CVE-2026-84695

BookStack before 26.05.4 fails to validate base64 content accepted by its drawing upload endpoint, storing attacker-supplied files without any content inspection. An attacker who already holds editor-level permissions can upload a malicious SVG containing embedded script. When an administrator later views that drawing through the image gallery API, the file is served without content-type validation or CSP headers, causing the script to execute in the administrator's browser. Successful exploitation could let the attacker act as the admin in that session, with high confidentiality and integrity impact per the CVSS 4.0 score of 9.3. Any self-hosted BookStack instance that grants editor rights to untrusted users is exposed; there are currently no known public proofs of concept, no KEV listing, and EPSS puts 30-day exploitation probability at just 0.3%.

What to do: Upgrade BookStack to version 26.05.4 or later. As interim mitigation, restrict drawing/upload permissions to fully trusted users and consider adding restrictive Content-Security-Policy headers or content-type enforcement for user-uploaded SVG and image files; check whether any editors on your instance have uploaded drawing files recently.