Summary
Email forging and replay attacks represent significant threats to email security and sender reputation. Forging, often referred to as email spoofing, involves crafting emails to appear as if they originate from a legitimate, trusted source. Replay attacks, on the other hand, involve intercepting a legitimate email, sometimes with a valid DKIM signature, and re-sending it to achieve malicious goals. Understanding these attack vectors is crucial for protecting your brand and ensuring your legitimate emails reach the inbox.
Key findings
- Email spoofing: Attackers fabricate email headers to deceive recipients, making an email appear to come from a different, often trusted, source.
- SPF passes: SPF (Sender Policy Framework) might pass for a forged email if the attacker's sending IP is authorized by their own return path domain, even if it's not authorized for your domain. This highlights the need for alignment checks provided by DMARC.
- Replay attacks: Malicious actors intercept legitimate, often authenticated, emails and resend them. These attacks sometimes involve reusing valid DKIM (DomainKeys Identified Mail) signatures from the original email.
- Compromised IPs: In some cases, the sending IPs used in forging attempts are owned by the spammers themselves or are part of a compromised network. These can exhibit 'snowshoe' characteristics, using many IPs across a broad range to evade detection.
- DMARC's role: DMARC (Domain-based Message Authentication, Reporting & Conformance) is critical for identifying and mitigating these attacks because it checks for alignment between the 'From' domain and the domains authenticated by SPF or DKIM. If alignment fails, DMARC can instruct receiving servers on how to handle the message.
Key considerations
- Implement DMARC: Ensure your domain has a DMARC policy in place, even if initially set to p=none. This allows you to monitor for unauthorized use of your domain and move towards an enforcement policy (like p=quarantine or p=reject) over time. You can learn more about how to safely transition your DMARC policy.
- Monitor DMARC reports: Regularly review your DMARC aggregate reports to identify sources attempting to send email on behalf of your domain that are not authorized. This helps in detecting forging attempts and suspicious activity. These reports are designed to help you detect and mitigate advanced phishing attacks.
- Strengthen SPF and DKIM: Ensure your SPF and DKIM records are correctly configured and updated for all legitimate sending sources. A robust email authentication setup is the foundation for DMARC enforcement.
- Educate users: Train your employees and customers to recognize signs of phishing and spoofing, such as mismatched sender domains, suspicious links, or unusual requests.
What email marketers say
Email marketers often face the direct impact of forging and replay attacks, which can harm their sender reputation, increase spam rates, and reduce campaign effectiveness. The discussion among marketers highlights the confusion and concern when discovering unauthorized email activity using their domain, especially from unfamiliar IPs or domains. They seek reassurance that their existing authentication measures are sufficient and discuss practical steps to mitigate harm, particularly during critical sending periods like Black Friday.
Key opinions
- Initial confusion: Marketers often struggle to identify the source and nature of suspicious email activity, questioning if unknown SPF domains or IPs indicate an attack.
- DMARC as primary defense: Many marketers rely on DMARC for its ability to enforce domain alignment and block forged emails, even if their policy is initially set to p=none.
- Impact on deliverability: Concerns arise regarding how such attacks might affect their email deliverability, especially when preparing for major sending events.
- IP ownership: There's a recognition that suspicious sending IPs are often owned by spammers, complicating traditional blocklisting efforts.
Key considerations
- Review SPF alignment: Even if SPF authentication passes, ensure the Return-Path domain (used for SPF checks) aligns with your 'From' domain. If not, DMARC will fail. This is crucial for understanding why emails might be getting to spam, as detailed in our guide on handling spoofed emails violating DMARC policies.
- Understand replay specifics: While DKIM should generally pass in a replay attack (as the signature is copied), if it fails, it may indicate manipulation or an expired signature. For more information, read about symptoms of a DKIM replay attack.
- Leverage DMARC reports: Use DMARC aggregate reports to quickly spot unauthorized sending IPs and domains attempting to spoof your brand. This proactive monitoring helps identify attacks early. The Purdue University Knowledge Base offers insights into identifying and dealing with email spoofs.
- Increase DMARC policy enforcement: Gradually move from p=none to p=quarantine or p=reject to strengthen protection against unauthorized email. You can find guidance on mitigating damage from email spoofing.
Marketer view
Email marketer from Email Geeks notes that SPF authentication may pass because the check is on the return path, but it will likely fail alignment, which is where DMARC becomes crucial.
Marketer view
Email marketer from Stack Overflow suggests preventing replay attacks by incorporating timestamps and unique one-time nonces into data, and implementing encryption or hashing.
What the experts say
Email deliverability experts consistently emphasize the foundational role of email authentication protocols like SPF, DKIM, and DMARC in combating email forging and replay attacks. They note that while SPF might pass for unauthorized senders due to return path configuration, DMARC's alignment checks are essential for detecting and blocking spoofed emails. Experts also highlight the evolving nature of these attacks, including the complexities of DKIM replay, and stress the importance of continuous monitoring and a robust DMARC enforcement strategy to protect email ecosystems.
