Internet DRAFT - draft-dkg-intarea-dangerous-labels
draft-dkg-intarea-dangerous-labels
intarea D. K. Gillmor
Internet-Draft ACLU
Intended status: Informational 13 October 2022
Expires: 16 April 2023
Dangerous Labels in DNS and E-mail
draft-dkg-intarea-dangerous-labels-03
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 16 April 2023.
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. DNS Labels . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. E-mail Local Parts . . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 7
4.1. Additional Risks Out of Scope . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5.1. Dangerous DNS Labels Registry . . . . . . . . . . . . . . 8
5.2. Dangerous E-mail Local Parts Registry . . . . . . . . . . 9
5.3. Shared Considerations . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 12
Appendix B. Document Considerations . . . . . . . . . . . . . . 12
B.1. Other types of labels? . . . . . . . . . . . . . . . . . 13
B.2. Document History . . . . . . . . . . . . . . . . . . . . 13
B.2.1. Substantive Changes from -02 to -03 . . . . . . . . . 13
B.2.2. Substantive Changes from -01 to -02 . . . . . . . . . 13
B.2.3. Substantive Changes from -00 to -01 . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
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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.
Note also that Section 2.2 of [RFC8820] makes it clear that depending
on specific forms of DNS labels in a given URI scheme in a protocol
is strongly discouraged.
Below are some normal-looking DNS labels that may grant some form of
administrative control over the domain that they are attached to.
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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].
+============+==================+===================================+
|DNS Label |Rationale | Reference |
+============+==================+===================================+
|autoconfig |Hijack mail user | [AUTOCONF], [THUNDERBIRD] |
| |agent | |
| |autoconfiguration | |
+------------+------------------+-----------------------------------+
|autodiscover|Hijack Microsoft | [AUTODISCOVER] |
| |Exchange client | |
| |configuration | |
+------------+------------------+-----------------------------------+
|imap |Hijack mail user | [AUTOCONF], [THUNDERBIRD] |
| |agent | |
| |autoconfiguration | |
+------------+------------------+-----------------------------------+
|imaps |Hijack mail user | [AUTOCONF] |
| |agent | |
| |autoconfiguration | |
+------------+------------------+-----------------------------------+
|mail |Hijack mail user | [AUTOCONF], [THUNDERBIRD] |
| |agent | |
| |autoconfiguration | |
+------------+------------------+-----------------------------------+
|mta-sts |Set SMTP transport| [RFC8461] |
| |security policy | |
+------------+------------------+-----------------------------------+
|openpgpkey |Domain-based | [I-D.koch-openpgp-webkey-service] |
| |OpenPGP | |
| |certificate lookup| |
| |and verification | |
+------------+------------------+-----------------------------------+
|pop3 |Hijack mail user | [AUTOCONF] |
| |agent | |
| |autoconfiguration | |
+------------+------------------+-----------------------------------+
|pop3s |Hijack mail user | [AUTOCONF] |
| |agent | |
| |autoconfiguration | |
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+------------+------------------+-----------------------------------+
|root-key- |Indicates which | [RFC8509] |
|sentinel-is-|DNSSEC root key is| |
|ta-<key-tag>|trusted | |
+------------+------------------+-----------------------------------+
|root-key- |Indicates which | [RFC8509] |
|sentinel- |DNSSEC root key is| |
|not-ta-<key-|not trusted | |
|tag> | | |
+------------+------------------+-----------------------------------+
|smtp |Hijack mail user | [AUTOCONF], [THUNDERBIRD] |
| |agent | |
| |autoconfiguration | |
+------------+------------------+-----------------------------------+
|smtps |Hijack mail user | [AUTOCONF] |
| |agent | |
| |autoconfiguration | |
+------------+------------------+-----------------------------------+
|submission |Hijack mail user | [AUTOCONF] |
| |agent | |
| |autoconfiguration | |
+------------+------------------+-----------------------------------+
|wpad |Automatic proxy | [I-D.ietf-wrec-wpad-01] |
| |discovery | |
+------------+------------------+-----------------------------------+
|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] |
+------------------+-------------------------+--------------------+
| openpgp-ca | Make authoritative- | [OPENPGP-CA] |
| | seeming OpenPGP | |
| | certifications for in- | |
| | domain e-mail addresses | |
+------------------+-------------------------+--------------------+
| 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 |
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| | Issuance (historic) | for root (software |
| | | config docs?) |
+------------------+-------------------------+--------------------+
| security | Receive reports of | Section 4 of |
| | technical | [RFC2142] |
| | 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.
