CVE-2021-34781: Cisco Firepower Threat Defense Software SSH Connections Denial of Service Vulnerability

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.4% 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-119

Description

A vulnerability in the processing of SSH connections for multi-instance deployments of Cisco Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on the affected device. This vulnerability is due to a lack of proper error handling when an SSH session fails to be established. An attacker could exploit this vulnerability by sending a high rate of crafted SSH connections to the instance. A successful exploit could allow the attacker to cause resource exhaustion, which causes a DoS condition on the affected device. The device must be manually reloaded to recover.

Affected products

Cisco Firepower Management Center Virtual Appliance: 6.3.0, 6.4.0, 6.4.0.13, 6.5.0, 6.6.0, 6.7.0.3 (+2)
Cisco Firepower Threat Defense Software: from 6.3.0 before 6.4.0.13; from 6.5.0 before 6.6.5; from 6.7.0 before 6.7.0.3; from 7.0.0 before 7.0.1 — fixed in 6.4.0.13, 6.6.5, 6.7.0.3, 7.0.1
Cisco Sourcefire Defense Center: 6.3.0, 6.4.0, 6.4.0.13, 6.5.0, 6.6.0, 6.7.0.3 (+2)

How to fix

Upgrade to a fixed version: Cisco Firepower Threat Defense Software 6.4.0.13, 6.6.5, 6.7.0.3, 7.0.1. Check the vendor advisory for mitigations.

References

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