Does ARC offer protection against man-in-the-middle attacks?
Michael Ko
Co-founder & CEO, Suped
Published 31 Dec 2024
Updated 29 Sep 2025
8 min read
When we talk about email security, one of the most insidious threats is the man-in-the-middle (MiTM) attack. This is where an attacker secretly relays and possibly alters the communication between two parties who believe they are communicating directly with each other. It's a critical concern, especially for sensitive email exchanges. Many wonder if the Authenticated Received Chain (ARC), a protocol designed to preserve email authentication results, offers direct protection against these types of sophisticated attacks.
The short answer is that ARC itself does not directly prevent man-in-the-middle attacks. Instead, ARC plays a crucial role in maintaining the integrity of email authentication results, particularly when emails are forwarded or modified by legitimate intermediaries, like mailing lists. While it doesn't stop an attacker from intercepting mail, it does help receiving mail servers detect if the message's authentication status has been tampered with or if the email was altered after it was initially signed by a trusted entity. Understanding its true function requires a look into how it operates alongside other email security protocols.
The nature of man-in-the-middle attacks
Understanding man-in-the-middle attacks in email
A man-in-the-middle attack in the context of email involves an attacker intercepting communication between a sender and a receiver. The goal is often to eavesdrop, tamper with the message content, or impersonate one of the parties without either side being aware of the interference. For email, this could involve intercepting email during transit between mail servers, potentially altering the message, and then forwarding it to the intended recipient or a different destination. Such attacks can lead to data breaches, financial fraud, or reputational damage.
These attacks exploit vulnerabilities in how email is transmitted and authenticated. For instance, if email transport lacks encryption, an attacker can easily read messages in plain text. If authentication protocols like DMARC, SPF, and DKIM are not properly implemented, it becomes easier for an attacker to spoof the sender's identity or forge email content without detection. Effective MiTM prevention strategies typically involve a multi-layered approach that includes strong encryption and robust authentication mechanisms. More information on how to protect your organization from MiTM attacks is available from Kiteworks.
Common MiTM attack vectors in email
DNS spoofing: Redirects legitimate email traffic to malicious servers.
SSL/TLS stripping: Downgrades encrypted connections to unencrypted ones, allowing interception.
Email content tampering: Modifies the email's body or attachments in transit.
Session hijacking: Steals authentication tokens to gain unauthorized access to email accounts.
For email, the transport layer security (TLS) is critical for encrypting communications between mail servers. Without it, messages are vulnerable to interception and reading. However, MiTM attacks can also involve tricking servers into not using TLS, or presenting fake certificates. This is why a defense-in-depth strategy is paramount, combining transport security with robust email authentication to verify sender identity and message integrity.
ARC's role in preserving email authentication
How ARC works for authentication integrity
ARC, or Authenticated Received Chain, was developed to address a specific problem with DMARC, SPF, and DKIM: their failure when emails are legitimately modified or forwarded. When an email passes through a mailing list or a forwarding service, its headers or body might be altered, which can break the original SPF and DKIM signatures. This often leads to DMARC failing authentication for legitimate messages, resulting in them being quarantined or rejected by the receiving server.
ARC works by providing a chain of custody for email authentication results. Each time an email is handled by an ARC-enabled intermediary, it adds three new headers: ARC-Authentication-Results, ARC-Message-Signature, and ARC-Seal. The ARC-Seal header contains a cryptographically signed copy of the message's state. These headers allow a recipient server to verify the previous authentication results and determine if the email was legitimate at the point it was last signed by a trusted ARC participant. Essentially, ARC re-authenticates an email in a way that accounts for forwarding.
This chain of cryptographic signatures allows the final recipient to see the email's authentication history. Even if SPF or DKIM fail at the final hop due to forwarding, the ARC chain provides a way to validate that the original message was authenticated and that any subsequent changes were made by legitimate intermediaries. This is why how to implement ARC is important for email deliverability, especially for organizations that frequently use mailing lists.
