CVE-2020-3565: Cisco Firepower Threat Defense Software TCP Intercept Bypass Vulnerability

Severity: medium (CVSS 5.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.9% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2020-10-21 · updated 2026-08-11
Weakness: CWE-284

Description

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 products

Cisco Firepower Threat Defense Software: before 6.4.0.8; from 6.5.0 before 6.5.0.4; from 6.5.0.5 before 6.6.0 — fixed in 6.4.0.8, 6.5.0.4, 6.6.0

How to fix

Upgrade to a fixed version: Cisco Firepower Threat Defense Software 6.4.0.8, 6.5.0.4, 6.6.0. Check the vendor advisory for mitigations.

References

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