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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].
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 filesystem 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].
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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,
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 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.
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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
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
[AUTOCONF] Wolf, A., "Mail Client Auto-Configuration", 22 February
2021,
<https://roll.urown.net/server/mail/autoconfig.html>.
[AUTODISCOVER]
Microsoft, "Autodiscover for Exchange", 15 January 2020,
<https://docs.microsoft.com/en-us/exchange/client-
developer/exchange-web-services/autodiscover-for-
exchange>.
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[CABForum-BR]
CA/Browser Forum, "CA/Browser Forum Baseline
Requirements", 23 April 2022, <https://cabforum.org/wp-
content/uploads/CA-Browser-Forum-BR-1.8.4.pdf>.
[Homograph]
Gabrilovich, E. and A. Gontmakher, "The homograph attack",
Communications of the ACM vol. 45, no. 2, pp. 128,
DOI 10.1145/503124.503156, February 2002,
<https://doi.org/10.1145/503124.503156>.
[I-D.hoffman-dns-special-labels]
Hoffman, P. E., "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.ietf-wrec-wpad-01]
Perkins, C. E., Cohen, J., Dunsmuir, M., Gauthier, P. A.,
Cooper, I., and J. W. C. M.A., "Web Proxy Auto-Discovery
Protocol", Work in Progress, Internet-Draft, draft-ietf-
wrec-wpad-01, 29 July 1999,
<https://www.ietf.org/archive/id/draft-ietf-wrec-wpad-
01.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>.
[OPENPGP-CA]
the OpenPGP CA project, "OpenPGP CA: Technical Details",
n.d., <https://openpgp-ca.org/background/details/>.
[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>.
[RFC8461] Margolis, D., Risher, M., Ramakrishnan, B., Brotman, A.,
and J. Jones, "SMTP MTA Strict Transport Security (MTA-
STS)", RFC 8461, DOI 10.17487/RFC8461, September 2018,
<https://www.rfc-editor.org/info/rfc8461>.
[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>.
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[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>.
[RFC8820] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 8820, DOI 10.17487/RFC8820, June 2020,
<https://www.rfc-editor.org/info/rfc8820>.
[THUNDERBIRD]
Bucksch, B., "Autoconfiguration in Thunderbird", 3 May
2021, <https://github.com/mdn/archived-
content/tree/main/files/en-us/mozilla/thunderbird/
autoconfiguration>.
[UTR36] Davis, M. and M. Suignard, "Unicode Security
Considerations", n.d.,
<https://unicode.org/reports/tr36/>.
[VU591120] CERT Coordination Center, "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].
Rasmus Dahlberg, yuki, and Carsten Bormann reviewed this draft and
gave feedback.
Tim Wicinski pointed out wpad.
Appendix B. Document Considerations
This section is to be removed before publishing as an RFC.
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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 that the author is
familiar with. 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. If you are aware of some other system with a
similar pattern, please send feedback.
B.2. Document History
B.2.1. Substantive Changes from -02 to -03
* Add mail DNS label (MUA autoconfiguration)
* Reference Thunderbird guidance about MUA autoconfiguration
* Add openpgp-ca e-mail local part
B.2.2. Substantive Changes from -01 to -02
* New dangerous DNS labels: WPAD, MS Exchange Autodiscover, common
MUA autoconfig (originally from Mozilla Thunderbird)
B.2.3. 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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