Internet-Draft Dangerous Labels in DNS and E-mail May 2022
Gillmor Expires 7 November 2022 [Page]
Workgroup:
intarea
Internet-Draft:
draft-dkg-intarea-dangerous-labels-00
Published:
Intended Status:
Informational
Expires:
Author:
D. K. Gillmor
ACLU

Dangerous Labels in DNS and E-mail

Abstract

This document establishes registries that list known security-sensitive labels in the DNS and in e-mail contexts.

It provides references and brief explanations about the risks associated with each known label.

The registries established here offer guidance to the security-minded system administrator, who may not want to permit registration of these labels by untrusted users.

About This Document

This note is to be removed before publishing as an RFC.

The latest revision of this draft can be found at https://dkg.gitlab.io/dangerous-labels/. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-dkg-intarea-dangerous-labels/.

Discussion of this document takes place on the Internet Area Working Group mailing list (mailto:intarea@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/intarea/.

Source for this draft and an issue tracker can be found at https://gitlab.com/dkg/dangerous-labels.

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 7 November 2022.

Table of Contents

1. Introduction

The Internet has a few places where seemingly arbitrary labels can show up and be used interchangeably.

Some choices for those labels have surprising or tricky consequences. Reasonable admnistrators may want to be aware of those labels to avoid an accidental allocation that has security implications.

This document registers a list of labels in DNS and e-mail systems that are known to have a security impact. It is not recommended to create more security-sensitive labels.

Offering a stable registry of these dangerous labels is not an endorsement of the practice of using arbitrary labels in this way. A new protocol that proposes adding a label to this list is encouraged to find a different solution if at all possible.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2. DNS Labels

Note that [RFC8552] defines the use of "underscored" labels which are treated differently than normal DNS labels, and often have security implications. That document also established the IANA registry for "Underscored and Globally Scoped DNS Node Names". That registry takes precedence to the list enumerated here, and any label in that list or with a leading underscore ("_") MUST NOT be included in this list.

Below are some normal-looking DNS labels that may grant some form of administrative control over the domain that the are attached to.

They are mostly "leftmost" or least-significant labels (in the sense used in Section 8 of [RFC8499]), in that if foo were listed here, it would be because granting control over the foo.example.net label (or control over the host pointed to by foo.example.net) to an untrusted party might offer that party some form of administrative control over other parts of example.org.

Note: where "<key-tag>" occurs in Table 1, it indicates any sequence of five or more decimal digits, as described in [RFC8509].

Table 1: Dangerous DNS labels
DNS Label Rationale and References
mta-sts Set SMTP transport security policy ([RFC8641])
openpgpkey Domain-based OpenPGP certificate lookup and verification ([I-D.koch-openpgp-webkey-service])
root-key-sentinel-is-ta-<key-tag> Indicates which DNSSEC root key is trusted ([RFC8509]
root-key-sentinel-not-ta-<key-tag> Indicates which DNSSEC root key is not trusted ([RFC8509]
www Popular web browsers guess this label (???)

3. E-mail Local Parts

Section 3.4.1 of [RFC5322] defines the local-part of an e-mail address (the part before the "@" sign) as "domain-dependent". However, allocating some specific local-parts to an untrusted party can have significant security consequences for the domain or other associated resources.

Note that all these labels are expected to be case-insensitive. That is, an administrator restricting registration of a local-part named "admin" MUST also apply the same constraint to "Admin" or "ADMIN" or "aDmIn".

[RFC2142] offers some widespread historical practice for common local-parts. The CA/Browser Forum's Baseline Requirements ([CABForum-BR]) constrain how any popular Public Key Infrastructure (PKI) Certification Authority (CA) should confirm domain ownership when verifying by e-mail. The public CAs used by popular web browsers ("web PKI") will adhere to these guidelines, but anyone relying on unusual CAs may still be subject to risk additional labels described here.

Table 2: Dangerous E-mail local-parts
E-mail local-part Rationale and References
abuse Receive reports of abusive public behavior (Section 2 of [RFC2142])
administrator PKI Cert Issuance (Section 3.2.2.4.4 of [CABForum-BR])
admin PKI Cert Issuance (Section 3.2.2.4.4 of [CABForum-BR])
hostmaster PKI Cert Issuance (Section 3.2.2.4.4 of [CABForum-BR]), DNS zone control (Section 7 of [RFC2142])
info PKI Cert Issuance (historical, see [VU591120])
is PKI Cert Issuance (historical, see [VU591120])
it PKI Cert Issuance (historical, see [VU591120])
noc Receive reports of network problems (Section 4 of [RFC2142])
postmaster Receive reports related to SMTP service (Section 5 of [RFC2142]), PKI Cert Issuance ( Section 3.2.2.4.4 of [CABForum-BR])
root Receive system software notifications (???), PKI Cert Issuance (historical, see [VU591120])
security Receive reports of technical vulnerabilities (Section 4 of [RFC2142])
ssladministrator PKI Cert Issuance (historical, see [VU591120])
ssladmin PKI Cert Issuance (historical, see [VU591120])
sslwebmaster PKI Cert Issuance (historical, see [VU591120])
sysadmin PKI Cert Issuance (historical, see [VU591120])
webmaster PKI Cert Issuance (Section 3.2.2.4.4 of [CABForum-BR]), Receive reports related to HTTP service (Section 5 of [RFC2142])
www Common alias for webmaster (Section 5 of [RFC2142])

