PowerDMARC

DMARC Bypass – Risks, Methods & Prevention

Milena Baghdasaryan
DMARC bypass

According to PowerDMARC’s 2024 DMARC Statistics Report, DMARC adoption worldwide remains low, with only 33.4% of domains having a valid DMARC record in place. Even those with DMARC implemented often fall victim to sophisticated attacks. 

DMARC bypass is a method used by cybercriminals to evade DMARC protection, allowing them to send spoofed emails that appear to come from a trusted domain, even when that domain has DMARC enabled. Misconfigured SPF, DKIM, or DMARC records and email forwarding practices can undermine the effectiveness of the protocol, making it easier for threat actors to bypass DMARC. These bypasses can lead to phishing attacks, data breaches, financial loss, reputational damage, and deliverability issues.

Key Takeaways

  • DMARC isn’t foolproof, and cybercriminals can still bypass protections by exploiting weak policies and gaps in forwarding mechanisms.
  • Mailing lists may unintentionally break DKIM or SPF, which can enable spoofed emails to slip through DMARC checks. 
  • ARC can be misused to preserve falsified authentication results on forwarded emails. 
  • You can detect DMARC bypass attempts by monitoring your DMARC reports, analyzing email headers, and checking suspicious subdomains. 
  • Properly setting up SPF and DKIM, and using strict DMARC policies may help prevent DMARC bypasses.

How Attackers Bypass DMARC (Top Methods)

Here are some of the most common methods attackers use to bypass DMARC.

Misconfigured Authentication Records

Misconfigured authentication records refer to errors or incomplete setups in a domain’s SPF, DKIM, or DMARC records that weaken email authentication. Common issues include:

These misconfigurations create loopholes that attackers can exploit to bypass DMARC and spoof emails.

Exploiting SPF Weaknesses

Attackers may exploit overly permissive SPF records, especially those that include several third-party services with limited control. By sending emails from these authorized IPs, they can spoof the domain while still passing SPF authentication.

DKIM Vulnerabilities

Attackers may exploit systems that improperly sign headers or modify messages in transit, breaking DKIM signature validation or alignment. This, in turn, can cause DKIM to fail or become misaligned with the From domain, which can undermine DMARC protection.

Subdomain Spoofing

DMARC policies are inherited by subdomains only if they don’t have their own DMARC record. Unsecured subdomains are another key gap that can easily be exploited. These subdomains often lack enforced DMARC policies or are excluded from the parent domain’s sp policy. Such subdomains may be hijacked and used to send spoofed emails. 

Forwarding Scenarios and Mailing Lists

Email forwarding often breaks SPF, and some mailing lists may alter message content, which can invalidate DKIM. If neither SPF nor DKIM aligns with the ‘From’ domain, DMARC will fail. Improper handling of forwarded mail can create deliverability issues or lead to false positives.

DMARC Policy Gaps 

Organizations that use weak DMARC policies like “p=none” fail to block suspicious messages. This allows unauthenticated emails to be delivered despite DMARC failures, offering no protection against spoofing. In May of 2024, the FBI, the U.S. Department of State, and the National Security Agency (NSA) issued a joint advisory warning about exploitation surrounding permissive DMARC policies to launch spearphishing attacks. 

Lookalike or Cousin Domain Spoofing

Hackers often use homograph attacks by registering domains with lookalike characters (e.g., “examp1e.com”). Since these are entirely separate domains, they are not covered by the target domain’s DMARC policy, effectively evading its protection.

How to Detect DMARC Bypass Attempts

Here are some useful strategies to detect DMARC bypass attempts. 

Monitor DMARC Reports 

Regularly review your DMARC aggregate (RUA) and forensic (RUF) reports. These can help you detect unusual patterns like SPF or DKIM authentication failures and domain misalignments. Such patterns may be indicative of bypass attempts or domain abuse. Detecting them in advance can help prevent future threats. 

⚠️Not all receivers send forensic reports.  

Check for Suspicious Subdomains

Regularly audit your DNS configuration and monitor for unauthorized subdomains using tools like passive DNS, certificate transparency logs, and subdomain enumeration services. This helps detect shadow IT or spoofable subdomains not protected by DMARC.

Analyze Email Headers

Analyzing your email headers can help you detect inconsistencies in SPF and DKIM authentication. Look at specific fields like Authentication-Results, Received-SPF, and DKIM-Signature. Note that alignment (not just result pass/fail) is what DMARC checks. 

How to Prevent DMARC Bypass (Actionable Fixes)

DMARC bypasses pose serious risks, but there are effective ways to reduce exposure.

Strengthen SPF

When you use a  hardfail or softfail policy (~all/-all) in your SPF record, it will help explicitly flag unauthorized senders as SPF fail. The neutral mechanism ?all does not instruct mail servers to take enforcement action, potentially allowing unauthorized emails to be accepted. Regularly audit IPs and third-party services listed in your SPF record to ensure only authorized sources are included.

Secure DKIM

Rotating DKIM keys periodically, ideally every 3 to 6 months, helps limit exposure if a key is compromised. Also, use strong 2048-bit DKIM keys instead of outdated 1024-bit keys whenever possible.

Harden DMARC Policies

p=none offers monitoring with no preventive action. On the contrary, p=quarantine instructs to send unauthorized emails to junk or spam, while p=reject blocks them outright. 

So while you can start with p=none in the initial stages to avoid delivery and deliverability issues, ensure to transition to stricter policies as soon as you are ready. Also, make sure you cover all subdomains by setting the sp tag appropriately. This tag helps specify what the policy should be for ALL of that domain’s subdomains.

You can verify your DMARC setup anytime using our free DMARC checker.

Additional Protections

Last but not least, implement BIMI (Brand Indicators for Message Identification). This will help add a layer of visual trust for recipients. Note that BIMI requires DMARC enforcement (p=reject/quarantine) to be configured and a SVG logo with specifications. 

FAQs

Can DMARC be 100% bypass-proof?

No, DMARC cannot guarantee absolute protection against all bypass attempts. But there are several steps you can take to reduce the risk of successful DMARC bypasses, spoofing, and phishing. These include:

Even with proper authentication and enforcement, DMARC is only effective against direct-domain spoofing attempts and cannot prevent lookalike or cousin domains.

Can forwarded emails create a DMARC bypass opportunity?

Forwarded emails can sometimes cause DMARC failures. Many forwarding services do not preserve the original SPF or DKIM authentication. As a result, the alignment that DMARC requires might break. 

This issue can sometimes be mitigated using the ARC (Authenticated Received Chain) protocol, which helps preserve authentication results across forwarding.

Is p=reject completely bypass-proof?

The short answer is no. p=reject blocks unauthorized emails more effectively than p=none or p=quarantine. However, p=reject too is not completely foolproof. Unsecured third-party services or compromised subdomains make it more vulnerable to bypasses. 

Is DMARC bypass easier than bypassing SPF or DKIM alone?

No, DMARC bypass is often harder than bypassing SPF or DKIM individually. The reason is quite straightforward; DMARC passes if either SPF or DKIM passes with proper alignment, which makes it more resilient than relying on either protocol alone. This means that, with DMARC, hackers have to overcome more layers of authentication. 

Summing Up

Simply setting up DMARC isn’t enough to protect your domain. You need to combine DMARC with other email security tools and techniques. Also, always monitor reports, update policies, and audit your DMARC setup. When everything is correctly configured and properly monitored, it’s much harder for hackers to bypass DMARC. 

PowerDMARC offers a complete package of tools and services for your DMARC and other email authentication protocols. Get in touch with our team of experts to strengthen your defenses and stay protected today and into the future.

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