Internet-Draft Sender Authentication BCP May 2023
Foster Expires 14 November 2023 [Page]
Intended Status:
D.E. Foster, Ed.

Sender Authentication Best Practices


Sender Authentication contributes to the goal of detecting and blocking maliciously impersonated email identifiers. Sender Policy Framework (SPF) (RFC 7208) validates the RFC5321.MailFrom address, and Domain-based Message Authentication, Reporting, and Conformance (DMARC) (RFC 7489) validates the RFC5322.From header address. Both techniques may produce a result other than PASS on a message that the recipient considers acceptable and wanted. This document describes best practices for integrating SPF and DMARC into an email filtering strategy.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 14 November 2023.

Table of Contents

1. Introduction

Sender Authentication contributes to the goal of detecting and blocking maliciously impersonated email identifiers. Sender Policy Framework (SPF) [RFC7208] validates the RFC5321.MailFrom address, and Domain-based Message Authentication, Reporting, and Conformance (DMARC) [RFC7489] validates the RFC5322.From header address. For both techniques, a result of PASS indicates that the message is judged to be free of impersonation, and therefore free of malicious impersonation risk. Any message that does not produce PASS can therefore be considered at risk of possible impersonation. From a risk standpoint, the evaluator has little reason to distinguish between results of NOPOLICY, PERMERROR, SOFTFAIL, or FAIL. However, malicious impersonation is only one of many reasons that a message does not produce PASS, and therefore malicious impersonation can only be concluded after all other possibilities have been ruled out.

2. Email Filtering Reference Model

2.1. Overview

A typical email filtering solution has two primary components, sender filtering and content filtering. Sender filtering evaluates the identifiers in a message to assign a reputation to the sender of the message. Upon completion of sender filtering, the message has three possible dispositons:

A basic assumption is that unwanted and malicious email originates from unwanted and malicious email senders. While content filtering and sender authentication will flag or block single messages that are suspicious, their greatest benefit accrues when they are used to identify malicious message sending entities, so that all messages from that sender can be blocked. Consider the situation where a message is identified as malicious and blocked, but no follow-up action is taken. When the attacker changes techniques, and a subsequent attack succeeds, the system administrator will appear negligent. This document assumes a continuous improvement process with these characteristics:

2.2. Privileged Mode Messages

Because any message may require Priviledged mode processing, any message must be verifiable, using either SPF and DMARC or a local policy which provides equivalent results. Local policy extends SPF to define acceptable relationships between a verfied server identifier and the RFC5321.MailFrom domain, and extends DMARC to define acceptable relationships between a verified RFC5321.MailFrom and the RFC5322.From domain. The supplemental authentication techniques for this purpose are explained later in this document.

2.3. Unwanted Messages

Some messages will be categorized as unwanted because of previously configured rules or trusted reference sources. These messages are blocked, but content may be preserved for subsequent review or other purposes.

2.4. Non-privileged Messages with Content Filtering Fail

Messages which fail Content Filtering are blocked. A subsequent review is used to determine if content filtering needs to be updated, if a sender block rule is needed for specific senders, or if a Privilege Mode exemption is needed for trusted senders.

When a message set is determined to be unwanted, an analysis is necessary to identify the identifiers which correctly represent the source of the attack. In most cases, one or more identifiers can be found which represent the source of the attack, and a single-attribute block rule is created for each such identifier. In less common situations, a system manager may determine that an identifier was fraudulent, but the underlying identifiers are not wholly malicious. If this occurs, a multiple-attribute block rule is needed to block the specific combination of identifiers. In either configuration, verification of the identifiers is not necessary to the purposes of a block rule.

2.5. Non-privileged Messages with Sender Authentication FAIL and Content Filtering PASS

Messages which produce an authentication FAIL result will carry the highest presumption of malicious impersonation, but exceptions will occur. Content Filtering will provide additional insight into the nature of the message, so disposition should be delayed until this data has been collected. For messages that pass Content Filtering, a sufficient defense is to quarantine these messages and prioritize them for review and categorization. Confirmed malice will justify a local policy to block all messages from the malicious source. Acceptable messages will be granted a local policy to provide alternate authentication.

