Cisco Adaptive Security Appliance Software and Firepower Threat Defense Software Software-Based SSL/TLS Denial of Service Vulnerability

published 2021-10-27 5 y ago modified 2026-08-11 2 mo ago also EUVD-2021-21433

cvss: 8.6; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 1.6%; EPSS 1.6% chance of exploitation in 30 days, percentile 76%
public exploits: 0; none known

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.

Patch / fix

fixed in 2

cisco secure_firewall_threat_defense 6.4.0.13, 6.6.5, 6.7.0.3, 7.0.1
cisco adaptive_security_appliance_software 9.8.4.40, 9.12.4.29, 9.14.3.9, 9.15.1.17, 9.16.2

CISA SSVC

Exploitation: none
Automatable: yes
Technical impact: partial
Assessed: 2 y ago (2024-11-07)

Exploit prediction

1.61% chance of exploitation in the next 30 days, higher than 76% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	8.6	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H
NVD	3.1	7.5	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD	2.0	7.8	AV:N/AC:L/Au:N/C:N/I:N/A:C

Affected products

Cisco Cisco Adaptive Security Appliance (ASA) Software

Classification

Weakness

CWE-119 Improper Restriction of Operations within the Bounds of a Memory Buffer
CWE-20 Improper Input Validation

References

tools.cisco.com /security/center/content/CiscoSecurityAdvisory/cisco-sa-ftd-tls-decrypt-dos-BMxYjm8M
vendor-advisory

Sources

CVE List CVE-2021-34783
NVD CVE-2021-34783

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Summary