Key opinions
- DMARC is key: Experts agree that DMARC is the most effective protocol for identifying and mitigating email forging and spoofing, particularly through its alignment requirements.
- SPF nuances: While SPF can pass authentication for a message, its utility against forging is limited without DMARC alignment, as the 'From' address can still be spoofed.
- Replay attack challenges: DKIM replay attacks are a specific concern, as a legitimate signature can be reused, making detection more complex.
- Proactive monitoring: Regular analysis of DMARC reports is essential to detect and respond to unauthorized email activity in real-time.
Key considerations
- Enforce DMARC: Move your DMARC policy to p=quarantine or p=reject as soon as feasible to prevent spoofed emails from reaching inboxes. Our guide explains how to safely transition your DMARC policy.
- Monitor for unusual activity: Pay attention to DMARC reports showing sending IPs or domains you don't recognize, particularly those with high volumes or unexpected SPF/DKIM authentication outcomes. DuoCircle notes that including timestamps and nonces can prevent replay attacks.
- Implement strong authentication: Beyond SPF and DKIM, consider implementing other security measures to protect against compromised accounts, which can be a source of replay attacks. Our advanced guide covers email authentication beyond the basics.
- Address DMARC failures: Investigate all DMARC failures to determine if they are legitimate but misconfigured sending sources or malicious forging attempts. This proactive approach helps to improve deliverability and protect your sender reputation. For instance, understanding why your emails get DMARC verification failed errors is key.
Expert view
Email expert from SpamResource explains that DMARC is essential for identifying and stopping unauthorized emails that spoof your domain, by requiring alignment of the 'From' header with authenticated domains.
Expert view
Email expert from Word to the Wise notes that a common misconception is that SPF alone can prevent spoofing; however, it only authenticates the return-path domain, leaving the visible 'From' address vulnerable without DMARC.
What the documentation says
Technical documentation provides comprehensive definitions and strategies for identifying and preventing email forging and replay attacks. These resources delve into the mechanisms of spoofing, the specifics of how authentication protocols like SPF, DKIM, and DMARC function, and how they contribute to email security. They offer structured guidance on implementing protective measures, recognizing attack patterns, and ensuring the integrity of email communications within various systems.
Key findings
- Email spoofing definition: It is the fabrication of an email header to trick recipients into believing the email originated from a different, often legitimate, source.
- Replay attack mechanism: Attackers intercept valid data (like session cookies or DKIM-signed emails) and resend them to bypass authentication or achieve malicious objectives.
- Authentication vulnerabilities: Even with authentication, systems can be vulnerable if proper checks (like DMARC alignment) are not in place, or if mechanisms like timestamps and nonces are not used to prevent replay.
- DMARC's effectiveness: DMARC provides a framework for domain owners to specify how receiving servers should handle emails that fail SPF or DKIM authentication and alignment, thereby directly addressing spoofing.
Key considerations
- Implement DMARC policies: Use DMARC with a policy of 'quarantine' or 'reject' to instruct mail servers to reject or quarantine unauthorized emails. This is a primary defense against email forging. Barracuda Networks provides a clear glossary on email spoofing.
- Use timestamps and nonces: To prevent replay attacks, incorporate unique, time-sensitive identifiers (timestamps or nonces) into email headers or bodies, making each email unique and preventing malicious re-sending. DuoCircle offers insights into DKIM replay attack prevention.
- Maintain authentication records: Regularly verify that SPF and DKIM records are accurate and cover all legitimate sending sources. Errors in these records can inadvertently lead to DMARC failures, even for legitimate mail.
- Monitor for anomalous behavior: Actively monitor email logs and DMARC reports for signs of spoofing or replay attempts, such as high volumes of unauthenticated mail from unexpected sources. Cisco Blogs provides information on what email spoofing is and how to detect it.
Technical article
Documentation from Microsoft Security Blog highlights how stolen session cookie replay attacks allow attackers to impersonate users by reusing valid stolen cookies, effectively bypassing authentication mechanisms.
Technical article
Documentation from Purdue University explains that email spoofing involves creating email messages with a forged sender address, intending to trick the recipient into providing sensitive information or money.
Related resources
12 resources
Related pages
What should i do if my email domain gets spoofed?
How to identify and handle spoofed emails violating DMARC policies?
How do i troubleshoot DMARC failures and potential DKIM replay attacks affecting email deliverability?
What steps can i take to mitigate damage from email spoofing and prevent future occurrences?
What are the symptoms of a DKIM replay attack and how can a compromised account be identified?
A simple guide to DMARC, SPF, and DKIM
How to safely transition your DMARC policy to quarantine or reject
Why your emails are getting a 'DMARC verification failed' error
An advanced guide to email authentication - beyond the basics
How to identify and handle email forging and replay attacks?
