Summary
DKIM replay attacks occur when malicious actors reuse legitimate, DKIM-signed emails to send fraudulent messages that bypass authentication checks. Identifying these attacks and discerning them from a simple compromised account requires careful observation of email authentication reports and sending patterns. While a compromised account might lead to unauthorized sending, a DKIM replay attack specifically leverages a valid DKIM signature, making it particularly insidious because the emails appear legitimate to receiving mail servers. Understanding the subtle differences and key indicators is crucial for timely detection and mitigation.
Key findings
- Volume surge: A significant and sudden increase in email volume from a specific DKIM domain, often 5 to 10 times the normal rate, is a strong indicator of a replay attack. This can be observed in tools like Google Postmaster Tools.
- DMARC reports: While a replay attack might bypass DMARC checks if the original signature is valid, you may still see unusual DMARC reports from recipient domains. This could include reports for your customers or domains you are signing for, even if your own DMARC reports show nothing amiss. Consider reviewing DMARC reports carefully.
- IP reputation drop: Reports of poor IP reputation from sending IPs that are not explicitly yours, but are sending on behalf of your domain or your customers' domains, can indicate a compromise or replay activity.
- Unusual content: Phishing or spam content appearing to come from your legitimate domain, even if authentication passes, is a clear sign of an issue.
- Compromised accounts: A DKIM replay attack often stems from a compromised account or system that allows attackers to intercept and resend legitimate emails. Identifying if a client's account has been compromised is a critical first step.
Key considerations
- BCC headers: The BCC header is typically not transmitted to the receiving SMTP server, meaning signing it won't prevent a replay attack. Focus should be on other aspects of the email header and body for protection.
- DKIM rotation: Rotating your DKIM keys can help mitigate ongoing replay attacks by invalidating old signatures, but it doesn't address the root cause of the compromise if one exists.
- DMARC policy: Implementing a strict DMARC policy (p=quarantine or p=reject) can help receiving servers handle emails that fail DMARC, even if they have a valid DKIM signature from a replay.
- Email header analysis: Thoroughly examining email headers for anomalies, such as unexpected ARC-Seal headers or routing paths, can provide clues to a replay attack or compromise.
- User awareness: Educating users to be wary of suspicious emails, even if they appear legitimate, is crucial, as replay attacks often precede phishing attempts.
What email marketers say
Email marketers often find themselves on the front lines when a domain or sending infrastructure is under attack, as deliverability rates and brand reputation are directly impacted. Their observations provide practical insights into the symptoms of unusual sending activity and the immediate steps taken to address them, even if the exact nature of the attack is initially unclear. The focus is often on real-time indicators such as unusual DMARC reports or a sudden decline in sending reputation.
Key opinions
- Reputation impacts: Marketers frequently notice a sudden drop in their sender reputation, with reports of IPs not belonging to them appearing in bad reputation lists. This is a primary red flag that something is amiss.
- Unexpected DMARC alerts: Receiving DMARC reports from customers or third parties showing high volumes of emails from their domain, without having received their own DMARC reports for the same activity, points to unauthorized sending or a compromise that is difficult to trace.
- Authentication complexity: When using multiple DKIM signatures (e.g., one for the sending domain and one for the client's domain), marketers note the complexity in pinpointing which signature or domain might be exploited in a replay scenario.
- Lack of direct evidence: It can be challenging for marketers to definitively classify an attack as a DKIM replay without deeper technical analysis, as symptoms often overlap with other forms of spoofing or account compromise.
Key considerations
- Client communication: Marketers recognize the importance of direct communication with affected clients to gather more information and collaborate on identifying the source of unauthorized sending.
- Proactive key rotation: Even without definitive proof of a replay attack, rotating DKIM signatures is considered a quick and necessary step to mitigate potential abuse of compromised keys. This can be done as part of general DKIM maintenance.
- Monitoring reputation: Continuous monitoring of IP and domain reputation is vital, especially when unusual sending activity is suspected. This helps identify unauthorized senders more quickly and understand the impact on deliverability.
- Investigating beyond DKIM: While DKIM is the focus, marketers also consider broader account compromise possibilities, checking for unauthorized access to their client's sending platforms or mail servers.
Marketer view
Email marketer from Email Geeks notes that they are experiencing attacks on one of their domains, suspecting a DKIM replay issue despite double-signing headers except for BCC.
Marketer view
Email marketer from Email Geeks indicates that they are seeing reports of bad IP reputation from IPs that are not their own, suggesting a problem with unauthorized sending. They also mentioned their customers are receiving large DMARC reports.
What the experts say
Email experts delve deeper into the technical nuances of email attacks, distinguishing between different types of compromises and authentication bypasses. Their insights often highlight specific technical indicators and provide a more definitive understanding of whether a DKIM replay attack is occurring or if another form of exploitation, such as an account compromise, is the root cause. They emphasize the role of tools like Google Postmaster Tools and the behavior of email headers.
Key opinions
- Volume is key: Experts strongly suggest that a true DKIM replay attack is characterized by an extremely high volume of forged emails, often an order of magnitude greater than normal sending, visible in aggregate reports.
