CVE-2021-3987 is a medium-severity (CVSS 4.3) CWE-284 vulnerability in calibreweb. 1 public exploit or proof-of-concept reference is indexed, so weaponization risk is real. A fix is available for calibreweb — see the affected versions and patch details below.
Improper Access Control in janeczku/calibre-web
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2021-3987.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
calibrewebReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
An improper access control vulnerability exists in janeczku/calibre-web. The affected version allows users without public shelf permissions to create public shelves. The vulnerability is due to the create_shelf method in shelf.py not verifying if the user has the necessary permissions to create a public shelf. This issue can lead to unauthorized actions being performed by users.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | calibreweb | all versions | 0.6.15pip install --upgrade 'calibreweb==0.6.15' |
Affected Products
calibre-webjaneczkuResearch 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 calibreweb, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update calibreweb to 0.6.15 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3987 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
Is CVE-2021-3987 in your dependencies?
Find it across PyPI, including transitive dependencies.