Docker Registry Token Scope and Access

Estimated 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.

Scope Components

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 that subject.

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 audience.

Resource Type

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 repository are pull for accessing the collection and push for adding to it.

Resource Name

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”.

Resource Actions

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 for the repository type are pull for read access and push for write access.

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 scope 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 scopes.

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 resourcename may 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 scope to be the resourcetype, resourcename, and a list of action would 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 WWW-AUTHENTICATE HTTP 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 sub and aud. The resource scope is put into the access field. The structure of the access field can be seen in the jwt documentation.

Refresh Tokens

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.

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