Does ARC offer protection against man-in-the-middle attacks?

To understand if Authenticated Received Chain (ARC) protects against man-in-the-middle (MITM) attacks, we first need to be clear on what these terms mean. A MITM attack is a type of cyberattack where an unauthorized third party intercepts communication between two systems. In the world of email, this could mean an attacker positioning themselves between your sending server and the recipient's mail server to eavesdrop on or even alter the contents of your messages.

Rapid7 says:

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Rapid7 provides a fundamental overview of what man-in-the-middle attacks are, how they work, and the various techniques used by attackers, along with methods for prevention and protection.

ARC, on the other hand, is an email authentication protocol designed to solve a very specific problem: the failure of DMARC, SPF, and DKIM when an email is forwarded. When an email passes through an intermediary, like a mailing list or a forwarding service, that intermediary can sometimes modify the email's headers or content. These changes often break the original SPF and DKIM signatures, causing an email that was perfectly legitimate to fail DMARC authentication upon final delivery. ARC works by creating a new, verifiable chain of custody, allowing the final receiving server to see that the email was authentic before it was forwarded.

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ARC does not directly offer protection against man-in-the-middle attacks. Its primary function is to preserve authentication results, not to encrypt the communication channel. A MITM attack typically involves intercepting data in transit. The primary defense against this is encryption.

Arctic Wolf says:

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This is table-stakes protection against MitM attacks. Encrypting data in transit ensures that, even if an attacker intercepts the communication, they can’t read it.

The protocol responsible for encrypting email in transit is Transport Layer Security (TLS). When your mail server connects to another to deliver a message, it can use a command called STARTTLS to upgrade the connection from a plain-text one to an encrypted one. If this is successful, anyone trying to intercept the message between those two servers will only see scrambled, unreadable data.

While ARC doesn't prevent MITM attacks, it is part of a broader security posture that makes such attacks harder to benefit from. Think of email security in layers:

• Authentication (SPF, DKIM, DMARC): These protocols verify that the sender is who they claim to be. This prevents an attacker from successfully spoofing your domain. DMARC tells a receiving server what to do if these checks fail.
• Preservation (ARC): ARC ensures that the original authentication results are preserved and remain valid even after an email is legitimately forwarded.
• Encryption (TLS): This protects the content of the email while it's traveling between mail servers, making it unreadable to an intercepting party. As noted by United States Cybersecurity Magazine, strong encryption is a cornerstone of network security.

ARC ensures the first layer remains intact through intermediaries. TLS provides the second, separate layer of protection against snooping. An email can be fully authenticated with DMARC and ARC, but if it is not transmitted over a TLS-encrypted connection, it could still be vulnerable to a MITM attack where the content is read or captured.

In summary, ARC is a crucial tool for email deliverability in a world with mailing lists and complex routing, as it helps maintain the integrity of DMARC authentication. However, it is not a defense mechanism against man-in-the-middle attacks. True protection from MITM attacks comes from robust transport-layer encryption using TLS, which secures the data channel itself. A comprehensive email security strategy requires both strong authentication (DMARC + ARC) and strong encryption (TLS).