Secure Firewall Threat DefenseApplication · Cisco
CVE-2020-3565
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 TCP Intercept functionality of Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to bypass configured Access Control Policies (including Geolocation) and Service Polices on an affected system. The vulnerability exists because TCP Intercept is invoked when the embryonic connection limit is reached, which can cause the underlying detection engine to process the packet incorrectly. An attacker could exploit this vulnerability by sending a crafted stream of traffic that matches a policy on which TCP Intercept is configured. A successful exploit could allow the attacker to match on an incorrect policy, which could allow the traffic to be forwarded when it should be dropped. In addition, the traffic could incorrectly be dropped.
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 · moderate confidenceA vulnerability in Cisco FTD's TCP Intercept feature allows attackers to bypass Access Control Policies when the embryonic connection limit triggers TCP Intercept processing. The detection engine incorrectly processes packets in this scenario, causing policy misapplication where traffic may be forwarded that should be dropped, or dropped that should be allowed.
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
< 6.4.0.8>= 6.5.0, < 6.5.0.4>= 6.5.0.5, < 6.6.0CVSS 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
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N
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 FTD versionRun the command `show version` on the Cisco FTD device to retrieve the installed software versionAffected if The displayed version falls into any of these ranges: < 6.4.0.8, >= 6.5.0 but < 6.5.0.4, >= 6.5.0.5 but < 6.6.0
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Verify TCP Intercept feature is enabledRun `show service-policy` or `show running-config | include tcp-intercept` to check if TCP Intercept is configured on any service policiesAffected if TCP Intercept is enabled on any service policy on the device
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Confirm Access Control Policies existRun `show access-list` or access-control policy commands to list configured Access Control PoliciesAffected if Any Access Control Policies are defined on the device
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Check for embryonic connection limit configurationExamine the TCP Intercept configuration for any embryonic connection limits using `show service-policy interface <interface> detail` or equivalent command to view the threshold settingsAffected if TCP Intercept is configured with embryonic connection limits that could trigger the intercept processing
You are affected if your FTD version is in the vulnerable ranges AND TCP Intercept with embryonic connection limits is enabled on any Access Control Policy, because this combination triggers the detection engine bug described in the CVE.
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 data6.4.0.86.5.0.46.6.0
Apply Cisco-provided patches for CVE-2020-3565 when available; until then, review and document all policies with TCP Intercept enabled and consider additional monitoring or compensating controls for affected rule matches.
Fix this in Secure Firewall Threat Defense
Scoped from the published advisory
- Consultation4.0 h
- Implementation6.0 h
- Testing8.0 h
- Review / QA4.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2020-3565 — 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-2020-3565 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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