How severe is CVE-2021-26600?

CVE-2021-26600 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-26600?

CVE-2021-26600 affects the following packages: impresscms/impresscms (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2021-26600?

Update impresscms/impresscms to 1.4.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-26600 is resolved across your whole dependency graph.

How do I detect CVE-2021-26600 in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for impresscms/impresscms, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-26600 if there is no patch (or I can't update yet)?

Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Is CVE-2021-26600 actively exploited in the wild?

Yes. There are 3 known exploit references for CVE-2021-26600, including 3 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-26600?

CVE-2021-26600 has an EPSS (Exploit Prediction Scoring System) score of 5.6%, placing it in the 93th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-26600?

CVE-2021-26600 is classified as CWE-843 (CWE-843). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-26600 published, and has it been updated?

CVE-2021-26600 was published on March 28, 2022 and was last updated on October 24, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘

🐘 Packagist

Not in CISA KEV

CRITICAL severity

CVE-2021-26600 — impresscms

CRITICAL

CVE-2021-26600 is a critical-severity (CVSS 9.8) CWE-843 vulnerability in impresscms/impresscms. 3 public exploit or proof-of-concept references are indexed, so weaponization risk is real. A fix is available for impresscms/impresscms — see the affected versions and patch details below.

Type Confusion in ImpressCMS

Also known asGHSA-m8xh-cqc2-5q6f

Published

Mar 28, 2022

Updated

Oct 24, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

3 known

Exploitation data as of Nov 11, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

5.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs93th percentile — riskier than 93% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

🐘impresscms/impresscms

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

Description

ImpressCMS before 1.4.3 has plugins/preloads/autologin.php type confusion with resultant Authentication Bypass (!= instead of !==).

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`impresscms/impresscms`	all versions	1.4.3`composer require impresscms/impresscms:^1.4.3`

Affected Products

1 product · 1 configurations

Application

impresscmsimpresscms

< 1.4.3

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

ImpressCMS before 1.4.3 has plugins/preloads/autologin.php type confusio…

seclists.orgNVDMar 2022

Report on packetstormsecurity.com

packetstormsecurity.comNVDMar 2022

Report on hackerone.com

hackerone.comNVDMar 2022

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for impresscms/impresscms, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update impresscms/impresscms to 1.4.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-26600 is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Frequently Asked Questions

ImpressCMS before 1.4.3 has plugins/preloads/autologin.php type confusion with resultant Authentication Bypass (!= instead of !==).

O3 Security · Impact-Aware SCA

Is CVE-2021-26600 in your dependencies?

Find it across Packagist, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

9.8/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

Raw vector

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

CWE Classification

CWE-843CWE-843

Weaponization

No Nuclei template or Metasploit module yet. 3 public proof-of-concept references are listed on this page.

Timeline

Disclosed

Mar 28, 2022

First exploit

Mar 30, 2022

3 days after disclosure — rapid exploitation

Last updated

Oct 24, 2026

Identifiers

CVE-2021-26600

GHSA-m8xh-cqc2-5q6f

References

Advisorykarmainsecurity.com · KIS 2022 01 Advisorynvd.nist.gov · CVE 2021 26600 Webgithub.com · v1.4.3 Packagegithub.com · impresscms Evidencepacketstormsecurity.com · ImpressCMS 1.4.2 Authentication Bypass Evidenceseclists.org Evidencehackerone.com

Related vulnerabilities

This CVE is critical severity — higher than any of the other 8 CVEs affecting impresscms/impresscms (highest there: high).

CVE-2023-37785CVE-2018-13983CVE-2014-4036CVE-2014-1836CVE-2010-4616CVE-2022-26986CVE-2021-26598CVE-2021-26601

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedOct 24, 2026

CVE-2021-26600 — impresscms

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Frequently Asked Questions

Is CVE-2021-26600 in your dependencies?