CVE-2026-54155

**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)** ``` 1. GetEndpoints (unauthenticated) → retrieve server public key and RSA token policy 2. OpenSecureChannel (SecurityMode=None) 3. CreateSession 4. Craft plaintext: [0x20, 0x00, 0x00, 0x00] (readUInt32LE = 32 = serverNonce.length) 5. RSA-OAEP encrypt with server public key → 256-byte ciphertext 6. ActivateSession with crafted UserNameIdentityToken 7