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RFC 8959 INFORMATIONAL SECURITY · URI

The "secret-token" URI Scheme

Registers a URI scheme that helps tools identify authentication tokens.

On rfc-editor.org ↗
PUBLISHED
January 2021
AUTHORS
M. Nottingham
STREAM
IETF
URI SCHEME
secret-token
MINIMUM TOKEN LENGTH ALLOWED BY SYNTAX
1 character
REGISTRATION STATUS
provisional
DOI
10.17487/RFC8959
ERRATA
Reported errata

RFC text

Contents
Original text · RFC Editor View on rfc-editor.org ↗ 
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.

Copyright Notice

   Copyright (c) 2021 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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/

ISO Plain Language

On this page
PROBLEMAccidentally exposed bearer tokens can be used by anyone possessing them, while arbitrary tokens can be hard for tools to identify.
MECHANISMThe RFC registers the secret-token URI scheme so generated secrets can use a recognizable prefix that scanning tools can detect.
THE CATCHThe scheme is intended to help prevent accidental disclosure; it cannot prevent intentional disclosure and is not intended for ephemeral tokens such as CSRF tokens.

§1Why Identify Secret Tokens

A bearer token is a security token that anyone possessing it can use, without proving possession of cryptographic key material. The RFC notes that accidental disclosure of these tokens has caused security incidents, including developers committing access tokens to public repositories.

A recognizable URI scheme can help continuous integration systems or repositories warn about tokens. The RFC intends secret-token URIs for generated secrets such as API keys, not for controlled uses of ephemeral tokens such as CSRF tokens.

A bearer token can be used by whoever has it. The recognizable scheme is meant to help tools find tokens that were exposed by accident.

§2The URI Format

A secret-token URI consists of the scheme secret-token, a colon and a token with at least one URI character. Disallowed characters, including non-ASCII characters, must first be encoded as UTF-8 and then percent-encoded.

When the URI is generated and later presented for authentication, the entire URI must be used without changes. The RFC gives an example that includes a percent-encoded space.

The URI begins with secret-token followed by a colon and the token. The full string must stay unchanged when it is used for authentication.

§4Limits of the Scheme

The token syntax allows a token as small as one character, which the RFC says is not recommended. Applications should assess their entropy needs and issue tokens accordingly.

The URI scheme is intended to reduce accidental disclosure, not prevent intentional disclosure. Users may remove the scheme or otherwise obfuscate secret material to evade detection, and the RFC notes that mitigating this can be beyond the reach of the system using the URI.

The label helps scanning tools, but it does not make a token secret by itself. Users can still remove the label or reveal the token intentionally.

§4References

[RFC3986]RFC 3986Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.January 2005[RFC3629]RFC 3629Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.November 2003[RFC6750]RFC 6750Jones, M. and D. Hardt, "The OAuth 2.0 Authorization Framework: Bearer Token Usage", RFC 6750, October 2012.October 2012