2.6. Non-privileged Messages with Sender Authentiction PASS and Content Filtering PASS

Messages which pass both Sender Authentication and Content Filtering have no detected risk and are consequently released to the user. User complaints could cause the sender to be reviewed and content filtering rules to be enhanced.

2.7. Non-privileged Messages with Sender Authentiction Unknown and Content Filtering PASS

Some messages will produce neither PASS nor FAIL, because of missing, misconfigured, or non-enforcing domain owner policies. These messages will be forwarded to content filtering, and may be blocked on that basis. The remainder will be messages that have no detected risk, but may have undetected impersonation. The evaluator must decide whether to release such messages to the user or quarantine them for prior review. The sheer volume of such messages will often cause an organization to release such messages to the final recipient, while retaining the option of after-the-fact review. The risk of doing so is proportional to the effectiveness of the content filtering system. As after-the-fact review identifies additional message sources to be blocked and additional messages sources to be granted local authentication policies, the volume of ambiguous results will steadily decline. When the ambiguous volume is sufficiently controlled, the evaluator can switch from after-the-fact review to quarantine with prior review.

3. Alternate Authentication

SPF and DMARC only provide results if the domain owner has published a policy. The policy is generally considered actionable if the policy returns a result of DMARC FAIL, or SPF FAIL without DMARC PASS. Even then, the FAIL result can be misleading if the domain owner's implementation is not consistent with its policy, or if the message transit has caused the message to lose authentication. For additional discussion of these issues, refer to Interoperability Issues between Domain-based Message Authentication, Reporting, and Conformance (DMARC) and Indirect Email Flows [RFC7960].

RFC 7208 and RFC 7489 provide no guidance for evaluators to cope with incorrect, ambiguous, or No Policy results. Without exception management, Sender Authentication dies as soon as an exception is necessary. A poorly designed exception process may enable the very impersonations that Sender Authentication is intended to prevent. To handle exceptions appropriately, and to ensure that any acceptable message can be configured as authenticated, additional authentication techniques are needed:

4. DMARC Authentication Types and Confidence Levels

DMARC combines eight different authentiction types into a single result, but the eight types do not provide equal confidence. Evaluators should be aware of the possibility of false PASS, and consider that risk when developing local policies, especially local alignment policies.

DKIM and SPF are the two underlying authentication techniques for DMARC, and within each technique there are four possible alignment methods: Self authenticates self, parent authenticates child, child authenticates parent, and sibling authenticates sibling,

When a message is sent from a multi-tenant server, SPF assumes that the server owner has administrative controls which prevent clients from impersonating each other, or that all clients are well behaved. DKIM, by contrast, has the much simpler expectation that the server owner is able to ensure that each client's private key data is kept private to themselves, out of reach from other clients in the same environment. Consequently, DKIM PASS always conveys a higher level of confidence than SPF PASS. Of course, a DKIM signature also ensures that authentication is preserved during forwarding (when done without content changes), so DKIM also provides confidence to a broader range of evaluators. In short, domain owners who wish to promote maximum trust by evaluators should ensure that all of their messages have verifiable DKIM signatures.

Similarly, the different types of alignment provide different levels of confidence. When the authenticating domain exactly matches the authenticated domain, trust in the result is maximized. The three varieties of relaxed alignment depend on correctly identifying the organizational domain. The Public Suffix List from is the reference used by most organizations, but it is not an authoritative source, is modified on a continuing basis, is developed for cross-site-scripting defenses rather than email defensies, and has detectable errors and omisssions. While the frequency of errors may be low, the impact of an error may be significant. If the PSL lands too low, the evaluator may not see a DMARC policy at all, or may see the wrong policy. If the PSL lands too high, the evaluator may authenticate sibling organizations as if they were sibling domains within a single organization. The impact of a PSL error varies with the type of authentication:

Domain owners who seek to maximize evaluator trust should utilize those authentication types which provide the highest confidence to evaluators. When configuring local authentication policies, evaluators have the option of applying any of four confidence levels, to obtain their desired balance between maximizing policy coverage and maximizing confidence in a PASS result.

