CVE-2021-40116: Multiple Cisco Products Snort Rule 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-241

Description

Multiple Cisco products are affected by a vulnerability in Snort rules that could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device.The vulnerability is due to improper handling of the Block with Reset or Interactive Block with Reset actions if a rule is configured without proper constraints. An attacker could exploit this vulnerability by sending a crafted IP packet to the affected device. A successful exploit could allow the attacker to cause through traffic to be dropped. Note: Only products with Snort3 configured and either a rule with Block with Reset or Interactive Block with Reset actions configured are vulnerable. Products configured with Snort2 are not vulnerable.

Affected products

Cisco Firepower Management Center: 3.1.0.1
Cisco Firepower Threat Defense Software: from 6.4.0 before 6.4.0.13; from 6.6.0 before 6.6.5.1; from 6.7.0 before 6.7.0.3; from 7.0.0 before 7.0.1 — fixed in 6.4.0.13, 6.6.5.1, 6.7.0.3, 7.0.1
Cisco Snort: from 3.0.0.0 before 3.1.0.100 — fixed in 3.1.0.100

How to fix

Upgrade to a fixed version: Cisco Firepower Threat Defense Software 6.4.0.13, 6.6.5.1, 6.7.0.3, 7.0.1; Cisco Snort 3.1.0.100. Check the vendor advisory for mitigations.

References

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