ARC's contribution to email security
ARC's indirect role in mitigating MiTM
While ARC doesn't directly prevent an attacker from intercepting an email, it does provide a mechanism for detecting if a message has been tampered with after being legitimately authenticated. If an email is altered during a man-in-the-middle attack, the cryptographic signatures in the ARC-Seal and ARC-Message-Signature headers will break. A receiving mail server can then use this broken chain to flag the email as suspicious, even if it appears to come from a legitimate sender and would have otherwise passed initial DMARC, SPF, and DKIM checks.
Therefore, ARC's protection is more about post-delivery detection rather than pre-delivery prevention. It helps maintain a verifiable audit trail of authentication decisions, making it harder for attackers to modify emails in transit without detection. This is a critical distinction, as it means ARC is not a standalone solution for MiTM attacks but a valuable component in a broader email security framework. It doesn't stop message alteration after signing, but it does make alterations detectable.
Direct MiTM prevention methods
TLS encryption: Encrypts data in transit between servers.
MTA-STS: Enforces TLS encryption for email, protecting against downgrade attacks.
VPNs: Creates a secure tunnel for network traffic.
ARC's contribution to security
Authentication chain: Preserves authentication results through forwarding.
Tampering detection: Helps identify if an email was altered post-initial signing.
Improved DMARC processing: Prevents legitimate forwarded mail from failing DMARC.
Therefore, when considering MiTM protection, it's crucial to understand that ARC acts as a forensic tool for authentication integrity rather than a direct barrier to interception. It ensures that even if an email travels through several hops and undergoes legitimate modifications, its original authentication status can still be verified by the final recipient.
A multi-layered approach to email security
Comprehensive protection beyond ARC
To effectively combat man-in-the-middle attacks in email, we need to implement a robust suite of security protocols. These include: DMARC, SPF, and DKIM for authenticating the sender and verifying message integrity. Implementing a DMARC policy of 'reject' provides the strongest protection against spoofed email, ensuring that unauthenticated messages are rejected at the mail gateway. This greatly reduces the success rate of phishing and spoofing attempts that often accompany MiTM attacks, as highlighted by CISA Insights.
Transport Layer Security (TLS) is fundamental. It encrypts the connection between mail servers, preventing eavesdropping and tampering during transit. However, TLS alone isn't enough, as attackers can sometimes force a downgrade to unencrypted connections. This is where MTA-STS (Mail Transfer Agent Strict Transport Security) comes in, enforcing the use of TLS and verifying server identities to prevent downgrade attacks and certificate spoofing. Combining these protocols creates a robust defense against various email-borne threats, including man-in-the-middle and email spoofing (or blocklisting) scenarios.
Enhance your email security with Suped
To effectively monitor and manage these complex email authentication protocols, a powerful DMARC reporting and monitoring tool is essential. Suped offers an intuitive platform that provides real-time alerts and AI-powered recommendations to help you fix issues and strengthen your policy. Our unified platform brings together DMARC, SPF, and DKIM monitoring with deliverability insights, ensuring your emails are always protected and reaching the inbox.
While ARC helps receiving mail servers determine if email sender authenticity was preserved through forwarding, it is part of a larger ecosystem of email authentication. Relying on a single protocol for protection against sophisticated attacks like MiTM is insufficient. A layered security approach, combining robust encryption, strong authentication policies, and continuous monitoring, is the most effective way to safeguard your email communications. This ensures that your domain's reputation remains intact and your messages consistently reach their intended recipients.
Protecting against man-in-the-middle attacks
In conclusion, ARC does not directly protect against man-in-the-middle attacks. Its primary function is to preserve the validity of DMARC, SPF, and DKIM authentication results when emails are legitimately handled by intermediaries like forwarding services or mailing lists. However, by providing a verifiable chain of custody for authentication, ARC indirectly contributes to detecting message tampering that might occur during a MiTM attack. For true protection against these threats, we must implement a comprehensive security strategy that includes TLS encryption, MTA-STS, and strong DMARC policies set to 'reject'. Monitoring your DMARC reports is essential for understanding your email ecosystem and identifying potential vulnerabilities.
Suped offers an intuitive platform that provides real-time alerts and AI-powered recommendations to help you fix issues and strengthen your policy. Our unified platform brings together DMARC, SPF, and DKIM monitoring with deliverability insights, ensuring your emails are always protected and reaching the inbox.
