CVE-2021-27516
HIGHCVE-2021-27516 is a high-severity (CVSS 7.5) vulnerability in urijs. 1 public exploit reference exists, so weaponization risk is real. O3 Security confirms whether CVE-2021-27516 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
URIjs Hostname spoofing via backslashes in URL
Blast Radius
Weekly download volume for affected packages — a proxy for how broadly this vulnerability is deployed.
urijsnpmDescription
URI.js (aka urijs) before 1.19.6 mishandles certain uses of backslash such as http:/ and interprets the URI as a relative path.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | urijs | all versions | 1.19.6 |
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 urijs. 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 urijs to 1.19.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-27516 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-2021-27516 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-2021-27516. 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-2021-27516 in your dependencies?
O3 detects CVE-2021-27516 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.