Does ARC actively prevent a malicious sender from altering an email?

No, ARC does not prevent a malicious sender from altering an email before its initial transmission or before it receives its first ARC seal. ARC's role is to detect alterations that occur after a legitimate ARC sealer has processed and signed the message, preserving the chain of authentication.

How does ARC detect if an email has been tampered with?

ARC detects tampering through cryptographic signatures. Each ARC sealer adds an ARC-Message-Signature header that signs parts of the email content and headers, and an ARC-Seal header that signs the entire chain. If the message is altered after a seal, the corresponding signature will no longer validate, indicating that the message has been tampered with.

Is ARC a replacement for SPF, DKIM, or DMARC?

No, ARC is not a replacement. It works in conjunction with SPF, DKIM, and DMARC to provide a complete email authentication solution. ARC specifically addresses the problem of authentication failures caused by legitimate message modifications by intermediaries like mailing lists or forwarding services.

Why is ARC important for email deliverability?

ARC is vital for email deliverability because it prevents legitimate emails from failing DMARC due to modifications by intermediate servers. By preserving the authentication chain, ARC ensures that these emails are trusted by recipient mail servers, allowing them to reach the inbox instead of being rejected or sent to spam.

Can ARC help with DMARC reporting?

Yes, ARC can provide valuable context for DMARC reporting . When DMARC fails, an active ARC chain can explain why the failure occurred (e.g., due to forwarding) and indicate that the original message was authenticated. This helps DMARC analyzers, like Suped, accurately interpret DMARC reports and identify true threats versus legitimate forwarding issues.

Does ARC prevent message alteration after signing? - ARC - Email authentication - Knowledge base

An illustration of an email message scroll with a broken chain, symbolizing message alteration after signing and how ARC aims to detect such changes.

The Authenticated Received Chain (ARC) is an essential email authentication protocol that addresses a specific challenge: what happens when legitimate intermediate mail servers, such as mailing lists or forwarding services, modify an email? These modifications, even minor ones like adding a footer, can cause standard authentication methods like SPF and DKIM to fail, leading to legitimate emails being marked as spam or rejected.

The core question is whether ARC prevents message alteration after signing. The answer is nuanced, as ARC doesn't prevent alteration in the same way a physical lock prevents tampering. Instead, it detects if a message has been altered after an ARC seal has been applied. It creates a cryptographic chain of trust that allows recipient mail servers to validate the authenticity of an email, even if it has undergone legitimate modifications in transit.

Understanding ARC is crucial for maintaining a strong sender reputation and ensuring email deliverability. While it doesn't stop every possible modification, its ability to preserve authentication results across various email processing stages makes it an indispensable component of modern email security. To fully grasp ARC's benefits, it's important to differentiate between preventing and detecting alteration.

How ARC signatures maintain integrity

How ARC signatures maintain email integrity

ARC works by introducing new email headers: ARC-Authentication-Results, ARC-Message-Signature, and ARC-Seal. When an email passes through an ARC-enabled intermediary (an ARC sealer), it captures the current authentication results (SPF, DKIM, DMARC status) in the ARC-Authentication-Results header. It then creates a cryptographic signature, contained in the ARC-Message-Signature header, which covers specific email headers and the message body. This is similar to how DKIM works.

The ARC-Seal header then cryptographically signs a copy of the message's state, including the ARC-Authentication-Results and ARC-Message-Signature headers that were just added by the current intermediary, along with any previous ARC headers. This creates a chain, where each link attests to the authenticity of the message and its authentication results at the point it passed through that specific intermediary.

If any part of the signed content (headers or body) is altered after an ARC seal has been applied, the corresponding cryptographic signature will become invalid. Recipient mail servers can then detect this discrepancy when re-authenticating the email. This detection mechanism is ARC's primary way of addressing alteration. It doesn't prevent the act of alteration itself, but rather flags it to the final receiver, who can then decide how to handle the message based on the broken chain of trust.

Understanding ARC Headers

ARC-Authentication-Results: Records the email's authentication status (SPF, DKIM, DMARC) at each hop.
ARC-Message-Signature (AMS): A cryptographic signature of key email headers and the message body.
ARC-Seal: A signature that authenticates the entire ARC chain, including the AMS and AAR headers from the current and previous hops.

