Difference between revisions of "Access Control List Tutorial"

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Revision as of 23:06, 25 September 2011

ACL Defined

ACL or Access Control List
According to Wikipedia, “An ACL specifies which users or system processes are granted access to objects, as well as what operations are allowed to be performed on given objects.”

In the case of Joomla!, we have two separate aspects to ACL.

  1. Which users can gain access to what parts of the website? For example, will a given menu choice be visible for a given user?
  2. What operations (or actions) a user can perform on any given object? For example, can a user submit or edit an article?

Overview of ACL in Version 1.6

This section outlines the major ACL changes between versions 1.5 and 1.6.

Users, Groups, and Access Levels

With the definition in mind, let's look at how we set up the ACL for our site in version 1.6. The table below summarizes the major changes from version 1.5.

Version 1.5 Version 1.6
Groups 7 fixed groups (Public, Registered, Author, Editor, Publisher, Manager, Administrator, and Super-Administrator) Unlimited user-defined Groups
Users & Groups A User can be assigned to only one group A User can be assigned to multiple groups
Access Levels 3 fixed Access Levels (Public, Registered, Special) Unlimited user-defined Access Levels
Access Levels & Groups Relationship between Groups and Access Levels was fixed. Groups are assigned to Access Levels. Any combination of Groups can be assigned to any Access Level.

We see that in every case the ACL has been made much more flexible, with unlimited Groups and Access Levels, and the ability to assign one User to multiple Groups and any Groups to any Access Level.

Actions, Groups, and Inheritance

The other side of ACL is granting permissions to users to take actions on objects. Here again there is a big change between Version 1.5 and 1.6. In 1.5, the actions allowed for a given group were fixed. For example, a User in the Author group could only submit an article whereas someone in the Publisher group could submit, edit, and publish articles. Also, in version 1.5 the permissions were all-or-nothing. A member of the Editor group could edit all articles on the site.

The table below shows what has changed between versions 1.5 and 1.6.

Version 1.5 Version 1.6
Groups and Actions Actions allowed by different groups are fixed. Actions allowed for each group are defined by site administrator.
Permission Scope Entire Site. User has same permissions for all objects on the site. Permissions can be set at multiple levels in hierarchy: Site, Component, Category, Object.
Permission Inheritance Not applicable Permissions can be inherited from parent Groups and parent Categories

How Permissions Work

There are four possible permissions for actions, as outlined below:

  • Not set: Defaults to "deny" but, unlike the Deny permission, this permission can be overridden by setting a child group or a lower level in the permission hierarchy to "Allow". This permission only applies to the Global Configuration permissions.
  • Inherit: Inherits the value from a parent Group or from a higher level in the permission hierarchy. This permission applies to all levels except the Global Configuration level.
  • Deny: Denies this action for this level and group. IMPORTANT: This also denies this action for all child groups and all lower levels in the permission hierarchy. Putting in Allow for a child group or a lower level will not have any effect. The action will always be denied for any child group member and for any lower level in the permission hierarchy.
  • Allow: Allows this action for this level and group and for lower levels and child groups. This does not have any effect if a higher group or level is set to Deny or Allow. If a higher group or level is set to Deny, then this permission will always be denied. If a higher group or level is set to Allow, then this permission will already be allowed.

Permission Hierarchy Levels

Action permissions in version 1.6 can be defined at up to four levels, as follows:

  1. Global Configuration: determines the default permissions for each action and group.
  2. Component Options->Permissions: can override the default permissions for this component (for example, Articles, Menus, Users, Banners, and so on)
  3. Category: can override the default permissions for objects in one or more categories. Applies to all components with categories, including Articles, Banners, Contacts, Newsfeeds, and Weblinks.
  4. Article: Can override the permissions for a specific article. This level only applies to articles. Other components only allow the first three levels.

Global Configuration

This is accessed from Site → Global Configuration → Permissions. This screen allows you set the top-level permission for each group for each action, as shown in the screenshot below.

Screenshot acl tutorial 20110111-01.png

The options for each value are Inherited, Allowed, or Denied. The Calculated Setting column shows you the setting in effect. It is either Not Allowed (the default), Allowed, or Denied.

You work on one Group at a time by opening the slider for that group. You change the permissions in the Select New Settings drop-down list boxes.

Note that the Calculated Setting column is not updated until you press the Save button in the toolbar. To check that the settings are what you want, press the Save button and check the Calculated Settings column.

Component Options->Permissions

This is accessed for each component by clicking the Options icon in the toolbar. This screen is similar to the Global Configuration screen above. For example, clicking the Options toolbar icon in the Menu Manager shows the Menus Configuration below.

