#MasteringEmailSecurity: Understanding DMARC to Prevent Spoofing Attacks
#MasteringEmailSecurity: Understanding DMARC to Prevent Spoofing Attacks
Introduction
Email spoofing is a common attack method where cybercriminals disguise an email to make it appear as though it is sent from a trusted domain. Without proper security measures like DMARC (Domain-based Message Authentication, Reporting & Conformance), attackers can exploit domain vulnerabilities, allowing malicious emails to bypass security checks. In this post, we’ll break down a real-world example of an attack and provide actionable steps to secure your domain.
Scenario Overview
Let’s consider a scenario where an attacker successfully spoofs an email using the victim’s domain due to insufficient DMARC policy enforcement.
- Attacker Domain: attacker.com
- Victim Domain: victim.com
- Attack Method: The attacker sends an email that looks as though it originates from a legitimate email account at victim.com, but it is spoofed.
- Victim’s DMARC Policy: No DMARC policy is in place, allowing emails to bypass security checks and get delivered.
In this case, the attacker is able to pass SPF and DKIM validation checks by manipulating the email but fails DMARC alignment. However, due to the absence of a strict DMARC policy, the email is still delivered.
Step-by-Step Breakdown of the Attack
1. Attacker Spoofs the Email from victim.com
The attacker sends a malicious email that appears to be from support@victim.com. To the recipient, it looks like the email comes from a legitimate source. The "From" address is manipulated to display the trusted domain, victim.com, but in reality, the email originates from the attacker’s domain, attacker.com.
Example of the spoofed email:
From: support@victim.com
To: user@example.com
Subject: Important Update: Action Required
[malicious content]
2. SPF and DKIM Pass, But Alignment Fails
The attacker sets up the email so that it passes SPF and DKIM validation, which are crucial email authentication protocols:
- SPF checks if the email is sent from an authorized IP for the domain. The attacker ensures their email passes by sending the email from an IP authorized by attacker.com.
- DKIM confirms the message's integrity by adding a digital signature, which the attacker sets up to pass for attacker.com.
However, DMARC alignment fails because the "From" domain (victim.com) does not align with the domains used for SPF and DKIM checks (attacker.com). DMARC ensures that the domain in the "From" field matches the domains verified by SPF/DKIM, which it doesn’t in this case.
3. Lack of DMARC Policy Allows Delivery
Despite the failed DMARC alignment, the email is still delivered to the recipient because victim.com has not implemented a DMARC policy to enforce actions on failed messages. Without a strict policy (e.g., p=reject), the email bypasses filters and reaches the user’s inbox.
Why DMARC is Critical in this Scenario
DMARC is an email authentication protocol designed to protect domain owners from unauthorized use of their domain (email spoofing). It adds an important layer of protection on top of SPF and DKIM by checking if the domains used in SPF/DKIM align with the domain in the "From" header of the email. If this alignment fails, DMARC can block or quarantine the email based on the configured policy.
The Solution: Implementing a Strict DMARC Policy
To prevent this type of attack, victim.com needs to implement a DMARC policy to enforce strict validation. Here's a step-by-step guide to how this can be done.
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1. Initial DMARC Setup
Start by setting up DMARC in monitoring mode with a p=none policy. This will not block any emails but will generate reports on how emails from your domain are being handled:
v=DMARC1; p=none; rua=mailto:dmarc-reports@victim.com
This allows you to observe the domain’s email flows and detect any issues in SPF/DKIM alignment without impacting email delivery.
2. Ensure Proper SPF and DKIM Alignment
Make sure that the SPF and DKIM records for victim.com are properly configured. Ensure that the domains used in SPF and DKIM checks match the "From" address domain in your legitimate emails.
For example, if your emails are sent from support@victim.com, ensure that:
- SPF authorizes legitimate IP addresses to send emails on behalf of victim.com.
- DKIM uses a signature tied to victim.com.
3. Move to a Stricter DMARC Policy
Once you have ensured that your email flows are correctly aligned with SPF and DKIM, move to a quarantine or reject DMARC policy to enforce stronger security:
- Quarantine Policy (`p=quarantine`): Suspicious emails that fail DMARC will be sent to the spam or junk folder instead of the inbox.
v=DMARC1; p=quarantine; rua=mailto:dmarc-reports@victim.com
- Reject Policy (`p=reject`): This is the most stringent policy, which will block any email that fails DMARC validation from being delivered.
v=DMARC1; p=reject; rua=mailto:dmarc-reports@victim.com
4. Monitor and Adjust Regularly
Set up DMARC reporting to receive feedback on email authentication results. This helps track emails that pass or fail DMARC checks and provides insights into potential threats.
Example setup for receiving reports:
rua=mailto:dmarc-reports@victim.com; ruf=mailto:dmarc-failures@victim.com
Conclusion
Without a properly configured DMARC policy, domains like victim.com are vulnerable to email spoofing attacks, even if SPF and DKIM are in place. In this scenario, the attacker was able to exploit the absence of a strict DMARC policy to spoof the victim’s domain and deliver malicious emails to unsuspecting recipients.
Implementing a strong DMARC policy, ensuring proper SPF/DKIM alignment, and regularly monitoring your domain’s email authentication flows are essential steps to prevent these types of attacks. By enforcing a strict DMARC policy, you can stop spoofed emails from being delivered and protect your domain's reputation.
Key Takeaways:
- Always set up DMARC with at least a monitoring policy (`p=none`) to gather insights into your email flows. (All though with this policy you are still vulnerable to attacks)
- Ensure that SPF and DKIM records align with the domain used in the "From" header of your emails.
- Move towards a stricter DMARC policy (`p=quarantine` or p=reject) once testing is complete.
- Monitor reports regularly to stay aware of threats and fine-tune your domain’s email security.
By taking these proactive steps, you can secure your domain from email spoofing and protect your users from phishing and other malicious activities.