.2) and fixed ASA release (9.20.x > 9.20.2.10) - obtain exact version from Cisco

  1. Contact Cisco TAC or visit sec.cloudapps.cisco.com for the most current fixed software releases for FTD version 7.4.x and ASA version 9.20.x
  2. Request the specific patch or fixed release that addresses CVE-2025-20127
  3. Apply the Cisco fix following standard upgrade procedures for FTD or ASA devices
  4. After upgrade, verify the device accepts new TLS 1.3 connections normally
  5. Test that the TLS_CHACHA20_POLY1305_SHA256 cipher is now properly handled without resource exhaustion
Caveat Standard upgrade risks apply; test in lab first; ensure compatibility with existing VPN/TLS configurations

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Secure Firewall Threat Defense Scoped from the published advisory
  • Consultation3.0 h
  • Implementation4.0 h
  • Testing4.0 h
  • Review / QA2.0 h
13.0 hours of engineering $2,280
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2025-20127 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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