Network Working Group J. Levine
Internet-Draft Taughannock Networks
Updates: 6376 (if approved) June 19, 2017
Intended status: Informational
Expires: December 21, 2017

A Message Header to Identify Subscription Form Mail


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

Discussion Venue:
For the time being, discussion about this draft is directed to the mailing list.

Many organizations have web forms that provoke an e-mail confirmation to the e-mail address provided in the form. Malicious entities sbmit 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. 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.


    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"

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.TBD to indicate that the rejection is due to the message being part of a mail flood that includes Form-Sub headers.

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.TBD 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 is requested to update registries as follows.

6.1. Provisional Message Header Registry

The following value is added to the Provisional Message Header Registry

Provisional Message Header Registry Added Value
Header Field name Template Protocol Status Reference
Form-Sub . mail . (this document)

6.2. Enhanced Status Codes

The following value is added to the Enhanced Status Codes Enumerated Status Codes Registry

Enunerated Status Codes Registry Added Value
Code Sample Text Associated Basic Status Code Description Reference Submitter Change Controller
X.7.TBD Mail flood detected The message appears to be part of a mail flood of similar abusive messages. . [this document] J. Levine

7. Acknowledgments

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

8. Normative References

[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008.
[RFC5322] Resnick, P., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, October 2008.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, DOI 10.17487/RFC5598, July 2009.
[RFC6376] Crocker, D., Hansen, T. and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, DOI 10.17487/RFC6376, September 2011.

Author's Address

John Levine Taughannock Networks PO Box 727 Trumansburg, NY 14886 Phone: +1 831 480 2300 EMail: