CVE-2021-34783: Cisco Adaptive Security Appliance Software

High severity, CVSS 7.5. EPSS: 1.6% chance of exploitation in the next 30 days.

A vulnerability in the software-based SSL/TLS message handler of Cisco Adaptive Security Appliance (ASA) Software and Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause an affected device to reload, resulting in a denial of service (DoS) condition. This vulnerability is due to insufficient validation of SSL/TLS messages when the device performs software-based SSL/TLS decryption. An attacker could exploit this vulnerability by sending a crafted SSL/TLS message to an affected device. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a DoS condition. Note: Datagram TLS (DTLS) messages cannot be used to exploit this vulnerability.

Affected products

Cisco Adaptive Security Appliance Software: from 9.8.0, before 9.8.4.40 (fixed in 9.8.4.40); from 9.12.0, before 9.12.4.29 (fixed in 9.12.4.29); from 9.14.0, 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 (fixed in 9.16.2)
Cisco ASA 5505 Firmware: version 009.016(001) only; version 009.016(001.025) only
Cisco ASA 5512-X Firmware: version 009.016(001) only; version 009.016(001.025) only
Cisco ASA 5515-X Firmware: version 009.016(001) only; version 009.016(001.025) only
Cisco ASA 5525-X Firmware: version 009.016(001) only; version 009.016(001.025) only
Cisco ASA 5545-X Firmware: version 009.016(001) only; version 009.016(001.025) only
Cisco ASA 5555-X Firmware: version 009.016(001) only; version 009.016(001.025) only
Cisco ASA 5580 Firmware: version 009.016(001) only; version 009.016(001.025) only
Cisco ASA 5585-X Firmware: version 009.016(001) only; version 009.016(001.025) only
Cisco Secure Firewall Threat Defense: from 6.4.0, before 6.4.0.13 (fixed in 6.4.0.13); from 6.6.0, before 6.6.5 (fixed in 6.6.5); from 6.7.0, before 6.7.0.3 (fixed in 6.7.0.3); from 7.0.0, before 7.0.1 (fixed in 7.0.1)

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