CVE-2021-23387 is a medium-severity (CVSS 6.1) Open Redirect vulnerability in trailing-slash. 1 public exploit reference exists, so weaponization risk is real. A fix is available for trailing-slash — see the affected versions and patch details below.
Open Redirect in trailing-slash
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2021-23387 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
trailing-slashnpmDescription
The package trailing-slash before 2.0.1 are vulnerable to Open Redirect via the use of trailing double slashes in the URL when accessing the vulnerable endpoint (such as https://example.com//attacker.example/). The vulnerable code is in index.js::createTrailing(), as the web server uses relative URLs instead of absolute URLs.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | trailing-slash | all versions | 2.0.1npm install trailing-slash@2.0.1 |
Affected Products
trailing-slashtrailing-slash_projectResearch 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 trailing-slash, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update trailing-slash to 2.0.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-23387 is resolved across your whole dependency graph.
Workarounds
Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.
Frequently Asked Questions
Is CVE-2021-23387 in your dependencies?
Find it across npm, including transitive dependencies.