Medium severity · published Oct 21, 2020
CVE-2020-3565
A vulnerability in the TCP Intercept functionality of Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to bypass configured Access…
5.8
CVSS
0.9%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
Low
Availability
None
Scope
Changed
Exploitation
Is it being used?
Exploit prediction (EPSS)0.9% · higher than 61% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
SF
Secure Firewall Threat Defense · Cisco
before 6.4.0.8; 6.5.0 before 6.5.0.4; 6.5.0.5 before 6.6.0 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2020-3565
CVE.org
https://www.cve.org/CVERecord?id=CVE-2020-3565
Vendor Advisory
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-tcp-intercept-bypass-xG9M3PbY
More from Secure Firewall Threat Defense
9.9
CVE-2025-20333
Cisco Secure Firewall Adaptive Security Appliance (ASA) and Secure Firewall Threat Defense (FTD) Buffer Overflow Vulnerability
9.1
CVE-2023-20269
Cisco Adaptive Security Appliance and Firepower Threat Defense Unauthorized Access Vulnerability
8.6
CVE-2026-20349
Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense Software Remote Access SSL VPN Denial of Service Vulnerability
8.6
CVE-2025-20362
Cisco Secure Firewall Adaptive Security (ASA) Appliance and Secure Firewall Threat Defense (FTD) Missing Authorization Vulnerability
8.6
CVE-2024-20353
Cisco ASA and FTD Denial of Service Vulnerability