- BCC irrelevance: The BCC header is typically not relevant to DKIM signing or replay attacks, as it's stripped before reaching the receiving SMTP server. Focusing on this header will not resolve a replay issue.
- Distinguishing attacks: Experts differentiate between DKIM replay and other forms of compromise, noting that symptoms like widespread bad IP reputation on unfamiliar IPs, without corresponding high volume on the DKIM domain, might point more towards a compromised account or third-party abuse.
- DMARC report analysis: While DKIM replay emails might pass DKIM, DMARC reports can still provide critical clues if the From domain doesn't align with the DKIM d= domain or the SPF domain.
Key considerations
- Google Postmaster Tools: This tool is a crucial resource for identifying massive volume increases associated with DKIM replay attacks. Its data can help confirm whether a large-scale spoofing event is occurring against a specific DKIM domain. Read our ultimate guide to Google Postmaster Tools V2.
- Account compromise verification: Experts advise verifying if client accounts were indeed compromised, as this often underlies DKIM replay or other spoofing techniques. This involves checking access logs and user activity.
- Double signing: While double signing (your domain's DKIM and your client's DKIM) is a common practice for ESPs, it doesn't inherently prevent replay attacks if one of the underlying keys is compromised.
- Beyond authentication: Even with perfect SPF and DKIM, compromised accounts can still send abusive mail. Experts stress the importance of monitoring content, recipient complaints, and DMARC aggregate reports (RUAs) for anomalies beyond simple authentication passes.
Expert view
Email expert from Email Geeks explains that it is difficult to assess the situation in the abstract, but a massive increase in volume for the DKIM domain in Google Postmaster Tools is a typical sign of a replay attack.
Expert view
Email expert from Word to the Wise suggests that an attacker leveraging a valid DKIM signature from a previous, legitimate email is the core of a DKIM replay attack, making detection challenging for receiving mail servers.
What the documentation says
Technical documentation and research papers offer the foundational understanding of DKIM, its vulnerabilities, and the mechanisms behind replay attacks. They detail how these attacks exploit the trust established by valid cryptographic signatures and outline the technical measures necessary for prevention and detection. This perspective is crucial for implementing robust security protocols and understanding the limitations of current email authentication standards.
Key findings
- Signature validity: DKIM replay attacks succeed because the intercepted email carries a legitimate DKIM signature, which, when replayed, still appears valid to the receiving server. This can allow phishing emails to bypass checks.
- Header and body integrity: DKIM signs specific headers and a portion of the body. If these elements remain unchanged, the signature will pass. Attackers often reuse emails where these elements are generic or can be recontextualized.
- Lack of expiry: The original DKIM specification (RFC 6376) did not include explicit mechanisms to prevent replay attacks, such as mandatory timestamping or nonces in the signed headers, making it possible for old valid signatures to be replayed.
- DMARC limitations: While DMARC relies on SPF and DKIM alignment, a DKIM replay attack can still pass DMARC if the DKIM d= domain aligns with the From header, making it difficult to detect purely from authentication results.
Key considerations
- Adding nonces/timestamps: To prevent replay attacks, documentation recommends including unique, per-message elements like timestamps (e.g., t= tag) or random numbers (nonces) within the signed portions of the email headers or body. This ensures each legitimate email has a unique signature.
- Header canonicalization: Using a 'relaxed' header canonicalization can make emails more robust to minor modifications but can also inadvertently aid replay attacks by being less sensitive to changes attackers might make.
- DMARC policy enforcement: While DMARC might not directly prevent a perfect DKIM replay, a strong p=reject policy provides the strongest defense against unauthorized sending, regardless of DKIM pass/fail.
- Authentication chain: Relying on a full authentication chain (SPF, DKIM, DMARC) and supplemental signals (like IP reputation, content analysis, and user complaints) is necessary for comprehensive email security, as no single protocol is foolproof against all attack vectors.
Technical article
Documentation from Proton.me describes a DKIM replay attack as an attacker intercepting a legitimate DKIM-signed email and resending it to multiple recipients, where it still passes authentication.
Technical article
Documentation from SocketLabs explains that attackers often sign up for free accounts with email service providers to generate legitimate DKIM-signed emails which they can then reuse in replay attacks.
Related resources
13 resources
Explore additional resources to deepen your understanding of DKIM replay attacks and broader email security.
Related pages
How do I troubleshoot DMARC failures and potential DKIM replay attacks affecting email deliverability?
How can a phishing email pass SPF and DKIM authentication checks?
A simple guide to DMARC, SPF, and DKIM
How to identify and handle email forging and replay attacks?
Ultimate Guide to Google Postmaster Tools V2
How to safely transition your DMARC policy to quarantine or reject
How to troubleshoot DKIM failures and which tools to use?
How long does it take to recover domain reputation from Bad/Low to High
Why Your Emails Are Going to Spam in 2024 and How to Fix It
What steps can I take to mitigate damage from email spoofing and prevent future occurrences?
