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

> Attackers can bypass hostname validation to force the esm.sh CDN to fetch internal localhost services, exposing private network resources.

- **Severity:** High 8.6 (`CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Feb 2026
- **Updated:** 25 Feb 2026
- **Also known as:** GHSA-p2v6-84h2-5x4r, GO-2026-4554

## 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 (Server-Side Request Forgery)

## 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.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/esm-dev/esm.sh` | Go | All versions | 0.0.0-20250616164159-0593516c4cfa |
| `github.com/esm-dev/esm.sh` | Go | All versions | 0.0.0-20250616164159-0593516c4cfa |
| `github.com/esm-dev/esm.sh` | Go | All versions | 0.0.0-20250616164159-0593516c4cfa |

Products: 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.

## 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](https://insights.sbomify.com/CVE-2025-50180/): esm.sh is vulnerable to full-response SSRF (high 8.7)
- [CVE-2025-65025](https://insights.sbomify.com/CVE-2025-65025/): esm.sh CDN service has arbitrary file write via tarslip (high 8.2)
- [CVE-2026-23644](https://insights.sbomify.com/CVE-2026-23644/): esm.sh has path traversal in `extractPackageTarball` that enables file writes from malicious packages (high 7.7)
- [CVE-2025-65026](https://insights.sbomify.com/CVE-2025-65026/): esm.sh CDN service has JS Template Literal Injection in CSS-to-JavaScript (medium 6.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-27730 and fix it if it is affected.

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

Affected packages and fixed versions:
- github.com/esm-dev/esm.sh (Go): affected all versions; fixed in 0.0.0-20250616164159-0593516c4cfa
- github.com/esm-dev/esm.sh (Go): affected all versions; fixed in 0.0.0-20250616164159-0593516c4cfa
- github.com/esm-dev/esm.sh (Go): affected all versions; fixed in 0.0.0-20250616164159-0593516c4cfa

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-27730/ (Markdown: https://insights.sbomify.com/CVE-2026-27730/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-27730
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-27730
- OSV: https://osv.dev/vulnerability/CVE-2026-27730

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-27730/
Summaries are generated and may contain mistakes; the official description above is the record.