The "secret-token" URI Scheme
Prahran
VIC
Australia
mnot@mnot.net
https://www.mnot.net/
General
bearer token
token scanning
This document registers the "secret-token" URI scheme to aid in the identification
of authentication tokens.
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.
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 when, and only when, they appear in all capitals, as shown here.
This document uses ABNF . It also uses the pchar rule from .
The "secret-token" URI Scheme
The "secret-token" URI scheme identifies a token that is intended to be a secret.
See for a definition of pchar. Disallowed characters -- including non-ASCII characters -- MUST be encoded into UTF-8 and then percent-encoded ().
When a token is both generated and presented for authentication, the entire URI MUST be used, without changes.
For example, given the URI:
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 , a client might send:
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
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 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.
References
Normative References
Informative References
Acknowledgements
The definition of bearer tokens is from .