esm.sh: Path Traversal via package.json browser field allows reading arbitrary server files

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

EPSS

• CVE-2026-44594: 0.39% estimated exploitation activity in the next 30 days; percentile 32.83 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary A Local File Inclusion (LFI) vulnerability exists in the esbuild plugin's handling of the browser field in package.json. An attacker can publish an npm package that causes the server to read and return arbitrary files from the host filesystem during the build process. Details The vulnerable code is in the OnResolve callback of the esbuild plugin: https://github.com/esm-dev/esm.sh/blob/main/server/build.go The plugin validates that resolved file paths stay within the package working directory. However, after this check, the browser field from package.json remaps the module path to an attacker-controlled value containing ../ sequences. No validation is performed after the remapping. File contents appear in both the bundled JS output and the source map sourcesContent array. Readable files are constrained by esbuild's loader selection based on file extension: .json files must be valid JSON, .txt/.html/.md are read as raw text, files without a recognized extension must be…

Published by the source

12 May 2026, 22:22 UTC

Last modified by the source

9 Jun 2026, 02:00 UTC

Source last checked

26 Oct 2026, 15:23 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-rg65-45m7-hq57

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# Patch Next advisory brief: CVE-2026-44594

Advisory: esm.sh: Path Traversal via package.json browser field allows reading arbitrary server files
IDs: CVE-2026-44594, GHSA-rg65-45m7-hq57
Record: https://patchnext.com/advisories/cve-2026-44594 (revision 1, source checked 26 Oct 2026, 15:23 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/esm-dev/esm.sh 0.0.0-20250616164159-0593516c4cfa. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/esm-dev/esm.sh: affected < 0.0.0-20250616164159-0593516c4cfa; fixed in 0.0.0-20250616164159-0593516c4cfa

## Suggested inspection (read-only)
- Go github.com/esm-dev/esm.sh: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/esm-dev/esm.sh.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-rg65-45m7-hq57 (GitHub-reviewed advisory GHSA-rg65-45m7-hq57 lists affected ranges: Go github.com/esm-dev/esm.sh >=0 <0.0.0-20250616164159-0593516c4cfa)

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