CVE-2026-27730 esm.sh has SSRF localhost/private-network bypass in `/http(s)` module route

What is CVE-2026-27730?

CVE-2026-27730 is a high-severity server-side request forgery vulnerability in esm-dev esm.sh, published on 25 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official 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.

Weakness: CWE-918

Which versions are affected by CVE-2026-27730?

3 open source packages ship an affected version, including github.com/esm-dev/esm.sh. It affects 1 product.

Products include Esm Esm.sh.

How do I fix CVE-2026-27730?

Upgrade github.com/esm-dev/esm.sh to 0.0.0-20250616164159-0593516c4cfa or later.

Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.

Is CVE-2026-27730 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-27730 exists.

Other vulnerabilities in esm-dev esm.sh

• CVE-2025-50180 esm.sh is vulnerable to full-response SSRF High 8.7
• CVE-2025-65025 esm.sh CDN service has arbitrary file write via tarslip High 8.2
• CVE-2026-23644 esm.sh has path traversal in `extractPackageTarball` that enables file writes from malicious packages High 7.7
• CVE-2025-65026 esm.sh CDN service has JS Template Literal Injection in CSS-to-JavaScript Medium 6.1

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-p2v6-84h2-5x4r, GO-2026-4554.