# CVE-2026-102657: esm.sh: web.NewHandler allows arbitrary file read via encoded dot-segment path traversal

Status: published. Published: 2026-11-06. Updated: 2026-11-06.

esm.sh is a no-build content delivery network (CDN) for web development. Prior to server version 137_7, the documented web.NewHandler static file server in web/handler.go joins the URL-decoded r.URL.Path with Config.AppDir without verifying that the result remains inside the configured application directory. URL-encoded dot segments such as %2e%2e bypass the plain traversal redirect performed by Go's ServeMux, allowing an unauthenticated remote attacker to make the server open and return files outside Config.AppDir that are readable by the server process. The confirmed impact is file disclosure only, and the issue is reachable with Dev disabled. This issue is fixed in server version 137_7.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- esm-dev / esm.sh: < 137_7 (affected). Source: CNA: GitHub_M

## Severity and prioritization

- CVSS 3.1: 7.5 (high); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N; source: CNA: GitHub_M

- CISA KEV: No listing in collected sources

- EPSS: Not available

- SSVC: Not available

- Weaknesses: CWE-22

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://github.com/esm-dev/esm.sh/security/advisories/GHSA-g3c2-rjq2-c763 (CNA: GitHub_M; x_refsource_CONFIRM)

- https://github.com/esm-dev/esm.sh/pull/1376 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/esm-dev/esm.sh/commit/468b6b2a4b4dfec334cf6b33b01fcd4e5cd93bed (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/esm-dev/esm.sh/commit/fc3b345e73aeaa0b7a05b6f27d91c05fdf5f9b53 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/esm-dev/esm.sh/releases/tag/v137_7 (CNA: GitHub_M; x_refsource_MISC)

## Provenance

- CNA: GitHub_M: https://www.cve.org/CVERecord?id=CVE-2026-102657; retrieved 2026-11-06; source updated 2026-11-06

Revision: 548218ead2749ea36ed1cdc1b146c6f6f63dc457ec680578308f967117171e26

Canonical: https://y2.dev/cve/CVE-2026-102657

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