Docker Registry Token Scope and AccessEstimated reading time: 5 minutes
Tokens used by the registry are always restricted what resources they may be used to access, where those resources may be accessed, and what actions may be done on those resources. Tokens always have the context of a user which the token was originally created for. This document describes how these restrictions are represented and enforced by the authorization server and resource providers.
Subject (Authenticated User)
The subject represents the user for which a token is valid. Any actions
performed using an access token should be considered on behalf of the subject.
This is included in the
sub field of access token JWT. A refresh token should
be limited to a single subject and only be able to give out access tokens for
Audience (Resource Provider)
The audience represents a resource provider which is intended to be able to
perform the actions specified in the access token. Any resource provider which
does not match the audience should not use that access token. The audience is
included in the
aud field of the access token JWT. A refresh token should be
limited to a single audience and only be able to give out access tokens for that
The resource type represents the type of resource which the resource name is intended to represent. This type may be specific to a resource provider but must be understood by the authorization server in order to validate the subject is authorized for a specific resource.
Example Resource Types
repository- represents a single repository within a registry. A repository may represent many manifest or content blobs, but the resource type is considered the collections of those items. Actions which may be performed on a
pullfor accessing the collection and
pushfor adding to it.
The resource name represent the name which identifies a resource for a resource provider. A resource is identified by this name and the provided resource type. An example of a resource name would be the name component of an image tag, such as “samalba/myapp” or “hostname/samalba/myapp”.
The resource actions define the actions which the access token allows to be
performed on the identified resource. These actions are type specific but will
normally have actions identifying read and write access on the resource. Example
repository type are
pull for read access and
push for write
Authorization Server Use
Each access token request may include a scope and an audience. The subject is
always derived from the passed in credentials or refresh token. When using
a refresh token the passed in audience must match the audience defined for
the refresh token. The audience (resource provider) is provided using the
service field. Multiple resource scopes may be provided using multiple
fields on the
GET request. The
POST request only takes in a single
scope field but may use a space to separate a list of multiple resource
Resource Scope Grammar
scope := resourcescope [ ' ' resourcescope ]* resourcescope := resourcetype ":" resourcename ":" action [ ',' action ]* resourcetype := /[a-z]*/ resourcename := [ hostname '/' ] component [ '/' component ]* hostname := hostcomponent ['.' hostcomponent]* [':' port-number] hostcomponent := /([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9-]*[a-zA-Z0-9])/ port-number := /[0-9]+/ action := /[a-z]*/ component := alpha-numeric [ separator alpha-numeric ]* alpha-numeric := /[a-z0-9]+/ separator := /[_.]|__|[-]*/
Full reference grammar is defined here. Currently the scope name grammar is a subset of the reference grammar.
NOTE: that the
resourcenamemay contain one
:due to a possible port number in the hostname component of the
resourcename, so a naive implementation that interprets the first three
:-delimited tokens of a
scopeto be the
resourcename, and a list of
actionwould be insufficient.
Resource Provider Use
Once a resource provider has verified the authenticity of the scope through JWT access token verification, the resource provider must ensure that scope satisfies the request. The resource provider should match the given audience according to name or URI the resource provider uses to identify itself. Any denial based on subject is not defined here and is up to resource provider, the subject is mainly provided for audit logs and any other user-specific rules which may need to be provided but are not defined by the authorization server.
The resource provider must ensure that ANY resource being accessed as the result of a request has the appropriate access scope. Both the resource type and resource name must match the accessed resource and an appropriate action scope must be included.
When appropriate authorization is not provided either due to lack of scope
or missing token, the resource provider to return a
header with the
realm as the authorization server, the
service as the
expected audience identifying string, and a
scope field for each required
resource scope to complete the request.
JWT Access Tokens
Each JWT access token may only have a single subject and audience but multiple
resource scopes. The subject and audience are put into standard JWT fields
aud. The resource scope is put into the
access field. The
structure of the access field can be seen in the
A refresh token must be defined for a single subject and audience. Further restricting scope to specific type, name, and actions combinations should be done by fetching an access token using the refresh token. Since the refresh token is not scoped to specific resources for an audience, extra care should be taken to only use the refresh token to negotiate new access tokens directly with the authorization server, and never with a resource provider.