What causes "Server nonce does not begin with client nonce" in mongodb/node-mongodb-native?

Connect directly to the MongoDB host, bypassing any proxy/VPN that could alter payloads.

mongodb/node-mongodb-native · error · MongoRuntimeError

Server nonce does not begin with client nonce

Error message

Server nonce does not begin with client nonce

What it means

Thrown during MONGODB-AWS SASL exchange when the first 32 bytes of the server's nonce do not equal the 32-byte client nonce the driver sent. Per the cross-driver auth spec, the server nonce must begin with the client nonce; a mismatch signals a protocol violation or potential attack, so the driver aborts.

Solutions

Connect directly to the MongoDB host, bypassing any proxy/VPN that could alter payloads.
Confirm the target is an authentic MongoDB deployment supporting MONGODB-AWS.
Gather driver version + server hello and file a bug if reproducible against real MongoDB.

Defensive patterns

Strategy: try-catch

Try / catch

try {
  await client.connect();
} catch (e) {
  if (e instanceof MongoRuntimeError && /Server nonce does not begin/.test(e.message)) {
    // protocol/security violation; do not retry against same endpoint blindly
  }
  throw e;
}

Prevention

Ensure a direct, unmodified path to the MongoDB server.
Treat nonce mismatches as security-sensitive; investigate the endpoint.
Do not retry-loop without diagnostics; the cause is server/proxy-side.

When it happens

Trigger: ByteUtils.equals(serverNonce.subarray(0,32), clientNonce) returns false. Fires at mongodb_aws.ts:90. A server-side response-construction bug or payload corruption is the usual cause.

Common situations: Intermediate proxy rewriting the conversation. A misbehaving MongoDB-compatible service. Extremely rare in normal operation against real MongoDB.

Related errors

AI-assisted analysis of mongodb/node-mongodb-native@dce7939f86 (2026-08-11). Data as JSON: /api/errors/2692174a3104ac8e. Report an issue: GitHub.

Appendix: source

Thrown at src/cmap/auth/mongodb_aws.ts:90

    const saslStartResponse = await connection.command(ns(`${db}.$cmd`), saslStart, undefined);

    const serverResponse = BSON.deserialize(saslStartResponse.payload.buffer, bsonOptions) as {
      s: Binary;
      h: string;
    };
    const host = serverResponse.h;
    const serverNonce = serverResponse.s.buffer;
    if (serverNonce.length !== 64) {
      // TODO(NODE-3483)
      throw new MongoRuntimeError(`Invalid server nonce length ${serverNonce.length}, expected 64`);
    }

    if (!ByteUtils.equals(serverNonce.subarray(0, nonce.byteLength), nonce)) {
      // throw because the serverNonce's leading 32 bytes must equal the client nonce's 32 bytes
      // https://github.com/mongodb/specifications/blob/master/source/auth/auth.md#conversation-5

      // TODO(NODE-3483)
      throw new MongoRuntimeError('Server nonce does not begin with client nonce');
    }

    if (host.length < 1 || host.length > 255 || host.indexOf('..') !== -1) {
      // TODO(NODE-3483)
      throw new MongoRuntimeError(`Server returned an invalid host: "${host}"`);
    }

    const body = 'Action=GetCallerIdentity&Version=2011-06-15';
    const headers = await aws4Sign(
      {
        method: 'POST',
        host,
        region: deriveRegion(serverResponse.h),
        service: 'sts',
        headers: {
          'Content-Type': 'application/x-www-form-urlencoded',
          'Content-Length': body.length,
          'X-MongoDB-Server-Nonce': ByteUtils.toBase64(serverNonce),

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