node-opcua missing nonce verification in UserNameIdentityToken authentication

Summary
A missing nonce verification in the UserNameIdentityToken authentication handler allows an unauthenticated remote attacker to forge a password token that extracts as an empty string, and to replay captured authentication tokens across sessions.

Affected versions: <= 2.165.0
Tested version: 2.165.0
CVSS Score: 8.1 (High)
CVSS Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:L
CWE: CWE-347 Improper Verification of Cryptographic Signature

Root Cause

In packages/node-opcua-server/source/opcua_server.ts at line 1886-1887, after RSA-OAEP decrypting the UserNameIdentityToken password blob, the server reads a 4-byte little-endian length field and extracts buff[4 : 4+length] as the password. It never verifies that the trailing bytes equal session.nonce.

This has two consequences:

1. Forged empty password: An attacker who retrieves the server's public key via an unauthenticated GetEndpoints call can craft a token where the 4-byte length field equals serverNonce.length (32). The server computes length = 32 - 32 = 0 and calls isValidUser(username, ""). Any account that accepts an empty password is compromised.

2. Unconditional replay attack: Because nonce binding is structurally absent, any captured UserNameIdentityToken ciphertext can be replayed in a different session unconditionally.

The impact is compounded by a second issue: when the channel uses SecurityMode=None, verifyClientSignature returns true unconditionally (security_policy.ts:697-700), bypassing the channel-level signature check entirely.

Proof of Concept (logic, no exploit code)

 11. GetEndpoints (unauthenticated) → retrieve server public key and RSA token policy
 22. OpenSecureChannel (SecurityMode=None)
 33. CreateSession
 44. Craft plaintext: [0x20, 0x00, 0x00, 0x00]  (readUInt32LE = 32 = serverNonce.length)
 55. RSA-OAEP encrypt with server public key → 256-byte ciphertext
 66. ActivateSession with crafted UserNameIdentityToken
 77. Server decrypts → length = 32 - 32 = 0 → password = ""
 88. isValidUser(username, "") is called

Dynamically confirmed: decryption produces password = "" with no error and no nonce verification.

Suggested Fix

After decrypting the password blob, verify that buff.slice(4 + passwordLength) equals session.nonce before extracting the password. Reject the token if verification fails.

I am following a 90-day responsible disclosure policy. I am happy to provide additional technical details under embargo. Please confirm receipt at your earliest convenience.

Reporter: Stanley Tobias
Discovery date: 2026-03-23