A Message Header to Identify Subscription Form Mail
draft-levine-mailbomb-header-01

Abstract

Many organizations have web forms that provoke an e-mail confirmation to the e-mail address provided in the form. Malicious entities do bulk form submissions with forged addresses, resulting in mail floods to the holders of those addresses. This document defines a message header to identify mail sent in response to web forms, so that recipient mail systems can better recognize and mitigate the mail floods.

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Table of Contents

• 1. Introduction
• 2. Conventions
• 3. The Form-Sub header field
• 4. Mail flood enhanced status code
• 5. Security Considerations
• 6. IANA Considerations
• 6.1. Provisional Message Header Registry
• 6.2. Enhanced Status Codes
• 7. Acknowledgments
• 8. Normative References
• Appendix A. Change log
• Author's Address

1. Introduction

Discussion Venue:

For the time being, discussion about this draft is directed to the collaboration@mailman.m3aawg.org mailing list.

Many organizations have web forms that provoke an e-mail confirmation to the e-mail address provided in the form. Malicious entities submit multiple forms with forged addresses, resulting in mail floods to those addresses. We define a message header that identifies mail sent in response to web forms, so that recipient mail systems can better recognize and mitigate the mail floods.

Mail systems that recognize a mail flood may defer or reject the mail. We also define an SMTP enhanced status code that a mail system can use in a message rejection SMTP reponse to alert the sending system that the message was rejected due to being part of a mail flood.

2. Conventions

The terms Message Submission Agent (MSA) and Message Transfer Agent (MTA) are defined as in [RFC5598].

The ABNF terms CRLF, FWS, and fields are imported from [RFC5322].

3. The Form-Sub header field

A MSA or an initial MTA adds a Form-Sub header field to indicate that the message was sent in response to a web form submission. The header consists of a semicolon-separated list of tag=value pairs. The first tag-value pair is "v=1" to indicate that the header uses the initial version of this specification. Receivers should ignore Form-Sub headers with a v= tag that indicates an unknown version. Subsequent tag-value pairs are optional, and receivers should ignore pairs with unknown tags.

The tags ip4 or ip6 contain the IPv4 or IPv6 address, respectively, from which the web form was submitted. The address may be partially redacted for privacy reasons, by replacing groups of digits with the letter "x", for example, 198.51.x.x or 2001:DB8::x or x::1234:abcd:5678:ef01. If the sender cannot determine the submitting IP address, it can include "ip=none". The goal of including the IP address is to help receiving mail systems recognize when a cluster of messages was provoked by the same submitter. Using "x" rather than a hash of the the address provides a redaction that cannot be reversed but still can be correlated among multiple messages.

    ABNF:

    fields =/ "Form-Sub:" FWS "v=1" *(FWS ";" FWS fsarg) CRLF

    fsarg = "ip4=" ip4redacted
    ip4redacted = IPv4 address with parts optionally replaced by "x"

    fsarg =/ "ip6=" ip6redacted 
    ip6redacted = IPv6 address with parts optionally replaced by "x"

    fsarg =/ "ip=none"

    fsarg =/ x-fsarg
    
    x-fsarg =/ ALPHA *(ALPHA / DIGIT) "=" tagdata
    tagdata = string of VCHAR excluding quote and semicolon

The Form-Sub header should be included within the set of the headers signed by any DKIM signature headers.

4. Mail flood enhanced status code

A mail receiver may choose to defer or reject mail that it recognizes as part of a mail flood. It can include the enhanced status code X.7.28 to indicate that the rejection is due to the message being part of a mail flood that includes Form-Sub headers.

A sender would typically interpret the code as a strong hint that their systems are being abused, so they should mitigate the abuse to stop the mail flood.

5. Security Considerations

IP addresses are sometimes considered to be personally identifable information. This specification allows partially redacted addresses as a compromise to avoid identifying individual persons, while still providing receivers a hint to recognize bulk submissions by the same party.

The Form-Sub header discloses information from a sender to a receiver, and the X.7.28 enhanced status code discloses information from a receiver to a sender that they would not otherwise have. If one party suspects the other is malicious, e.g., a receiver fears that a sender is probing to see what its mail volume limits are, it might not include the header or the status code for the possibly malicious other party.

6. IANA Considerations

IANA has updated registries as follows.

6.1. Provisional Message Header Registry

The following value has been added to the Provisional Message Header Registry

6.2. Enhanced Status Codes

The following value has been added to the Enhanced Status Codes Enumerated Status Codes Registry

7. Acknowledgments

Kurt Andersen and the M3AAWG Collaboration Committee provided the good parts.

8. Normative References

Appendix A. Change log

00 to 01:

Fix ABNF to allow arbitrary tags. Fix typos. .

Author's Address