The short answer is no, Authenticated Received Chain (ARC) does not directly prevent email spoofing. However, it plays a critical supporting role that makes anti-spoofing measures like DMARC more effective and practical to implement. To understand why, we first need to look at the problem ARC was designed to solve.
Standard email authentication protocols like SPF and DKIM work wonderfully in simple A-to-B email delivery. The challenge arises when an email passes through an intermediary, like a mailing list or a forwarding service, before reaching its final destination. These intermediaries often modify the email, for example, by adding a subject line prefix or an unsubscribe footer. These changes can break the DKIM signature. Similarly, the forwarding server's IP address will not be listed in the original sender's SPF record, causing the SPF check to fail. When these checks fail, DMARC also fails, and a legitimate email can be incorrectly marked as spam or rejected.
This is where ARC comes in. It was created to preserve the original authentication results as an email travels through various hops.
How ARC creates a chain of trust
ARC works by adding a new set of headers to an email at each step of its journey. Think of it as a digital chain of custody. Each intermediary that handles the email inspects the original authentication results (SPF, DKIM, DMARC) and then adds its own ARC header, which includes a cryptographic signature.
This process involves three key headers:
- ARC-Authentication-Results (AAR): Captures the initial authentication results, like spf=pass and dkim=pass.
- ARC-Message-Signature (AMS): A DKIM-like signature that covers the entire message, including the AAR header.
- ARC-Seal (AS): A signature that validates the previous ARC headers, creating the 'chain'.
When the final mail server receives the email, even if the current SPF and DKIM checks fail, it can look at the ARC chain. If the chain is valid, the server can choose to trust the original authentication results that were recorded at the start of the chain.
The indirect link to preventing spoofing
So, how does this help prevent spoofing? The connection is that ARC makes DMARC more reliable. DMARC is the protocol that tells receiving servers what to do with unauthenticated mail, allowing you to instruct them to quarantine or reject spoofed messages. The fear of blocking legitimate forwarded mail often prevents domain owners from implementing a strict DMARC policy.
By preserving the authentication verdicts for legitimate forwarded emails, ARC reduces these 'false positives'. This gives organizations the confidence to enforce a strict DMARC policy (p=reject), which is the most effective tool against direct domain spoofing. Without ARC, senders might stick to a less secure p=none policy, leaving their domain vulnerable.
In conclusion, while ARC's primary function isn't to stop spoofing, it is an essential piece of the email security puzzle. It fixes a known weakness in the authentication system, thereby enabling the widespread and confident use of DMARC at its strictest enforcement level, which absolutely does prevent spoofing.
