CVE-2026-27730
HIGHCVE-2026-27730 is a high-severity (CVSS 8.6) Server-Side Request Forgery (SSRF) vulnerability in github.com/esm-dev/esm.sh. O3 Security confirms whether CVE-2026-27730 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
esm.sh has SSRF localhost/private-network bypass in `/http(s)` module route
Real-World Exposure
github.com/esm-dev/esm.shReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
esm.sh is a no-build content delivery network (CDN) for web development. Versions up to and including 137 have an SSRF vulnerability (CWE-918) in esm.sh’s /http(s) fetch route. The service tries to block localhost/internal targets, but the validation is based on hostname string checks and can be bypassed using DNS alias domains. This allows an external requester to make the esm.sh server fetch internal localhost services. As of time of publication, no known patched versions exist.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/esm-dev/esm.sh | all versions | 0.0.0-20250616164159-0593516c4cfa |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/esm-dev/esm.sh. 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 github.com/esm-dev/esm.sh to 0.0.0-20250616164159-0593516c4cfa or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27730 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-2026-27730 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-2026-27730. 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-2026-27730 in your dependencies?
O3 detects CVE-2026-27730 across Go dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.