The role of ARC sealers in preserving trust

ARC's primary benefit lies in enabling legitimate intermediate services (ARC sealers) to process emails without inadvertently causing DMARC failures. These services, such as mailing lists or forwarding systems, often need to alter an email, for example, by adding a custom header, a subject prefix, or a footer. Without ARC, these changes would break the original DKIM signature and could cause SPF alignment issues, leading to DMARC failure.

When an ARC sealer receives an email, it first verifies the existing authentication results. If the email passes its initial checks, the sealer then applies its own ARC headers, signing the message's state at that point. This effectively creates a new, trusted 'link' in the email's authentication history. The final recipient mail server can then review this chain of authentication results and decide to trust the email, even if the original SPF or DKIM fails due to the intermediary's modifications. Microsoft Defender documentation highlights ARC's role in reducing inbound email authentication failures from message modification.

This process is vital for ensuring that emails from legitimate sources, particularly those that use mailing lists or forwarding, still reach the inbox. Without ARC, many perfectly valid emails would be mistakenly classified as suspicious, leading to significant deliverability issues. This is especially true as DMARC adoption increases, requiring stricter alignment of SPF and DKIM.

Without ARC

SPF/DKIM break: Modifications by intermediate servers cause original SPF/DKIM to fail.
DMARC failure: Lack of alignment leads to messages being rejected or quarantined, even if legitimate.
Loss of context: Recipient servers lose visibility into the original sender's authentication status.

With ARC

Authentication preserved: Intermediate servers add new ARC seals to maintain the chain of trust.
DMARC pass: Recipient servers can validate the ARC chain, treating forwarded emails as legitimate.
Full visibility: Complete history of authentication results is available to the final receiver.

What ARC does not prevent

It's important to clarify that ARC does not prevent malicious actors from altering an email before it receives its first ARC seal. If a sender's mail server itself is compromised, or if an email is tampered with before it reaches the first ARC-enabled intermediary, ARC cannot retroactively secure the message. Its protection begins with the first ARC seal and extends through the chain of trusted intermediaries. ARC does not prevent email spoofing directly; that's the role of SPF, DKIM, and DMARC.

Moreover, ARC doesn't replace other email authentication standards. Instead, it complements DMARC, SPF, and DKIM, providing a layer of transparency for mail servers that are legitimately modifying messages. The cryptographic signatures within ARC are designed to detect unauthorized changes during transit between ARC-enabled hops, ensuring that any break in the chain can be identified.

Therefore, while ARC is a powerful tool for preserving email authentication across legitimate intermediaries, it's not a standalone solution for all email security concerns. It must be implemented as part of a comprehensive email authentication strategy that includes SPF, DKIM, and DMARC.

Stylised email headers ARC-Message-Signature and ARC-Seal as glowing digital chains, symbolizing cryptographic links.

Implementing ARC correctly can be complex, especially alongside other authentication protocols. Tools like Suped offer integrated DMARC, SPF, and DKIM monitoring, providing actionable recommendations to ensure your entire email authentication setup is robust. Our free plan makes DMARC accessible for organizations of all sizes, ensuring that messages maintain their integrity across their journey.

Summary

ARC's primary function is not to outright prevent message alteration after signing, but rather to provide a mechanism for detecting if alterations occur after an ARC seal has been applied by a legitimate intermediary. It builds a verifiable chain of trust, allowing recipient mail servers to assess the authenticity of an email that has undergone legitimate modifications. This capability is critical for environments where emails are frequently forwarded or processed by mailing lists, preventing valid communications from being erroneously flagged as suspicious.

By cryptographically linking authentication results across multiple hops, ARC ensures that DMARC policies can still pass for forwarded messages. Without ARC, the legitimate changes made by intermediaries would break SPF and DKIM alignment, leading to increased spam classifications and deliverability issues.

For robust email security and deliverability, an ARC implementation should always be paired with comprehensive DMARC monitoring. Platforms like Suped provide AI-powered recommendations and real-time alerts, simplifying the process of understanding your email authentication landscape and quickly resolving any issues. This ensures that your emails are not only delivered but also maintain their integrity and authenticity throughout their journey.