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

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-2026-102657: 0.72% estimated exploitation activity in the next 30 days; percentile 54.10 (FIRST EPSS, 2026-11-10). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The documented `web.NewHandler` static file server can read files outside `Config.AppDir` when the request path contains URL-encoded dot-segments such as `%2e%2e`. Go's `ServeMux` redirects plain `/../` paths, but it passes encoded dot-segments to the handler after decoding `r.URL.Path`. The handler then calls `filepath.Join(s.config.AppDir, r.URL.Path)` without checking that the result remains under `AppDir`, allowing unauthenticated arbitrary file reads with the server process privileges.

Details `web/README.md:34-40` documents a network-facing setup: ```go http.Handle("GET /", web.NewHandler(web.Config{ AppDir: "/path/to/webapp", Fallback: "/index.html", Dev: false, })) log.Fatal(http.ListenAndServe(":8080", nil)) ``` In `web/handler.go:62-75`, the handler takes `r.URL.Path` and joins it directly with `s.config.AppDir`: ```go pathname := r.URL.Path filename := filepath.Join(s.config.AppDir, pathname) fi, err := os.Lstat(filename) ``` Later the same resolved…

Published by the source

9 Nov 2026, 22:03 UTC

Last modified by the source

9 Nov 2026, 22:15 UTC

Source last checked

10 Nov 2026, 06:45 UTC

1. Revision 2 · amended · 10 Nov 2026

   Amended from osv record GHSA-g3c2-rjq2-c763

2. Revision 1 · created · 9 Nov 2026

   Published from ghsa record GHSA-g3c2-rjq2-c763

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# Patch Next advisory brief: CVE-2026-102657

Advisory: esm.sh: web.NewHandler allows arbitrary file read via encoded dot-segment path traversal
IDs: CVE-2026-102657, GHSA-g3c2-rjq2-c763
Record: https://patchnext.com/advisories/cve-2026-102657 (revision 2, source checked 10 Nov 2026, 06:45 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/web 0.2.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/esm-dev/esm.sh/web: affected < 0.2.0; fixed in 0.2.0

## Suggested inspection (read-only)
- Go github.com/esm-dev/esm.sh/web: 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/web.
- 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-g3c2-rjq2-c763 (GitHub-reviewed advisory GHSA-g3c2-rjq2-c763 lists affected ranges: Go github.com/esm-dev/esm.sh/web >=0 <0.2.0)
- Verified: https://osv.dev/vulnerability/GHSA-g3c2-rjq2-c763 (OSV record GHSA-g3c2-rjq2-c763 lists affected ranges: Go github.com/esm-dev/esm.sh/web >=0 <0.2.0 (fixed))

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