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NVD / CVE Program· CNA cisco

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

Noroxi assessment

Priority: Monitor (action score 22/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 2.1% (FIRST EPSS); higher than 82.5% of all records.
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: snort (Debian:10) 2.9.20-0+deb10u1; snort (Debian:11) 2.9.20-0+deb11u1.

Low priority for now. Upgrade to snort (Debian:10) 2.9.20-0+deb10u1; snort (Debian:11) 2.9.20-0+deb11u1 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

MediumCVSS 5.3 · v3.1—No exploit Fix available
Published
Jan 13, 2021
Updated
Aug 11, 2026
EPSS
2.1% · 82th percentile

Report tools

JSON

Action score

22

Monitor

Low priority for now.

CVSS
21 / 40 · 5.3 / 10
CISA KEV
0 / 30 · Not listed
EPSS
1 / 30 · 2.1%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
5.3
CNA · cisco
4.0
1.3 point gap

CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2021-1236

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
ciscoios xe
cisco1100-4p integrated services router
cisco1100-8p integrated services router
cisco1101-4p integrated services router
cisco1109-2p integrated services router
cisco1109-4p integrated services router
cisco1111x-8p integrated services router
cisco4221 integrated services router
cisco4321 integrated services router
cisco4331 integrated services router
cisco4351 integrated services router
cisco4431 integrated services router

and 7 more

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • cisco ios xebefore 17.4.1
  • cisco secure firewall management center2.9.14.0
  • cisco secure firewall management center2.9.14.14
  • cisco secure firewall management center2.9.15
  • cisco secure firewall management center2.9.16
  • cisco secure firewall management center2.9.17
  • cisco secure firewall threat defensebefore 6.5.0.5
  • snort snortbefore 2.9.14

Versions reported by the vendor

Affected version ranges reported by the assigning authority (cisco). Independent of NVD's CPE analysis and usually ahead of it.

  • Cisco Cisco Firepower Threat Defense Software

    • n/aaffected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Debian:10snortbefore 2.9.20-0+deb10u12.9.20-0+deb10u1
Debian:11snortbefore 2.9.20-0+deb11u12.9.20-0+deb11u1

Same product

cisco: all records

Other highest-scoring records for the same primary product.

  • CVE-2023-20198Cisco is providing an update for the ongoing investigation into observed exploitation of the web UI feature in Cisco IOS XE Software.KEV
    100Now
  • CVE-2023-20273A vulnerability in the web UI feature of Cisco IOS XE Software could allow an authenticated, remote attacker to inject commands with the priKEV
    86Now
  • CVE-2018-0151A vulnerability in the quality of service (QoS) subsystem of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, reKEV
    73This week

Published before and after this one in the same product

  • ← EarlierCVE-2020-3527Sep 24, 2020 · Cisco Catalyst 9200 Series Switches Jumbo Frame Denial of Service Vulnerability
    34Monitor
  • ← EarlierCVE-2020-3444Nov 6, 2020 · Cisco SD-WAN Software Packet Filtering Bypass Vulnerability
    30Monitor
  • Later →CVE-2021-1382Mar 24, 2021 · Cisco IOS XE SD-WAN Software Command Injection Vulnerability
    26Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
debian:snort2.9.20-0+deb10u1 · Debian:10Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
none
Integrity
low · data or configuration can be modified
Availability
none
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality impact
None
Integrity impact
Low
Availability impact
None

Weakness class (CWE)

CWE-670 · Always-Incorrect Control Flow Implementation

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

nvd-primary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (2 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records