CVE-2021-1224 [Medium] | Security Bypass in Firepower Threat Defense

Multiple Cisco products are affected by a vulnerability with TCP Fast Open (TFO) when used in conjunction with the Snort detection engine that could allow an unauthenticated, remote attacker to bypass a configured file policy for HTTP. The vulnerability is due to incorrect detection of the HTTP payload if it is contained at least partially within the TFO connection handshake. An attacker could exploit this vulnerability by sending crafted TFO packets with an HTTP payload through an affected device. A successful exploit could allow the attacker to bypass configured file policy for HTTP packets and deliver a malicious payload.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-06-15	0.65%	2.00%	+1.36%
2	2026-05-24	0.48%	0.65%	+0.17%
3	2026-05-18	—	0.48%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-06-15

0.65%

2.00%

+1.36%

2026-05-24

0.48%

0.65%

+0.17%

2026-05-18

—

0.48%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
5.8	3.1	MEDIUM	`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N` 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:L) Attackers could change some data, but it’s limited—not everything goes. Availability (A:N) Service keeps running; no real outage angle.	3.9	1.4	[email protected]
5.3	3.1	MEDIUM	`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N` 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:L) Attackers could change some data, but it’s limited—not everything goes. Availability (A:N) Service keeps running; no real outage angle.	3.9	1.4	[email protected]
5.0	2.0	MEDIUM	`AV:N/AC:L/Au:N/C:N/I:P/A:N` Click to expand Access vector (AV:N) Can be exploited remotely over network reachability. Access complexity (AC:L) Exploitation conditions are straightforward and predictable. Authentication (AU:N) No authentication is required. Confidentiality impact (C:N) No confidentiality impact. Integrity impact (I:P) Partial integrity impact. Availability impact (A:N) No availability impact.	10.0	2.9	[email protected]

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

5.8

3.1

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N 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:L): Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N): Service keeps running; no real outage angle.

3.9

1.4

[email protected]

5.3

3.1

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N 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:L): Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N): Service keeps running; no real outage angle.

3.9

1.4

[email protected]

5.0

2.0

MEDIUM

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

Access vector (AV:N): Can be exploited remotely over network reachability.
Access complexity (AC:L): Exploitation conditions are straightforward and predictable.
Authentication (AU:N): No authentication is required.
Confidentiality impact (C:N): No confidentiality impact.
Integrity impact (I:P): Partial integrity impact.
Availability impact (A:N): No availability impact.

10.0

2.9

[email protected]

OS Trackers for CVE-2021-1224

vendor	priority	summary	link
`ubuntu`	medium	CVE-2021-1224 medium priority: Ubuntu including 1 source packages (snort), 14 status rows across 14 suites (bionic, focal, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, questing, trusty, upstream, xenial): needed 5, ignored 4, DNE 3, not-affected 1, released 1.	https://ubuntu.com/security/CVE-2021-1224

Affected software / configurations for CVE-2021-1224

Vendor	Product	Version	Raw CPE
cisco	firepower_threat_defense	< 6.7.0	cpe:2.3:a:cisco:firepower_threat_defense::::::::
cisco	secure_firewall_management_center	2.9.14.0	cpe:2.3:a:cisco:secure_firewall_management_center:2.9.14.0:::::::*
cisco	secure_firewall_management_center	2.9.15	cpe:2.3:a:cisco:secure_firewall_management_center:2.9.15:::::::*
cisco	secure_firewall_management_center	2.9.16	cpe:2.3:a:cisco:secure_firewall_management_center:2.9.16:::::::*
cisco	secure_firewall_management_center	2.9.17	cpe:2.3:a:cisco:secure_firewall_management_center:2.9.17:::::::*
cisco	secure_firewall_management_center	2.9.18	cpe:2.3:a:cisco:secure_firewall_management_center:2.9.18:::::::*
cisco	secure_firewall_management_center	3.0.1	cpe:2.3:a:cisco:secure_firewall_management_center:3.0.1:::::::*
cisco	ios_xe	< 17.4.1	cpe:2.3:o:cisco:ios_xe::::::::
snort	snort	< 2.9.17	cpe:2.3:a:snort:snort::::::::
cisco	meraki_mx64_firmware	—	cpe:2.3:o:cisco:meraki_mx64_firmware:-:::::::*
cisco	meraki_mx64w_firmware	—	cpe:2.3:o:cisco:meraki_mx64w_firmware:-:::::::*
cisco	meraki_mx67_firmware	—	cpe:2.3:o:cisco:meraki_mx67_firmware:-:::::::*
cisco	meraki_mx67c_firmware	—	cpe:2.3:o:cisco:meraki_mx67c_firmware:-:::::::*
cisco	meraki_mx67w_firmware	—	cpe:2.3:o:cisco:meraki_mx67w_firmware:-:::::::*
cisco	meraki_mx68_firmware	—	cpe:2.3:o:cisco:meraki_mx68_firmware:-:::::::*
cisco	meraki_mx68cw_firmware	—	cpe:2.3:o:cisco:meraki_mx68cw_firmware:-:::::::*
cisco	meraki_mx68w_firmware	—	cpe:2.3:o:cisco:meraki_mx68w_firmware:-:::::::*
cisco	meraki_mx100_firmware	—	cpe:2.3:o:cisco:meraki_mx100_firmware:-:::::::*
cisco	meraki_mx84_firmware	—	cpe:2.3:o:cisco:meraki_mx84_firmware:-:::::::*
cisco	meraki_mx250_firmware	—	cpe:2.3:o:cisco:meraki_mx250_firmware:-:::::::*
cisco	meraki_mx450_firmware	—	cpe:2.3:o:cisco:meraki_mx450_firmware:-:::::::*

References for CVE-2021-1224

URL	Tags
https://lists.debian.org/debian-lts-announce/2023/02/msg00011.html
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-snort-tfo-bypass-MmzZrtes	Vendor Advisory
https://www.debian.org/security/2023/dsa-5354

URL

CVE-2021-1224 | Multiple Cisco Products Snort TCP Fast Open File Policy Bypass Vulnerability

Exploit prediction scoring system (EPSS) score for CVE-2021-1224

Common vulnerability scoring system (CVSS) metrics for CVE-2021-1224

Weakness enumeration for CVE-2021-1224

OS Trackers for CVE-2021-1224

Affected software / configurations for CVE-2021-1224

References for CVE-2021-1224