pnpm Has an Integrity Check Bypass via Missing Lockfile Integrity Field

Summary

pnpm's tarball extraction worker skips integrity verification when the integrity field is absent from the lockfile resolution. If an attacker can both modify pnpm-lock.yaml to remove the integrity: field and cause the referenced registry URL to serve altered package content, pnpm install --frozen-lockfile can install the altered package without an integrity error. npm's npm ci enforces integrity by default; pnpm's behavior of silently skipping verification is a pnpm-specific fail-open gap.

Vulnerability Details

The addTarballToStore function in worker/src/start.ts (lines 189-204) checks if (integrity) before verifying the tarball hash. The TarballResolution type declares integrity as optional (integrity?: string). When the lockfile omits the integrity field, the guard evaluates to false, skipping hash verification entirely. The worker then computes a new hash from the unverified content and stores it as legitimate.

// worker/src/start.ts:189-204
function addTarballToStore ({ buffer, storeDir, integrity, ... }: TarballExtractMessage) {
  if (integrity) {           // false when integrity is undefined
    const { algorithm, hexDigest } = parseIntegrity(integrity)
    const calculatedHash = crypto.hash(algorithm, buffer, 'hex')
    if (calculatedHash !== hexDigest) {
      return { status: 'error', error: { type: 'integrity_validation_failed', ... } }
    }
  }
  return {
    status: 'success',
    value: { integrity: integrity ?? calcIntegrity(buffer) },
  }
}

Proof of Concept

bash autofyn_audit/exploits/vuln1_integrity_bypass/exploit.sh
# Publishes a package, generates lockfile, republishes tampered version,
# strips integrity field, re-runs install --frozen-lockfile.
# Result: PASS -- tampered package installed without integrity error.

Impact

Supply chain compromise in environments where an attacker can both alter the lockfile and cause the referenced registry URL to serve altered package content. The --frozen-lockfile flag does not fail closed when the integrity field is missing.

Suggested Remediation

Require an integrity field for remote tarball resolutions. Change the if (integrity) guard to fail when integrity is absent for non-local packages. When --frozen-lockfile is active, reject lockfile entries that lack integrity for remote packages.

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