CVE-2021-3988
MEDIUMCross-site Scripting (XSS) - DOM in janeczku/calibre-web
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Blast Radius
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
A Cross-site Scripting (XSS) vulnerability exists in janeczku/calibre-web, specifically in the file edit_books.js. The vulnerability occurs when editing book properties, such as uploading a cover or a format. The affected code directly inserts user input into the DOM without proper sanitization, allowing attackers to execute arbitrary JavaScript code. This can lead to various attacks, including stealing cookies. The issue is present in the code handling the #btn-upload-cover change event.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | calibreweb | all versions | 0.6.15 |
Research 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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update calibreweb to 0.6.15 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-3988 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2021-3988 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2021-3988. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-3988 in your dependencies?
O3 detects CVE-2021-3988 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.