Auth bypass in Bookstackapp Bookstack
CVE-2026-67204
BookStack before 26.05.4 contains a broken access control vulnerability that allows authenticated API users with image-update or image-delete permissions to manipulate other users' avatars by exploiting missing content-type restrictions in the Image Gallery API endpoints. Attackers can supply a user avatar's ID to the API controller, which loads any image type without the web controller's gallery and drawio restrictions, and when the avatar's uploaded_to field matches a page ID accessible to the attacker, the authorization check passes allowing the attacker to rename, replace, or delete the target user's avatar without requiring user-management permission.
Vulnerability class: Broken Access Control
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L.
EPSS exploit prediction
EPSS: 0.004 (33.6th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 34th percentile — 33.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.004 · change: +0.002
Affected products
Weakness classification (CWE)
References
- www.bookstackapp.com/blog/bookstack-release-v26-05-4/ (vendor-advisory)
- codeberg.org/bookstack/bookstack/releases/tag/v26.05.4 (release-notes)
- github.com/BookStackApp/BookStack/releases/tag/v26.05.4 (release-notes)
- www.vulncheck.com/advisories/bookstack-broken-access-control-via-image-gallery-… (third-party-advisory)
Frequently asked questions
- What is CVE-2026-67204?
- CVE-2026-67204 is a medium-severity vulnerability in Bookstackapp Bookstack, classified under Incorrect Authorization. CVSS score: 5.4/10. Published 2026-08-24.
- How severe is CVE-2026-67204?
- Medium severity. CVSS v3 base score is 5.4 out of 10.