What is CVE-2021-4026?

CVE-2021-4026 is a medium-severity vulnerability in Bookstackapp Bookstack, classified under Improper Access Control. CVSS score: 4.3/10. Published 2021-11-30.

How severe is CVE-2021-4026?

Medium severity. CVSS v3 base score is 4.3 out of 10.

Is CVE-2021-4026 known to be exploited?

3 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Auth bypass in Bookstackapp Bookstack

CVE-2021-4026

bookstack is vulnerable to Improper Access Control

Published 2021-11-30 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N.

EPSS exploit prediction

EPSS: 0.009 (58.9th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 59th percentile — 58.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.009 · newest: 0.009 · change: +0.000

Affected products

Bookstackapp Bookstack
Bookstackapp Bookstackapp/bookstack — versions unspecified

Weakness classification (CWE)

Public proof-of-concept exploits

References

huntr.dev/bounties/c6dfa80d-43e6-4b49-95af-cc031bb66b1d (Exploit, Third Party Advisory)
github.com/bookstackapp/bookstack/commit/b4fa82e3298a15443ca40bff205b7a16a1031d… (Patch, Third Party Advisory)

Frequently asked questions

What is CVE-2021-4026?: CVE-2021-4026 is a medium-severity vulnerability in Bookstackapp Bookstack, classified under Improper Access Control. CVSS score: 4.3/10. Published 2021-11-30.
How severe is CVE-2021-4026?: Medium severity. CVSS v3 base score is 4.3 out of 10.
Is CVE-2021-4026 known to be exploited?: 3 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.