intarea                                                    D. K. Gillmor
Internet-Draft                                                      ACLU
Intended status: Informational                               21 May 2022
Expires: 22 November 2022


                   Dangerous Labels in DNS and E-mail
                 draft-dkg-intarea-dangerous-labels-01

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/.





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   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 22 November 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  DNS Labels  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  E-mail Local Parts  . . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     4.1.  Additional Risks Out of Scope . . . . . . . . . . . . . .   6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Dangerous DNS Labels Registry . . . . . . . . . . . . . .   7
     5.2.  Dangerous E-mail Local Parts Registry . . . . . . . . . .   8
     5.3.  Shared Considerations . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  10
   Appendix B.  Document Considerations  . . . . . . . . . . . . . .  10
     B.1.  Other types of labels?  . . . . . . . . . . . . . . . . .  10
     B.2.  Document History  . . . . . . . . . . . . . . . . . . . .  11
       B.2.1.  Substantive Changes from -00 to -01 . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

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




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   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].








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     +============+==============+===================================+
     | DNS Label  | Rationale    | Reference                         |
     +============+==============+===================================+
     | mta-sts    | Set SMTP     | [RFC8641]                         |
     |            | transport    |                                   |
     |            | security     |                                   |
     |            | policy       |                                   |
     +------------+--------------+-----------------------------------+
     | openpgpkey | Domain-based | [I-D.koch-openpgp-webkey-service] |
     |            | OpenPGP      |                                   |
     |            | certificate  |                                   |
     |            | lookup and   |                                   |
     |            | verification |                                   |
     +------------+--------------+-----------------------------------+
     | root-key-  | Indicates    | [RFC8509]                         |
     | sentinel-  | which DNSSEC |                                   |
     | is-ta-     | root key is  |                                   |
     | <key-tag>  | trusted      |                                   |
     +------------+--------------+-----------------------------------+
     | root-key-  | Indicates    | [RFC8509]                         |
     | sentinel-  | which DNSSEC |                                   |
     | not-ta-    | root key is  |                                   |
     | <key-tag>  | not trusted  |                                   |
     +------------+--------------+-----------------------------------+
     | www        | Popular web  | FIXME: find a reference           |
     |            | browsers     |                                   |
     |            | guess this   |                                   |
     |            | label        |                                   |
     +------------+--------------+-----------------------------------+

                       Table 1: Dangerous DNS labels

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



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   (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.

    +==================+=========================+====================+
    | E-mail local-    | Rationale               | References         |
    | part             |                         |                    |
    +==================+=========================+====================+
    | abuse            | Receive reports of      | Section 2 of       |
    |                  | abusive public behavior | [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, DNS  | Section 3.2.2.4.4  |
    |                  | zone control            | of [CABForum-BR],  |
    |                  |                         | Section 7 of       |
    |                  |                         | [RFC2142]          |
    +------------------+-------------------------+--------------------+
    | info             | PKI Cert Issuance       | [VU591120]         |
    |                  | (historical)            |                    |
    +------------------+-------------------------+--------------------+
    | is               | PKI Cert Issuance       | [VU591120]         |
    |                  | (historical)            |                    |
    +------------------+-------------------------+--------------------+
    | it               | PKI Cert Issuance       | [VU591120]         |
    |                  | (historical)            |                    |
    +------------------+-------------------------+--------------------+
    | noc              | Receive reports of      | Section 4 of       |
    |                  | network problems        | [RFC2142]          |
    +------------------+-------------------------+--------------------+
    | postmaster       | Receive reports related | Section 5 of       |
    |                  | to SMTP service, PKI    | [RFC2142],         |
    |                  | Cert Issuance           | Section 3.2.2.4.4  |
    |                  |                         | of [CABForum-BR]   |
    +------------------+-------------------------+--------------------+
    | root             | Receive system software | [VU591120], FIXME: |
    |                  | notifications, PKI Cert | find a reference   |
    |                  | Issuance (historic)     | for root (software |
    |                  |                         | config docs?)      |
    +------------------+-------------------------+--------------------+
    | security         | Receive reports of      | Section 4 of       |
    |                  | technical               | [RFC2142]          |



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    |                  | vulnerabilities         |                    |
    +------------------+-------------------------+--------------------+
    | ssladministrator | PKI Cert Issuance       | [VU591120]         |
    |                  | (historical)            |                    |
    +------------------+-------------------------+--------------------+
    | ssladmin         | PKI Cert Issuance       | [VU591120]         |
    |                  | (historical)            |                    |
    +------------------+-------------------------+--------------------+
    | sslwebmaster     | PKI Cert Issuance       | [VU591120]         |
    |                  | (historical)            |                    |
    +------------------+-------------------------+--------------------+
    | sysadmin         | PKI Cert Issuance       | [VU591120]         |
    |                  | (historical)            |                    |
    +------------------+-------------------------+--------------------+
    | webmaster        | PKI Cert Issuance,      | Section 3.2.2.4.4  |
    |                  | Receive reports related | of [CABForum-BR],  |
    |                  | to HTTP service         | Section 5 of       |
    |                  |                         | [RFC2142]          |
    +------------------+-------------------------+--------------------+
    | www              | Common alias for        | Section 5 of       |
    |                  | webmaster               | [RFC2142]          |
    +------------------+-------------------------+--------------------+

                   Table 2: Dangerous E-mail local-parts

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.

