CVE-2021-34793: Cisco Adaptive Security Appliance Software and Firepower Threat Defense Software Transparent Mode Denial of Service Vulnerability

Severity: high (CVSS 8.6 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-10-27 · updated 2026-08-11
Weakness: CWE-924

Description

A vulnerability in the TCP Normalizer of Cisco Adaptive Security Appliance (ASA) Software and Firepower Threat Defense (FTD) Software operating in transparent mode could allow an unauthenticated, remote attacker to poison MAC address tables, resulting in a denial of service (DoS) vulnerability. This vulnerability is due to incorrect handling of certain TCP segments when the affected device is operating in transparent mode. An attacker could exploit this vulnerability by sending a crafted TCP segment through an affected device. A successful exploit could allow the attacker to poison the MAC address tables in adjacent devices, resulting in network disruption.

Affected products

Cisco Adaptive Security Appliance: before 9.8.4.40 — fixed in 9.8.4.40
Cisco ASA Software: from 9.9.0 before 9.12.4.29; from 9.13.0 before 9.14.3.9; from 9.15.0 before 9.15.1.17; from 9.16.0 before 9.16.2.3 — fixed in 9.12.4.29, 9.14.3.9, 9.15.1.17, 9.16.2.3
Cisco Asa 5505 Firmware: 009.008(004.025)
Cisco Asa 5512-X Firmware: 009.008(004.025)
Cisco Asa 5515-X Firmware: 009.008(004.025)
Cisco Asa 5525-X Firmware: 009.008(004.025)
Cisco Asa 5545-X Firmware: 009.008(004.025)
Cisco Asa 5555-X Firmware: 009.008(004.025)
Cisco Asa 5580 Firmware: 009.008(004.025)
Cisco Asa 5585-X Firmware: 009.008(004.025)
Cisco Firepower Threat Defense Software: before 6.4.0.13; from 6.5.0 before 6.6.5; from 6.7.0 before 6.7.0.3 — fixed in 6.4.0.13, 6.6.5, 6.7.0.3

How to fix

Upgrade to a fixed version: Cisco Adaptive Security Appliance 9.8.4.40; Cisco ASA Software 9.12.4.29, 9.14.3.9, 9.15.1.17, 9.16.2.3; Cisco Firepower Threat Defense Software 6.4.0.13, 6.6.5, 6.7.0.3. Check the vendor advisory for mitigations.

References

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