CVE-2021-3915 is a medium-severity (CVSS 5.7) Unrestricted File Upload 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 Unrestricted Upload of File with Dangerous Type
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-3915 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
ssddanbrown/bookstackReal-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 Unrestricted Upload of File with Dangerous Type
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | ssddanbrown/bookstack | all versions | 21.0.3composer require ssddanbrown/bookstack:^21.0.3 |
Affected Products
bookstackbookstackappResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
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 21.0.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3915 is resolved across your whole dependency graph.
Workarounds
Treat uploaded files as untrusted until proven otherwise: validate the actual content type rather than the supplied extension, store uploads outside the web root on a volume mounted without execute permission, and rename them to server-generated identifiers so an attacker cannot choose the path a request will later resolve.
Frequently Asked Questions
Is CVE-2021-3915 in your dependencies?
Find it across Packagist, including transitive dependencies.