Screenshot acl tutorial 20110111-02.png

Access to Options is only available to members of groups who have permission for the Configure action in for each component. In the example above, the Administrator group has Allowed permission for the Configure option, so members of this group can access this screen.


Category permissions are accessed in the Category Manager: Edit Category screen, in a slider at the bottom of the screen. This screen has five permissions, as shown below.

Screenshot acl tutorial 20110111-03.png

In these screens, you work on the permissions for one User Group at a time. In the example above, we are editing the permissions for the Administrator group.

Note that the Configure and Access Component actions do not apply at the category level, so those actions are not included.

Note also that Categories can be arranged in a hierarchy. If so, then action permissions in a parent category are inherited automatically by a child category. For example, if you had a category hierarchy of Animals → Pets → Dogs, then the full permission level hierarchy for an article in the Dogs category would be as follows:

  • Global Configuration
  • Article Manager → Options → Permission
  • Animals Category
  • Pets Category
  • Dogs Category
  • specific article


Permissions for a single article are access in the Article Manager: Edit Article screen, again in a slider at the bottom of the screen. This screen has three actions, as shown below.

Screenshot acl tutorial 20110111-04.png

Again, you edit each group by clicking on it to open the slider for that group. You can then change the permissions under the Select New Setting column. To see the effect of any changes, press the Save button to update the Calculated Setting column.

Note that the Configure, Access Component, and Create actions do not apply at the article level, so these actions are not included. Permission to create an article is set at one of the higher levels in the hierarchy.

Access Levels

Access Levels in version 1.6 are simple and flexible. The screen below shows the Special Access Level.

Screenshot acl tutorial 20110111-05.png

Simply check the box for each group you want included in that level. The Special Access Level includes the Manager, Author, and Super Users groups. It also includes child groups of those groups. So, Administrator group is included, since it is a child group of the Manager group. The Editor, Publisher, and Shop Suppliers groups are included, since they are child groups of Author. (Note that we could check all of the child groups if we wanted and it wouldn't hurt anything.)

Once Access Levels are created, they are used in the same way as in version 1.5. Each object in the front end is assigned an Access Level. If the level is Public, then anyone may access that object. Otherwise, only members of groups assigned to that access level may access that object. Access levels are assigned to Menu Items and to Modules. Each one can only be assigned to one access level.

For example, the screen below shows the Edit Menu Item screen with the list of available access levels.

Screenshot acl tutorial 20110111-06.png

Default ACL Setup

When Joomla! is installed, these are set to their initial default settings. We will discuss these initial settings as a way to understand how the ACL works.

Default Groups

Version 1.6 allows you to define your own Groups. When you install version 1.6, it includes a set of default groups, as shown below.

Screenshot acl tutorial 20110111-07.png

The arrows indicate the child-parent relationships. As discussed above, when you set a permission for a parent group, this permission is automatically inherited by all child groups. The Inherited, and Allowed permissions can be overridden for a child group. The Denied permission cannot be overridden and will always deny an action for all child groups.

Global Configuration

Joomla! version 1.6 will install with the same familiar back-end permissions as that of version 1.5. However, with 1.6, you can easily change these to suit the needs of your site.

As discussed earlier, the permissions for each action are inherited from the level above in the permission hierarchy and from a group's parent group. Let's see how this works. The top level for this is the entire site. This is set up in the Site->Global Configuration->Permissions, as shown below.

Screenshot acl tutorial 20110111-08.png

The first thing to notice are the nine Actions: Site Login, Admin Login, Super Admin, Access Component, Create, Delete, Edit, Edit State. and Edit Own. These are the actions that a user can perform on an object in Joomla. The specific meaning of each action depends on the context. For the Global Configuration screen, they are defined as follows:

Site Login 
Login to the front end of the site
Admin Login 
Login to the back end of the site
Super Admin 
Grants the user "super user" status. Users with this permission can do anything on the site. Only users with this permission can change Global Configuration settings (this screen). These permissions cannot be restricted. It is important to understand that, if a user is a member of a Super Admin group, any other permissions assigned to this user are irrelevant. The user can do any action on the site. However, Access Levels can still be assigned to control what this group sees on the site. (Obviously, a Super Admin user can change Access Levels if they want to, so Access Levels do not totally restrict what a Super Admin user can see.)
Access Component
Open the component manager screens (User Manager, Menu Manager, Article Manager, and so on)
Create new objects (for example, users, menu items, articles, weblinks, and so on)
Delete existing objects
Edit existing objects
Edit State 
Change object state (Publish, Unpublish, Archive, and Trash)
Edit Own 
Edit objects that you have created.

