CVE-2021-34791 [Medium] | Input Validation Bypass in Adaptive Security Appliance

Multiple vulnerabilities in the Application Level Gateway (ALG) for the Network Address Translation (NAT) feature of Cisco Adaptive Security Appliance (ASA) Software and Firepower Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to bypass the ALG and open unauthorized connections with a host located behind the ALG. For more information about these vulnerabilities, see the Details section of this advisory. Note: These vulnerabilities have been publicly discussed as NAT Slipstreaming.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-06-15	0.99%	1.10%	+0.11%
2	2026-06-06	0.73%	0.99%	+0.26%
3	2025-11-21	—	0.73%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-06-15

0.99%

1.10%

+0.11%

2026-06-06

0.73%

0.99%

+0.26%

2025-11-21

—

0.73%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
4.7	3.1	MEDIUM	`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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:R) A real person has to do something—click, install, enable—otherwise it doesn’t land. 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.	2.8	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

4.7

3.1

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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:R): A real person has to do something—click, install, enable—otherwise it doesn’t land.
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.

2.8

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]

Affected software / configurations for CVE-2021-34791

Vendor	Product	Version	Raw CPE
cisco	adaptive_security_appliance	< 9.8.4.40	cpe:2.3:a:cisco:adaptive_security_appliance::::::::
cisco	firepower_threat_defense	< 6.4.0.12	cpe:2.3:a:cisco:firepower_threat_defense::::::::
cisco	firepower_threat_defense	>= 6.5.0, < 6.6.5	cpe:2.3:a:cisco:firepower_threat_defense::::::::
cisco	firepower_threat_defense	>= 6.7.0, < 6.7.0.2	cpe:2.3:a:cisco:firepower_threat_defense::::::::
cisco	adaptive_security_appliance_software	>= 9.12.0, < 9.12.4.18	cpe:2.3:o:cisco:adaptive_security_appliance_software::::::::
cisco	adaptive_security_appliance_software	>= 9.13.0, < 9.14.2.15	cpe:2.3:o:cisco:adaptive_security_appliance_software::::::::
cisco	adaptive_security_appliance_software	>= 9.15.0, < 9.15.1.15	cpe:2.3:o:cisco:adaptive_security_appliance_software::::::::
cisco	asa_5512-x_firmware	009.008	cpe:2.3:o:cisco:asa_5512-x_firmware:009.008:::::::*
cisco	asa_5512-x_firmware	009.015	cpe:2.3:o:cisco:asa_5512-x_firmware:009.015:::::::*
cisco	asa_5505_firmware	009.008	cpe:2.3:o:cisco:asa_5505_firmware:009.008:::::::*
cisco	asa_5505_firmware	009.015	cpe:2.3:o:cisco:asa_5505_firmware:009.015:::::::*
cisco	asa_5515-x_firmware	009.008	cpe:2.3:o:cisco:asa_5515-x_firmware:009.008:::::::*
cisco	asa_5515-x_firmware	009.015	cpe:2.3:o:cisco:asa_5515-x_firmware:009.015:::::::*
cisco	asa_5525-x_firmware	009.008	cpe:2.3:o:cisco:asa_5525-x_firmware:009.008:::::::*
cisco	asa_5525-x_firmware	009.015	cpe:2.3:o:cisco:asa_5525-x_firmware:009.015:::::::*
cisco	asa_5545-x_firmware	009.008	cpe:2.3:o:cisco:asa_5545-x_firmware:009.008:::::::*
cisco	asa_5545-x_firmware	009.015	cpe:2.3:o:cisco:asa_5545-x_firmware:009.015:::::::*
cisco	asa_5555-x_firmware	009.008	cpe:2.3:o:cisco:asa_5555-x_firmware:009.008:::::::*
cisco	asa_5555-x_firmware	009.015	cpe:2.3:o:cisco:asa_5555-x_firmware:009.015:::::::*
cisco	asa_5580_firmware	009.008	cpe:2.3:o:cisco:asa_5580_firmware:009.008:::::::*
cisco	asa_5580_firmware	009.015	cpe:2.3:o:cisco:asa_5580_firmware:009.015:::::::*
cisco	asa_5585-x_firmware	009.008	cpe:2.3:o:cisco:asa_5585-x_firmware:009.008:::::::*
cisco	asa_5585-x_firmware	009.015	cpe:2.3:o:cisco:asa_5585-x_firmware:009.015:::::::*

References for CVE-2021-34791

URL	Tags
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-natalg-bypass-cpKGqkng	Vendor Advisory

URL

CVE-2021-34791 | Cisco Adaptive Security Appliance Software and Firepower Threat Defense Software Application Level Gateway Bypass Vulnerabilities

Exploit prediction scoring system (EPSS) score for CVE-2021-34791

Common vulnerability scoring system (CVSS) metrics for CVE-2021-34791

Weakness enumeration for CVE-2021-34791

Affected software / configurations for CVE-2021-34791

References for CVE-2021-34791