CVE-2021-40116 [High] | Denial of Service in Firepower Threat Defense

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.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-06-15	0.78%	1.35%	+0.56%
2	2025-11-21	1.08%	0.78%	-0.30%
3	2025-11-18	—	1.08%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-06-15

0.78%

1.35%

+0.56%

2025-11-21

1.08%

0.78%

-0.30%

2025-11-18

—

1.08%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
8.6	3.1	HIGH	`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H` Click to expand Attack vector (AV:N) Could be attacked over the internet or any normal routed network—not just someone sitting at the machine. Attack complexity (AC:L) Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup. Privileges required (PR:N) No account or special rights needed—anonymous or random user is enough. User interaction (UI:N) Nobody has to click “OK” or open a trap file; it can work without a victim helping. Scope (S:C) Breaking this can reach past the original component and bite other resources—bigger blast radius. Confidentiality (C:N) Doesn’t really leak secrets in a meaningful way. Integrity (I:N) Data isn’t meaningfully altered or forged. Availability (A:H) Could take the service down hard or make it unusable for people who depend on it.	3.9	4.0	[email protected]
7.5	3.1	HIGH	`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H` Click to expand Attack vector (AV:N) Could be attacked over the internet or any normal routed network—not just someone sitting at the machine. Attack complexity (AC:L) Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup. Privileges required (PR:N) No account or special rights needed—anonymous or random user is enough. User interaction (UI:N) Nobody has to click “OK” or open a trap file; it can work without a victim helping. Scope (S:U) Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems. Confidentiality (C:N) Doesn’t really leak secrets in a meaningful way. Integrity (I:N) Data isn’t meaningfully altered or forged. Availability (A:H) Could take the service down hard or make it unusable for people who depend on it.	3.9	3.6	[email protected]
7.1	2.0	HIGH	`AV:N/AC:M/Au:N/C:N/I:N/A:C` Click to expand Access vector (AV:N) Can be exploited remotely over network reachability. Access complexity (AC:M) Exploitation needs some favorable conditions, but not exceptional ones. Authentication (AU:N) No authentication is required. Confidentiality impact (C:N) No confidentiality impact. Integrity impact (I:N) No integrity impact. Availability impact (A:C) Complete availability impact.	8.6	6.9	[email protected]

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

8.6

3.1

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H Click to expand

Attack vector (AV:N): Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L): Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N): No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N): Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C): Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:N): Doesn’t really leak secrets in a meaningful way.
Integrity (I:N): Data isn’t meaningfully altered or forged.
Availability (A:H): Could take the service down hard or make it unusable for people who depend on it.

3.9

4.0

[email protected]

7.5

3.1

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand

Attack vector (AV:N): Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L): Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N): No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N): Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U): Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N): Doesn’t really leak secrets in a meaningful way.
Integrity (I:N): Data isn’t meaningfully altered or forged.
Availability (A:H): Could take the service down hard or make it unusable for people who depend on it.

3.9

3.6

[email protected]

7.1

2.0

HIGH

AV:N/AC:M/Au:N/C:N/I:N/A:C Click to expand

Access vector (AV:N): Can be exploited remotely over network reachability.
Access complexity (AC:M): Exploitation needs some favorable conditions, but not exceptional ones.
Authentication (AU:N): No authentication is required.
Confidentiality impact (C:N): No confidentiality impact.
Integrity impact (I:N): No integrity impact.
Availability impact (A:C): Complete availability impact.

8.6

6.9

[email protected]

Affected software / configurations for CVE-2021-40116

Vendor	Product	Version	Raw CPE
cisco	firepower_threat_defense	>= 6.4.0, < 6.4.0.13	cpe:2.3:a:cisco:firepower_threat_defense::::::::
cisco	firepower_threat_defense	>= 6.6.0, < 6.6.5.1	cpe:2.3:a:cisco:firepower_threat_defense::::::::
cisco	firepower_threat_defense	>= 6.7.0, < 6.7.0.3	cpe:2.3:a:cisco:firepower_threat_defense::::::::
cisco	firepower_threat_defense	>= 7.0.0, < 7.0.1	cpe:2.3:a:cisco:firepower_threat_defense::::::::
cisco	secure_firewall_management_center	3.1.0.1	cpe:2.3:a:cisco:secure_firewall_management_center:3.1.0.1:::::::*
cisco	snort	>= 3.0.0.0, < 3.1.0.100	cpe:2.3:a:cisco:snort::::::::

References for CVE-2021-40116

URL	Tags
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-snort-dos-RywH7ezM	Vendor Advisory

URL

CVE-2021-40116 | Multiple Cisco Products Snort Rule Denial of Service Vulnerability

Exploit prediction scoring system (EPSS) score for CVE-2021-40116

Common vulnerability scoring system (CVSS) metrics for CVE-2021-40116

Weakness enumeration for CVE-2021-40116

Affected software / configurations for CVE-2021-40116

References for CVE-2021-40116