node-opcua is an OPC UA implementation for TypeScript and Node.js. Prior to 2.166.0, the process-global g_alreadyUsedNonce cache used by nonceAlreadyBeenUsed in packages/node-opcua-secure-channel/source/server/server_secure_channel_layer.ts records nonces from OpenSecureChannelRequest and CreateSession without expiration or a size limit. An unauthenticated remote attacker can repeatedly create sessions with unique nonces, causing entries to persist after session expiry and accumulate across connection cycles even when maxSessions=10 limits concurrent sessions. The resulting unbounded heap growth can exhaust the default Node.js heap and crash the node-opcua server process. This issue is fixed in version 2.166.0.

Affected: npm: node-opcua

**Summary** A process-global nonce cache with no eviction policy allows an unauthenticated remote attacker to exhaust server heap memory by repeatedly opening sessions, causing the node-opcua server process to crash.

**Affected versions:** <= 2.165.0 **Tested version:** 2.165.0 **CVSS Score:** 7.5 (High) **CVSS Vector:** CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H **CWE:** CWE-770 Allocation of Resources Without Limits or Throttling

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**Root Cause**

In `packages/node-opcua-secure-channel/source/server/server_secure_channel_layer.ts` at line 156, `g_alreadyUsedNonce` is a process-global object used to track previously seen nonces for replay detection. Entries are added on every `OpenSecureChannelRequest` and every `CreateSession` request but are never removed or expired.

An unauthenticated attacker can exploit the `CreateSession` path (which requires no certificate) to accumulate nonce entries indefinitely. Even with `maxSessions=10` limiting concurrent sessions, nonces persist after session expiry, allowing slow but reliable heap exhaustion across repeated connection cycles.

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**Measured Impact**

Dynamically confirmed heap growth: - 5,000 unique nonces → +1.23 MB resident heap, no eviction after explicit GC - Projected: 10^6 nonces → ~246 MB resident heap - Achievable OOM on default Node.js heap limits

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**Suggested Fix**

Add a TTL-based eviction policy to `g_alreadyUsedNonce`. Nonces should be expired after the maximum session timeout (or a reasonable fixe…