4.1.  Additional Risks Out of Scope

   The lists of security concerns in this document cover security risks
   and concerns associated with interoperable use of specific labels.
   They do not cover every possible security concern associated with any
   DNS label or e-mail localpart.

   For example, DNS labels with an existing underscore are likely by
   construction to be sensitive, and are registered separately in the
   registry defined by [RFC8552].





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   Similarly, where humans or other systems capable of transcription
   error are in the loop, subtle variations of the labels listed here
   may also have security implications, due to homomgraphic confusion
   ([Homograph]), but this document does not attempt to enumerate all
   phishing, typosquatting, or similar risks of visual confusion, nor
   does it exhaustively list all other potential risks associated with
   variant encodings.  See [UTR36] for a deeper understanding of these
   categories of security concerns.

   Additionally, some labels may be associated with security concerns
   that happen to also commonly show up as DNS labels or e-mail local
   parts, but their risk is not associated with their use in
   interoperable public forms of DNS or e-mail.  For example, on some
   systems, a local user account named backup has full read access to
   the local filesytsem so that it can transfer data to the local backup
   system.  And in some cases, the list of local user accounts is also
   aliased into e-mail local parts.  However, permitting the
   registration of backup@example.net as an e-mail address is not itself
   an interoperable security risk -- no external third party will treat
   any part of the example.net domain differently because of the
   registration.  This document does not cover any risk entirely related
   to internal configuration choices.

5.  IANA Considerations

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

5.1.  Dangerous DNS Labels Registry

   The table of Dangerous DNS Labels (in Table 1) has three columns:

   *  DNS Label (text string)

   *  Rationale (human-readable short explanation)

   *  References (pointer or pointers to more detailed documentation)

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

   Following the guidance in [BCP26], any new entry to this registry
   will be assigned using Specification Required.  The IESG will assign
   one or more designated experts for this purpose, who will consult
   with the IETF DNSOP working group mailing list or its designated
   successor.  The Designated Expert will support IANA by clearly
   indicating when a new DNS label should be added to this table,



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   offering the label itself, a brief rationale, and a pointer to the
   permanent and readily available documentation of the security
   consequences of the label.  Updates or deletions of of DNS Labels
   will follow the same process.

5.2.  Dangerous E-mail Local Parts Registry

   The table of Dangerous E-mail Local Parts (in Table 2 also has three
   columns:

   *  E-mail local part (text string)

   *  Rationale (human-readable short explanation)

   *  References (pointer or ponters to more detailed documentation)

   Following the guidance in [BCP26], any new entry to this registry
   will be assigned using Specification Required.  The IESG will assign
   one or more designated experts for this purpose, who will consult
   with the IETF EMAILCORE working group mailing list or its designated
   successor.  The Designated Expert will support IANA by clearly
   indicating when a new e-mail local part should be added to this
   table, offering the local part itself, a brief rationale, and a
   pointer to the permanent and readily available documentation of the
   security consequences of the local part.  Updates or deletions of of
   E-mail Local Parts will follow the same process.

5.3.  Shared Considerations

   Having to add 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
   control over a separate resource based on control over an arbitrary
   label, administrators need a central place to keep track of which
   labels are dangerous.

   If such a practice cannot be avoided, it is better to ensure that the
   risk is documented clearly and referenced in the appropriate
   registry, rather than leaving it up to each administrator to re-
   discover the problem.

6.  References




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6.1.  Normative References

   [BCP26]    Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, June 2017.

              <https://www.rfc-editor.org/info/bcp26>

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <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,
              May 2017, <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, March 2019,
              <https://www.rfc-editor.org/info/rfc8552>.

6.2.  Informative References

   [CABForum-BR]
              Forum, C., "CA/Browser Forum Baseline Requirements", 23
              April 2022, <https://cabforum.org/wp-content/uploads/CA-
              Browser-Forum-BR-1.8.4.pdf>.

   [Homograph]
              "*** BROKEN REFERENCE ***".

   [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, 1 October 2018,
              <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-14, 13
              May 2022, <https://www.ietf.org/archive/id/draft-koch-
              openpgp-webkey-service-14.txt>.



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   [RFC2142]  Crocker, D., "Mailbox Names for Common Services, Roles and
              Functions", RFC 2142, DOI 10.17487/RFC2142, May 1997,
              <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,
              January 2019, <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, December 2018,
              <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,
              September 2019, <https://www.rfc-editor.org/info/rfc8641>.

   [UTR36]    Davis, M. and M. Suignard, "Unicode Security
              Considerations", n.d.,
              <https://unicode.org/reports/tr36/>.

   [VU591120] Center, C. C., "Multiple SSL certificate authorities use
              predefined email addresses as proof of domain ownership",
              7 April 2015, <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.





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B.2.  Document History

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

B.2.1.  Substantive Changes from -00 to -01

   *  explicitly define IANA tables

   *  indicate that the tables use Specification Required

   *  clarify scope

Author's Address

   Daniel Kahn Gillmor
   American Civil Liberties Union
   United States of America
   Email: dkg@fifthhorseman.net

































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