CVE-2025-20136
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 · uneditedA vulnerability in the function that performs IPv4 and IPv6 Network Address Translation (NAT) DNS inspection for Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause the device to reload unexpectedly, resulting in a denial of service (DoS) condition. This vulnerability is due to an infinite loop condition that occurs when a Cisco Secure ASA or Cisco Secure FTD device processes DNS packets with DNS inspection enabled and the device is configured for NAT44, NAT64, or NAT46. An attacker could exploit this vulnerability by sending crafted DNS packets that match a static NAT rule with DNS inspection enabled through an affected device. A successful exploit could allow the attacker to create an infinite loop and cause the device to reload, resulting in a DoS condition.
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 confidenceA vulnerability in Cisco Secure Firewall ASA and FTD Software's NAT DNS inspection function allows crafted DNS packets matching static NAT rules to trigger an infinite loop, causing unexpected device reloads and denial of service. The issue occurs specifically when DNS inspection is enabled alongside NAT44, NAT64, or NAT46 configurations.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
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
- None
- User interaction
- None
- Scope
- Changed
- Confidentiality
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/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 checksWork through these to decide whether this CVE applies to you.
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Identify device model and software versionRun 'show version' on the ASA/FTD CLI to confirm the device is a Cisco Secure Firewall ASA or FTD appliance and record the software version numberAffected if Device runs ASA or FTD software - compare your installed version to Cisco's official affected version list for this CVE
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Check if DNS inspection is enabledRun 'show service-policy | include dns' or inspect the running configuration with 'show running-config | include dns' to determine if DNS inspection is configuredAffected if DNS inspection is globally enabled or applied to specific traffic policies
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Verify NAT44, NAT64, or NAT46 configuration existsRun 'show nat' or 'show running-config nat' to list all NAT rules configured on the deviceAffected if Any NAT44, NAT64, or NAT46 rules are present on the device
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Identify static NAT rules in the configurationExamine the NAT configuration output for 'static' NAT rule entries, as the vulnerability specifically relates to static NAT in combination with DNS inspectionAffected if Static NAT rules are configured alongside DNS inspection
The device is affected if it runs a vulnerable ASA/FTD version AND has DNS inspection enabled AND has NAT44, NAT64, or NAT46 static NAT rules configured, creating the conditions for the infinite loop when crafted DNS packets match the NAT rules.
Generated from the published advisory. Verify against your own configuration.
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.
From vendor dataApply Cisco-provided software patches when available; meanwhile, temporarily disable DNS inspection on affected NAT rules or remove the problematic NAT/DNS inspection combination to mitigate exploitation.
- Consultation4.0 h
- Implementation3.0 h
- Testing6.0 h
- Review / QA3.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2025-20136 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2025-20136 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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