CVE-2021-1236 [Medium] | Security Bypass in Ios Xe

Multiple Cisco products are affected by a vulnerability in the Snort application detection engine that could allow an unauthenticated, remote attacker to bypass the configured policies on an affected system. The vulnerability is due to a flaw in the detection algorithm. An attacker could exploit this vulnerability by sending crafted packets that would flow through an affected system. A successful exploit could allow the attacker to bypass the configured policies and deliver a malicious payload to the protected network.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-06-15	0.27%	2.15%	+1.87%
2	2026-05-24	0.20%	0.27%	+0.07%
3	2026-05-18	—	0.20%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-06-15

0.27%

2.15%

+1.87%

2026-05-24

0.20%

0.27%

+0.07%

2026-05-18

—

0.20%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
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]
4.0	3.0	MEDIUM	`CVSS:3.0/AV:N/AC:H/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:H) Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work. 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.	2.2	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

5.3

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]

4.0

3.0

MEDIUM

CVSS:3.0/AV:N/AC:H/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:H): Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
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.

2.2

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-2021-1236

vendor	priority	summary	link
`ubuntu`	medium	CVE-2021-1236 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): not-affected 6, needed 4, DNE 3, released 1.	https://ubuntu.com/security/CVE-2021-1236

Affected software / configurations for CVE-2021-1236

Vendor	Product	Version	Raw CPE
cisco	ios_xe	< 17.4.1	cpe:2.3:o:cisco:ios_xe::::::::
cisco	firepower_threat_defense	< 6.5.0.5	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	2.9.14.14	cpe:2.3:a:cisco:secure_firewall_management_center:2.9.14.14:::::::*
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:::::::*
snort	snort	< 2.9.14	cpe:2.3:a:snort:snort::::::::

References for CVE-2021-1236

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

URL

CVE-2021-1236 | Multiple Cisco Products Snort Application Detection Engine Policy Bypass Vulnerability

Exploit prediction scoring system (EPSS) score for CVE-2021-1236

Common vulnerability scoring system (CVSS) metrics for CVE-2021-1236

Weakness enumeration for CVE-2021-1236

OS Trackers for CVE-2021-1236

Affected software / configurations for CVE-2021-1236

References for CVE-2021-1236