How severe is CVE-2021-4026?

CVE-2021-4026 has a CVSS score of 4.3/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-4026?

CVE-2021-4026 affects the following packages: ssddanbrown/bookstack (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2021-4026?

Update ssddanbrown/bookstack to 21.11.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-4026 is resolved across your whole dependency graph.

How do I detect CVE-2021-4026 in my Packagist dependencies?

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

How do I mitigate CVE-2021-4026 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-2021-4026 actively exploited in the wild?

Yes. There are 1 known exploit references for CVE-2021-4026, including 1 in-the-wild exploitation 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-2021-4026?

CVE-2021-4026 has an EPSS (Exploit Prediction Scoring System) score of 0.9%, placing it in the 59th 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-2021-4026?

CVE-2021-4026 is classified as CWE-284 (CWE-284), CWE-863 (CWE-863). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-4026 published, and has it been updated?

CVE-2021-4026 was published on November 30, 2021 and was last updated on August 7, 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

MEDIUM severity

CVE-2021-4026 — bookstack

MEDIUMFix: bookstackapp/bookstack@b4fa82e

CVE-2021-4026 is a medium-severity (CVSS 4.3) CWE-284 vulnerability in ssddanbrown/bookstack. 1 public exploit reference exists, so weaponization risk is real. A fix is available for ssddanbrown/bookstack — see the affected versions and patch details below.

bookstack is vulnerable to Improper Access Control

Also known asGHSA-jm6p-wfjg-xm7x

Published

Nov 30, 2021

Updated

Aug 7, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

1 known

Exploitation data as of Oct 4, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.9%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs59th percentile — riskier than 59% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-4026 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

🐘ssddanbrown/bookstack

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

bookstack is vulnerable to Improper Access Control

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`ssddanbrown/bookstack`	all versions	21.11.2`composer require ssddanbrown/bookstack:^21.11.2`

Affected Products

1 product · 1 configurations

Application

bookstackbookstackapp

< 21.11.2

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.

bookstack is vulnerable to Improper Access Control

huntr.devNVDDec 2021

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for ssddanbrown/bookstack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update ssddanbrown/bookstack to 21.11.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-4026 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

bookstack is vulnerable to Improper Access Control

O3 Security · Impact-Aware SCA

Is CVE-2021-4026 in your dependencies?

Find it across Packagist, including transitive dependencies.

Scan my dependencies How O3 SCA works

CVSS Score

4.3/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

Low

User Interaction

None

Scope

Unchanged

Confidentiality

Low

Integrity

None

Availability

None

Raw vector

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

CWE Classification

CWE-284CWE-284 CWE-863CWE-863

Weaponization

No known public exploit tooling found.

Timeline

Disclosed

Nov 30, 2021

First exploit

Dec 1, 2021

1 day after disclosure — rapid exploitation

Last updated

Aug 7, 2026

Identifiers

CVE-2021-4026

GHSA-jm6p-wfjg-xm7x

References

Advisorynvd.nist.gov Fixgithub.com Webgithub.com Webgithub.com Evidencehuntr.dev

Related vulnerabilities

This CVE is medium severity; 7 of 8 other CVEs affecting ssddanbrown/bookstack are rated as high or higher (top: high).

CVE-2024-36676CVE-2020-26211CVE-2022-0877CVE-2021-4194CVE-2021-4119CVE-2021-3944CVE-2021-3915CVE-2020-11055

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedAug 7, 2026

CVE-2021-4026 — bookstack

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-2021-4026 in your dependencies?