CVE-2021-34793: Cisco Adaptive Security Appliance

High severity, CVSS 8.6. EPSS: 0.6% chance of exploitation in the next 30 days.

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

Published 2021-10-27. Last modified 2026-08-11.