Keywords: bearer token, token scanning
Internet Engineering Task Force (IETF) M. Nottingham
Request for Comments: 8959 January 2021
Category: Informational
ISSN: 2070-1721
The "secret-token" URI Scheme
Abstract
This document registers the "secret-token" URI scheme to aid in the
identification of authentication tokens.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8959.
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Table of Contents
1. Introduction
1.1. Notational Conventions
2. The "secret-token" URI Scheme
3. IANA Considerations
4. Security Considerations
5. References
5.1. Normative References
5.2. Informative References
Acknowledgements
Author's Address
1. Introduction
It has become increasingly common to use bearer tokens as an
authentication mechanism in various protocols.
A bearer token is a security token with the property that any party
in possession of the token (a "bearer") can use the token in any way
that any other party in possession of it can. Using a bearer token
does not require a bearer to prove possession of cryptographic key
material (proof-of-possession).
Unfortunately, the number of security incidents involving accidental
disclosure of these tokens has also increased. For example, we now
regularly hear about a developer committing an access token to a
public source code repository, either because they didn't realize it
was included in the committed code or because they didn't realize the
implications of its disclosure.
This specification registers the "secret-token" URI scheme to aid
prevention of such accidental disclosures. When tokens are easier to
unambiguously identify, they can trigger warnings in continuous
integration systems or be used in source code repositories
themselves. They can also be scanned for separately.
For example, if cloud.example.net issues access tokens to its clients
for later use, and it does so by formatting them as "secret-token"
URIs, tokens that "leak" into places that they don't belong are
easier to identify. This could be through a variety of mechanisms;
for example, if repo.example.com can be configured to refuse commits
containing "secret-token" URIs, it helps its customers avoid
accidental disclosures.
"secret-token" URIs are intended to aid in identification of
generated secrets, like API keys and similar tokens. They are not
intended for use in controlled situations where ephemeral tokens are
used, such as things like Cross-Site Request Forgery (CSRF) tokens.
1.1. Notational Conventions
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.
This document uses ABNF [RFC5234]. It also uses the pchar rule from
[RFC3986].
2. The "secret-token" URI Scheme
The "secret-token" URI scheme identifies a token that is intended to
be a secret.
secret-token-URI = secret-token-scheme ":" token
secret-token-scheme = "secret-token"
token = 1*pchar
See [RFC3986], Section 3.3 for a definition of pchar. Disallowed
characters -- including non-ASCII characters -- MUST be encoded into
UTF-8 [RFC3629] and then percent-encoded ([RFC3986], Section 2.1).
When a token is both generated and presented for authentication, the
entire URI MUST be used, without changes.
For example, given the URI:
secret-token:E92FB7EB-D882-47A4-A265-A0B6135DC842%20foo
This (character-for-character, case-sensitive) string will both be
issued by the token authority and required for later access.
Therefore, if the example above were used as a bearer token in
[RFC6750], a client might send:
GET /authenticated/stuff HTTP/1.1
Host: www.example.com
Authorization: Bearer
secret-token:E92FB7EB-D882-47A4-A265-A0B6135DC842%20foo
3. IANA Considerations
This document registers the following value in the "Uniform Resource
Identifier (URI) Schemes" registry:
Scheme name: secret-token
Status: provisional
Applications/protocols that use this scheme: none yet
Contact: iesg@iesg.org
Change Controller: IESG
References: RFC 8959
4. Security Considerations
The token ABNF rule allows tokens as small as one character. This is
not recommended practice; applications should evaluate their
requirements for entropy and issue tokens correspondingly. See
[RFC4086] for more information.
This URI scheme is intended to reduce the incidence of accidental
disclosure; it cannot prevent intentional disclosure.
If it is difficult to correctly handle secret material, or unclear as
to what the appropriate handling is, users might choose to obfuscate
their secret tokens in order to evade detection (for example,
removing the URI scheme for storage). Mitigating this risk is often
beyond the reach of the system using the "secret-token" URI; users
can be cautioned against such practices and be provided tools to
help.
5. References
5.1. Normative References
[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>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[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>.
5.2. Informative References
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<https://www.rfc-editor.org/info/rfc6750>.
Acknowledgements
The definition of bearer tokens is from [RFC6750].
Author's Address
Mark Nottingham
Prahran VIC
Australia
Email: mnot@mnot.net
URI: https://www.mnot.net/