This page is being updated
The examples on this page are incompatible with Fedora 5, as they do not follow the SOLID WebAC specification. This page is being updated to bring it into alignment with the current specification
The Fedora WebAC authorization module is an implementation of the W3C's still evolving draft of an RDF-based decentralized authorization policy mechanism.
W3C's definition of WebAccessControl
From the WebAccessControl description at the W3C website:
WebAccessControl is a decentralized system for allowing different users and groups various forms of access to resources where users and groups are identified by HTTP URIs.
The WebAC module will enforce access control based on the Access Control List (ACL) RDF resource associated with the requested resource. In WebAC, an ACL consists of a set of Authorizations. Each Authorization is a single rule for access, such as "users alice and bob may write to resource foo", described with a set of RDF properties. Authorizations have the RDF type http://www.w3.org/ns/auth/acl#Authorization
.
For the remainder of this document, the
http://www.w3.org/ns/auth/acl#
namespace will be abbreviated with the prefix acl:
.
Access Control Lists (ACLs)
An ACL is an RDF document (RDFSource) that contains WebAC statements that authorize access to repository resources. Each resource may have their own ACL, or implicitly be subject to the ACL of a parent container. The location of the acl for a given resource may be discovered via a Link
header with relation rel=acl
.
If a resource does not have an individual ACL (and therefore relies on an implicit ACL from a parent), this link header will still be present, but will return a 404. This is because the location of ACLs is solely determined by the server, much like the automatically-created LDP-RS descriptions for binary resources. The key difference is that Fedora does not create ACLs automatically, only their location.
Therefore, to discover whether a resource has an individual ACL, a client would need to:
- Perform a
HEAD
orGET
against the resource, - Find the link header
- Do a
GET
orHEAD
against the ACL location, and see if returns 200 or 404.
To create an ACL for a resource that does not already have one, a client needs to discover the ACL location (via HEAD
or GET
), then PUT
to that location.
Authorizations
Authorizations are the permissions statements contained within an ACL document. An ACL may contain many authorizations, each of which must share the same subject. The way this is achieved is via hash URIs:
@prefix acl: <http://www.w3.org/ns/auth/acl#> <#auth1> a acl:Authorization .
The properties that may be used on an acl:Authorization
are:
Property | Meaning |
---|---|
acl:accessTo | the URI of the protected resource |
acl:accessToClass | an RDF class of protected resources. (While the WebAC specification does not support acl:accessToClass, servers are required to support it according to the Fedora specification) |
acl:agent | the user (in the W3C WebAC ontology, the user is named with a URI, but Fedora's implementation supports both URI- and string-based usernames) |
acl:agentClass | Identifies a class of agents, rather than a specific agent. Usage according to the the WebAC spec is limited to foaf:Agent (meaning "everybody"), and acl:AuthenticatedAgent (meaning "any authenticated agent"). |
acl:agentGroup | a group of users (defined as a foaf:Group resource listing its users with the foaf:member property) |
acl:default | signifies that an authorization for a container may be inherited by children of that container, if they do not otherwise define their own ACLs. |
acl:mode | the type of access (WebAC defines several modes: acl:Read , acl:Write , acl:Append , and acl:Control ; Fedora implements acl:Read and acl:Write ) |
For a more detailed explanation of Authorizations and their properties, see WebAC Authorizations.
Agents
Agents are the users of Fedora. These identify the principals (in a security sense) have made authenticated requests to the repository. In ACL Authorizations used by Fedora, these may be represented as strings or as URIs. The SOLID WebAC spec stipulates that agents are identified by URIs, and suggests (but does not have any normative language requiring) that these URIs are intended to be WebIDs. The Fedora specification does not comment on the topic of identifying agents. Nevertheless, for legacy purposes, the Fedora 5.x software allows strings or URIs to identify agents (e.g. "bob"
or <http://example.org/people/bob>
). When using URIs, there is no expectation be Fedora that these URIs be resolvable, or have a representation. It is highly recommended that you use URIs
The mapping of a logged-on principal to a string or URI depends on the selection and configuration of a Principal Provider, which may provide the identity of users as strings or URIs depending on its implementation. Because agents are recommended to be represented as URIs, Fedora can be configured to automatically prefix any principals that are provided as strings with a baseURI. This is achieved by setting the system property fcrepo.auth.webac.userAgent.baseUri
. For example:
fcrepo.auth.webac.userAgent.baseUri=http://example.org/agent/
Continuing with this example, if a user comes in as user "dra2"
, the user's identity will be converted to the URI http://example.org/agent/dra2 before applying ACLs.
Examples of Authorizations
The user userA can Read document foo
@prefix acl: <http://www.w3.org/ns/auth/acl#> <#auth1> a acl:Authorization ; acl:accessTo </fcrepo/rest/foo> ; acl:mode acl:Read; acl:agent "userA" .
Users in NewsEditor group can Write to any resource of type ex:News
@prefix acl: <http://www.w3.org/ns/auth/acl#> . @prefix ex: <http://example.org/ns#> . <#auth2> a acl:Authorization ; acl:accessToClass ex:News ; acl:mode acl:Read, acl:Write; acl:agentClass <fcrepo/rest/agents/NewsEditors> .
/agents/NewsEditors@prefix foaf: <http://xmlns.com/foaf/0.1/> . <> a foaf:Group; foaf:member "editor1", "editor2".
The user userB can Read document foo (This involves setting a system property for the servlet container, e.g.
-Dfcrepo.auth.webac.userAgent.baseUri=http://example.org/agents/)
@prefix acl: <http://www.w3.org/ns/auth/acl#> <#auth3> a acl:Authorization ; acl:accessTo </fcrepo/rest/foo> ; acl:mode acl:Read; acl:agent <http://example.org/agents/userB> .
Protecting Resources
A resource specifies the location of its ACL using the acl:accessControl
property. If a resource itself does not specify an ACL, its parent containers are inspected, and the first specified ACL found is used as the ACL for the requested resource. If no ACLs are found, a filesystem-based ACL will be checked, the default policy of which is to deny access to the requested resource.
Example Scenarios
These scenarios assume that Fedora has been configured to use fcrepo.auth.webac.userAgent.baseUri=http://example.org/agent/ and
fcrepo.auth.webac.groupAgent.baseUri=http://example.org/group/
I want to allow a user with username "smith123" to have read, write access to resource http://localhost:8080/rest/webacl_box1.
I want to let the group "Editors" have read, write access on all the items in the collection "http://localhost:8080/rest/box/bag/collection"
I would like the collection http://localhost:8080/rest/dark/archive to be viewable only by the groupId "Restricted", but I would like to allow anyone to view the resource http://localhost:8080/rest/dark/archive/sunshine.
The collection http://localhost:8080/rest/public_collection should be readable by anyone but only editable by users in the group Editors.
Only the ex:publicImage type objects in the container http://localhost:8080/rest/mixedCollection are viewable by anyone, all others are only viewable by the group Admins.