4. Security Considerations

Allowing untrusted parties to allocate names with the labels associated in this document may grant access to administrative capabilities.

The administrator of a DNS or E-mail service that permits any untrusted party to register an arbitrary DNS label or e-mail local-part for their own use SHOULD reject attempts to register the labels listed here.

5. IANA Considerations

This document asks IANA to establish two registries, from Table 1 and Table 2.

Note that the DNS table in Table 1 does not include anything that should be handled by the pre-existing "Underscored and Globally Scoped DNS Node Names" registry defined by [RFC8552].

It's not clear how these registries should be updated.

Adding a new security-sensitive entry to either of these tables is likely to be a bad idea, because existing DNS zones and e-mail installations may have already made an allocation of the novel label, and cannot avoid the security implications. For a new protocol that wants to include a label in either registry, there is almost always a better protocol design choice.

Yet, if some common practice permits any form of administrative access to a resource based on control over an arbitrary label, administrators need a central place to keep track of which labels are dangerous.

6. References

6.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC5322]
Resnick, P., Ed., "Internet Message Format", RFC 5322, DOI 10.17487/RFC5322, , <https://www.rfc-editor.org/info/rfc5322>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
[RFC8552]
Crocker, D., "Scoped Interpretation of DNS Resource Records through "Underscored" Naming of Attribute Leaves", BCP 222, RFC 8552, DOI 10.17487/RFC8552, , <https://www.rfc-editor.org/info/rfc8552>.

6.2. Informative References

[CABForum-BR]
Forum, C., "CA/Browser Forum Baseline Requirements", , <https://cabforum.org/wp-content/uploads/CA-Browser-Forum-BR-1.8.4.pdf>.
[I-D.hoffman-dns-special-labels]
Hoffman, P., "IANA Registry for Special Labels in the DNS", Work in Progress, Internet-Draft, draft-hoffman-dns-special-labels-00, , <https://www.ietf.org/archive/id/draft-hoffman-dns-special-labels-00.txt>.
[I-D.koch-openpgp-webkey-service]
Koch, W., "OpenPGP Web Key Directory", Work in Progress, Internet-Draft, draft-koch-openpgp-webkey-service-13, , <https://www.ietf.org/archive/id/draft-koch-openpgp-webkey-service-13.txt>.
[RFC2142]
Crocker, D., "Mailbox Names for Common Services, Roles and Functions", RFC 2142, DOI 10.17487/RFC2142, , <https://www.rfc-editor.org/info/rfc2142>.
[RFC8499]
Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, , <https://www.rfc-editor.org/info/rfc8499>.
[RFC8509]
Huston, G., Damas, J., and W. Kumari, "A Root Key Trust Anchor Sentinel for DNSSEC", RFC 8509, DOI 10.17487/RFC8509, , <https://www.rfc-editor.org/info/rfc8509>.
[RFC8641]
Clemm, A. and E. Voit, "Subscription to YANG Notifications for Datastore Updates", RFC 8641, DOI 10.17487/RFC8641, , <https://www.rfc-editor.org/info/rfc8641>.
[VU591120]
Center, C. C., "Multiple SSL certificate authorities use predefined email addresses as proof of domain ownership", , <https://www.kb.cert.org/vuls/id/591120/>.

Appendix A. Acknowledgements

Many people created these dangerous labels or the authorization processes that rely on them over the years.

Dave Crocker wrote an early list of special e-mail local-parts, from [RFC2142].

Paul Hoffman tried to document a wider survey of special DNS labels (not all security-sensitive) in [I-D.hoffman-dns-special-labels].

Appendix B. Document Considerations

RFC Editor: please remove this section before publication.

B.1. Other types of labels?

This document is limited to leftmost DNS labels and e-mail local-parts because those are the arbitrary labels There may be other types of arbitrary labels on the Internet with special values that have security implications that the author is not aware of.

B.2. Document History

/No history yet/

Author's Address

Daniel Kahn Gillmor
American Civil Liberties Union
United States of America