Blog | G5 Cyber Security

DMARC SPF Spoofing: How it Works

G5 Cyber Security

TL;DR

Yes, DMARC’s SPF alignment can be spoofed, but it requires specific conditions and isn’t a simple task. Attackers typically need control over subdomains or the ability to send emails that appear to originate from legitimate domains (e.g., through open relays or compromised accounts). Understanding how SPF works and its limitations is crucial for effective DMARC implementation.

Understanding SPF Alignment

DMARC uses SPF alignment to verify if an email was sent from a server authorized by the domain owner. SPF (Sender Policy Framework) specifies which mail servers are permitted to send emails on behalf of your domain. Alignment means that the ‘From’ address in the email matches the domain used for SPF checks.

How Spoofing Can Occur

  1. Subdomain Control: An attacker controlling a subdomain can publish an SPF record for that subdomain allowing emails to be sent from any server. If they then send an email with a ‘From’ address that includes the main domain (e.g., user@example.com) but originates from the attacker’s controlled subdomain, SPF alignment can pass if your DMARC policy isn’t strict enough.
  2. Open Relays/Compromised Accounts: If an open mail relay exists or an email account is compromised, attackers can send emails appearing to come from legitimate addresses without needing control of the domain’s DNS records. This bypasses SPF if the relay doesn’t enforce SPF checks or the compromised account has valid sending permissions.
  3. Forwarding Loops: Emails forwarded through multiple servers can sometimes alter the ‘From’ address and headers, potentially leading to misaligned SPF results.

Step-by-Step Guide to Mitigating Spoofing Risks

  1. Implement a Strict DMARC Policy: Start with a monitoring policy (v=DMARC1; p=none;) to gather data, then move to quarantine (v=DMARC1; p=quarantine;) and finally reject (v=DMARC1; p=reject;). Rejecting is the strongest protection.
  2. Review SPF Records Regularly: Ensure your SPF records only list authorized sending sources. Remove any unnecessary entries. A typical record might look like this: 
    v=spf1 include:_spf.google.com mx a ip4:192.0.2.0/24 ~all
  3. Use DKIM (DomainKeys Identified Mail): DKIM provides cryptographic authentication and is more resistant to spoofing than SPF alone. Implement DKIM alongside SPF for stronger email security.
  4. Monitor DMARC Reports: Regularly analyze DMARC aggregate reports to identify potential issues, such as unauthorized sending sources or misconfigured SPF records. These reports will show you which servers are attempting to send emails on your behalf and whether they’re passing or failing authentication checks.
  5. Subdomain Control Checks: Audit your subdomains regularly to ensure no rogue subdomains are being used for malicious email activity.
  6. Account Security Best Practices: Enforce strong passwords, multi-factor authentication (MFA), and regular security awareness training for all email accounts to prevent compromise.

Checking SPF Records

You can use online tools or command-line utilities to check your SPF records:

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