CVE-2021-40116Network attack vector

Traffic denial via unconstrained Snort reset rules

Published Oct 27, 2021 · Updated Nov 7, 2024

Denial of service in Snort 3 rules in Cisco Firepower Threat Defense 7.0.0 allows remote attackers to drop through traffic. The inspection engine mishandles crafted IP packets when a reset-action rule omits both an application and a TCP or UDP protocol constraint. Exposure requires FMC management, Snort 3, and a Block with Reset or Interactive Block with Reset rule lacking those constraints; Snort 2 and FDM-managed devices are unaffected.

CVSS severity8.6
High
EPSS probability1.39%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in Snort 3 rules in Cisco Firepower Threat Defense 7.0.0 allows remote attackers to drop through traffic. The inspection engine mishandles crafted IP packets when a reset-action rule omits both an application and a TCP or UDP protocol constraint. Exposure requires FMC management, Snort 3, and a Block with Reset or Interactive Block with Reset rule lacking those constraints; Snort 2 and FDM-managed devices are unaffected.

The record

CVE
CVE-2021-40116
Published
Oct 27, 2021
Updated
Nov 7, 2024
Vendor
ClamAV
Product
Cisco Firepower Threat Defense Software
Classifications
CWE-241, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 27, 2021CVE publishedPublication date reported by the CVE source.
  2. Nov 7, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=n/a

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

ClamAV · Cisco Firepower Threat Defense Softwareversion=n/a

Published CVSS scores

8.6Cisco Systems, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H
7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
7.1NIST NVDAV:N/AC:M/Au:N/C:N/I:N/A:C

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-241Improper Handling of Unexpected Data Type
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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