5. Forwarding Considerations

Forwarding creates significant challengers for the evaluator seeking to eliminate impersonation. The evaluator needs to assess the reputation of both the originator and the forwarder, but in most cases, the greater concern applies to the originator. The process of forwarding:

Consequently, an ideal evaluation process needs to include:

Forwarding may be easier to detect in aggregate than on single messages, because a forwarding configuration will produce a stream of messages from a single server organization, on behalf of a wide variety of RFC5322.From domains. Within a single message, these data sources may be available to help detect forwarding:

While these are potentially useful clues, this document does not attempt to provide an algorithm for optimal interpretation of these clues. Evaluators must consider that Received headers may be forged and ARC Sets may mislead. Consequently, header parsing is most reliable when it is used to identify messages that are unwanted because of detected identifiers with an unacceptable reputation.

Evaluators who wish to choose to accept forwarded mail, but cannot perform detailed header parsing, will need to authenticate based on the forwarder identity alone. For messages forwarded without RFC5321.MailFrom rewrite, this requires a local policy to provide alternate authentication for the RFC5321.MailFrom address. The policy would need to specify that any MailFrom domain is acceptable as long as the forwarding server identity matches the expected Source IP or the server host domain matches the expected domain name and is forward confirmed. For messages forwarded with RFC5321.MailFrom rewrite to provide SPF PASS based on the forwarder's domain, the local policy would need to specify that all RFC5322.From domains are acceptable as long as the RFC5321.MailFrom domain has the expected value and is confirmed by SPF PASS or a local policy equivalent. In either case, the evaluator is implicitly assuming that either the forwarder will intercept and block malicious impersonation, or that the evaluator's content filtering will be strong enough to intercept and block any malicious impersonation in the forwarded stream.

Forwarders should consider that many evaluators will not attempt to navigate the complexity of originator detection. Instead, any unacceptable messages will be assigned to the reputation of the forwarder, which could lead to obstruction of messages unrelated to the forwarded stream. Consequently, forwarders should apply the same rigorous filtering to forwarded messages as they use to protect their own domain.

Additionally, forwarders should be aware of the impact that content-altering spam filters will have on forwarded mail. An increasing number of spam filters add disclaimers to distinguish external mail from internal mail, or rewrite embedded links to provide click-time protection. Any such change will invalidate the originator's DKIM signatures, causing an authentication problem if the message is forwarded in its altered state. Consequently, organizations that use content-altering spam filters should not forward mail, and organizations that routinely forward mail should not use content-altering featues of their spam filter.

6. IANA Considerations

This memo includes no request to IANA.

7. Security Considerations

The goal of sender authentication is to separate well-identified messages from potentially-impersonated messages. Successful sender authentication is a necessary pre-requisite to privileged-mode handling of a document, but it is not a reason to grant that privilege. Sender authentication simply ensures that any reputation information, obtianed by other methods, can be applied correctly.

Evaluators must remember that both sender infrastructure and sender reputation can change over time and therefore they must remain vigilant. Previously trusted sources may become compromised or even retired and reassigned to other, less trustworthy, uses. Similarly, a previously untrusted source may have earned that designation because of an infection which has been subsequntly corrected. Coping with these sources of variability is outside the scope of this document.

Becuase of the additional risk associated with sibling relationships, evaluators may wish to treat authentication based on sibling alignment more carefully than other forms of relaxed alignment.

8. References

8.1. Normative References

Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, , <>.
Kitterman, S., "Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1", RFC 7208, DOI 10.17487/RFC7208, , <>.
Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based Message Authentication, Reporting, and Conformance (DMARC)", RFC 7489, DOI 10.17487/RFC7489, , <>.

8.2. Informative References

Martin, F., Ed., Lear, E., Ed., Draegen, T., Ed., Zwicky, E., Ed., and K. Andersen, Ed., "Interoperability Issues between Domain-based Message Authentication, Reporting, and Conformance (DMARC) and Indirect Email Flows", RFC 7960, DOI 10.17487/RFC7960, , <>.

Author's Address

Douglas Foster (editor)
Virginia Beach, VA 23464
United States of America