pnpm Vulnerable to Arbitrary File Write/Delete via Malicious Patch File (Path Traversal)

Summary

pnpm's patch application pipeline (@pnpm/patch-package) performs no path validation on file paths extracted from .patch files. An attacker who contributes a malicious patch file via a pull request can write attacker-controlled content to or delete arbitrary files on the filesystem during pnpm install, as the user running the install. The diff --git header paths containing ../../ sequences traverse out of the package directory, and the traversal is difficult to catch in code review because patch file diff headers are opaque to most reviewers.

Vulnerability Details

During pnpm install, when a patchedDependencies entry is present in pnpm-workspace.yaml, pnpm reads the referenced .patch file and applies it via the embedded @pnpm/patch-package library. The applyPatchToDir function at patching/apply-patch/src/index.ts:12-13 calls process.chdir(opts.patchedDir), setting the working directory to the installed package location deep inside node_modules/.pnpm/.

The patch parser at @pnpm/patch-package/dist/patch/parse.js:88 extracts file paths from diff --git a/(.*?) b/(.*?) headers using a regex with no path sanitization. The executeEffects function in apply.js then operates on these unsanitized paths:

File write (apply.js:35-49):

case 'file creation': {
  const eff = effect
  fs.ensureDirSync(dirname(eff.path))
  fs.writeFileSync(eff.path, fileContents, { mode: eff.mode })
  break
}

File delete (apply.js:13-22):

case 'file deletion': {
  const eff = effect
  // TODO: integrity checks
  if (!opts.dryRun) {
    fs.unlinkSync(eff.path)
  }
  break
}

A path like ../../../../../../../../../../home/user/.ssh/authorized_keys in the patch header traverses out of the package directory to an arbitrary location.

Proof of Concept

# Write variant:
bash autofyn_audit/exploits/vuln6_patch_traversal_write/exploit.sh
# Result: PASS -- /tmp/vuln6_pwned created with attacker-controlled content

# Delete variant:
bash autofyn_audit/exploits/vuln7_patch_traversal_delete/exploit.sh
# Result: PASS -- /tmp/vuln7_target deleted by malicious patch

# Combined chain (delete + replace SSH authorized_keys):
bash autofyn_audit/exploits/chain2_patch_ssh_backdoor/exploit.sh
# Result: PASS -- authorized_keys replaced with attacker's public key

Impact

Arbitrary file write and delete as the user running pnpm install, limited to paths writable by that user. An attacker who submits a PR adding a .patch file and patchedDependencies config can target SSH authorized_keys, shell configuration, CI/CD files, or other writable files. Patch files may receive less review scrutiny than package.json changes because the ../ traversal sequences are in diff --git headers that look like patch metadata.

Suggested Remediation

Validate parsed patch file paths against the package root directory. Reject any path that resolves outside the patched package directory via path.resolve + prefix check. Alternatively, sanitize at parse time by rejecting paths containing .. components in parse.js.

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Discovered by AutoFyn Full audit report: audit_report.md Exploit script: exploit.sh