---
id: CVE-2020-3565
title: >-
  A 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 …
summary: >-
  A 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 …
severity: medium
cvss: 5.8
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N'
cwe:
  - CWE-284
  - CWE-287
vendor: cisco
product: secure_firewall_threat_defense
affected:
  - secure_firewall_threat_defense < 6.4.0.8
  - 'secure_firewall_threat_defense >= 6.5.0, < 6.5.0.4'
  - 'secure_firewall_threat_defense >= 6.5.0.5, < 6.6.0'
patched:
  - secure_firewall_threat_defense 6.6.0
published: '2020-10-21'
updated: '2026-08-11'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2020-3565'
references:
  - url: >-
      https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-tcp-intercept-bypass-xG9M3PbY
    label: psirt@cisco.com
  - url: >-
      https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-tcp-intercept-bypass-xG9M3PbY
    label: af854a3a-2127-422b-91ae-364da2661108
tags:
  - nvd
epss: 0.00939
epssPercentile: 0.59394
ingestedAt: '2026-08-18T12:28:06.339Z'
---

## Overview

A 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.

## Affected

- `secure_firewall_threat_defense < 6.4.0.8`
- `secure_firewall_threat_defense >= 6.5.0, < 6.5.0.4`
- `secure_firewall_threat_defense >= 6.5.0.5, < 6.6.0`

## Remediation

Upgrade past the affected range:

- `secure_firewall_threat_defense 6.6.0`