What is a DKIM replay attack?

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DKIM (DomainKeys Identified Mail) is an email authentication method that helps verify the authenticity of emails and detect email spoofing and phishing attempts. DKIM adds a digital signature to emails on the sending server, which the recipient's email server can verify to ensure that the message has not been tampered with in transit. DKIM works by the following process: 1. When an email is sent from a domain that uses DKIM, the sending mail server generates a unique cryptographic signature for the message. The signature is based on the content of the email (header and body) and some specific header fields, such as the "From" address and the "Date" field
The signing process usually requires the use of a private key. 2. The sending domain publishes a public DKIM key in its DNS (Domain Name System) records. The recipient's email server verifies the signature using this public key. 3. The email message containing the DKIM signature is transmitted over the Internet to the recipient's email server. 4. When the recipient's email server receives the email, it retrieves the DKIM signature from the message headers and looks up the sender's public DKIM in the DNS records for the sender'

s domain name. key. If the signature matches the content of the email, the recipient can be reasonably certain that the email has not been tampered with in transit and that it indeed comes from the purported phone number data
sender domain. 5. Depending on the results of the verification process, the recipient's server can mark the email as DKIM verified or DKIM failed. DKIM provides a mechanism to verify the authenticity of the sender's domain name, helping to prevent various email-based attacks such as phishing and spoofing.
3. DKIM replay attack principle
In a DKIM replay attack, a malicious person can exploit the laxity of the DKIM signature to deceive email recipients and potentially spread harmful content or commit fraud. Let's break down how a DKIM replay attack works step by step: DKIM allows the signing domain (the domain that signs the email) to be different from the domain mentioned in the email's "From" header. This means that even if the email's "From" header claims to be from a certain domain, the DKIM signature can be associated with a different domain. When the email recipient's server receives an email with a DKIM signature, it checks the signature to make sure that the email has not been tampered with after it was signed by the domain's mail server. If the DKIM signature is valid, it verifies that the email passed through the signing domain's mail server and was not tampered with in transit. Now, the attack comes into play. If an attacker successfully takes over or hacks a mailbox,

or creates a mailbox with a high reputation for the domain (which means it is a trustworthy source in the eyes of the email server), they can use the reputation of the domain to their advantage. The attacker sends a message from their own high-reputation domain to another mailbox that they control. This initial message could have been harmless or even legitimate in order to avoid suspicion. Now, the attacker can use the recorded email to re-disseminate the same message to different recipients, who are often not the intended recipients of the legitimate sender. Because the email's DKIM signature comes from a high-reputation domain, the email server is more likely to trust it as a legitimate message, thereby bypassing the validation filters.
Strategies to mitigate DKIM replay attacks
To ensure that key headers such as date, subject, from, to, and cc cannot be added or modified after signing, consider over-signing them. This safeguard prevents malicious actors from tampering with these critical information components. Keep the expiration time (x=) as short as possible. This reduces the chances of replay attacks. Newly created domains must have a shorter expiration time than old domains because they are more vulnerable. To further prevent replay attacks, include timestamps and nonces (random numbers) in the email headers or body. This makes it difficult for an attacker to resend the same email at a later time because these values ​​have changed. And update DNS records accordingly. This minimizes the exposure of long-term keys that could be compromised and used in replay attacks. Receivers can implement rate limits on received email messages to prevent attackers from flooding your system with replayed emails. To do this, you can set a limit on the number of emails received from a specific sender in a given time. Educate your email recipients on the importance of DKIM and encourage them to verify DKIM signatures on the emails they receive. This helps reduce the impact of any potential replay attacks on the recipients. Implement network security measures to detect and block traffic from known malicious IP addresses and sources that may be involved in replay attacks. We offer a comprehensive solution to make DKIM key management easy for domain owners. We help you monitor email flows and DKIM signing practices so you can quickly spot discrepancies while staying one step ahead of attackers. Record optimization on our dashboard is automated, eliminating the need to visit your DNS multiple times for manual updates. With SecurityGateway, you can automate changes to signatures, handle multiple selectors, and rotate DKIM keys, eliminating the need for tedious manual work. Sign up today!