Each Group for the site has its own slider which is opened by clicking on the group name. In this case (with the sample data installed), we have the standard 7 groups that we had in version 1.5 plus two additional groups called "Shop Suppliers" and "Customer Group". Notice that our groups are set up with the same permissions as they had in version 1.5. Keep in mind that we can change any of these permissions to make the security work the way we want. Let's go through this to see how it works.

  • Public has everything set to "Not set", as shown below.
Screenshot acl tutorial 20110112-06.png
This can be a bit confusing. Basically, "Not Set" is the same as "Inherited". Because Public is our top-level group, and because Global Configuration is the top level of the component hierarchy, there is nothing to inherit from. So "Not Set" is used instead of "Inherit".
The default in this case is for no permissions. So, as you would expect, the Public group has no special permissions. Also, it is important to note that, since nothing is set to Denied, all of these permissions may be overridden by child groups or by lower levels in the permission hierarchy.
  • Manager is a "child" group of the Public group. It has Allowed permissions for everything except Access Component and Super Admin. So a member of this group can do everything in the front and back end of the site except change Global Permissions and Component Options.
  • Administrator group members inherit all of the Manager permissions and also have Allowed for Access Component. So members of this group by default can access the Options screens for each component.
  • Registered is the same a Public except for the Allow permission for the Site Login action. This means that members of the Registered group can login to the site. Since default permissions are inherited, this means that, unless a child group overrides this permission, all child groups of the Registered group will be able to login as well.
  • Author is a child of the Registered group and inherits its permissions and also adds Create and Edit Own. Since Author, Editor, and Publisher have no back-end permissions, we will discuss them below, when we discuss front-end permissions.
  • Editor is a child of the Authors group and adds the Edit permission.
  • Publisher is a child of Editor and adds the Edit State permission.
  • Shop Suppliers is an example group that is installed if you install the sample data. It is a child group of Author.
  • Customer Group is an example group that is installed if you install the sample data. It is a child group of Registered.
  • Super Users group has the Allow permission for the Super Admin action. Because of this, members of this group have super user permissions throughout the site. They are the only users who can access and edit values on the Global Configuration screen. Users with permission for the Super Admin action have some special characteristics:
    • If a user has Super Admin permissions, no other permissions for this user matter. The user can perform any action on the site.
    • Only Super Admin users can create, edit, or delete other Super Admin users or groups.

There are two very important points to understand from this screen. The first is to see how the permissions can be inherited from the parent Group. The second is to see how you can control the default permissions by Group and by Action.

This provides a lot of flexibility. For example, if you wanted Shop Suppliers to be able to have the ability to login to the back end, you could just change their Admin Login value to "Allowed". If you wanted to not allow members of Administrator group to delete objects or change their state, you would change their permissions in these columns to Inherited (or Denied).

It is also important to understand that the ability to have child groups is completely optional. It allows you to save some time when setting up new groups. However, if you like, you can set up all groups to have Public as the parent and not inherit any permissions from a parent group.

Component Options & Permissions

Now, let's continue to see how the default back-end permissions for version 1.6 mimic the permissions for version 1.5. The Super Users group in 1.6 is equivalent to the Super Administrator group in 1.5.

Just looking at the Global Configuration screen above, it would appear that the Administrator group and the Manager group have identical permissions. However, in version 1.5 Administrators can do everything except Global Configuration, whereas Managers are not permitted to add users or work with menu items. That is also true in the default version 1.6 configuration. Let's see how this is accomplished.

If we navigate to Users->User Manager and click the Options button in the toolbar, we see the screen below:

Screenshot acl tutorial 20110111-09-en.png
Screenshot acl tutorial 20110111-10-en.png

This screen is the same as the Global Configuration Permissions screen, except that these values only affect working with Users. Let's look at how this works.

First, notice that the Administrator group has Allow permission for the Admin action and the Manager group has Deny permission for this action. Remember that the Admin action in the Global Configuration screen gives the group "super user" permissions. In this screen, the Admin action allows you to edit the Options values. So, the Administrator group can do this but the Manager group cannot.

Next, notice that the Administrator has Inherit for the Manage action and the Manager group has Deny permission. In this screen, the Manage action gives a group access to the User Manager. Since the Administrator has Allow for the Manage action by default, then the Inherit permission here means they inherit the Allow permission for the Manage action. Since the Manager group has Deny permission for the Manage action, members of the Manager group cannot access the User Manager and therefore cannot do any of the other user-related actions.

If you look at the Options for Menus->Menu Manager, you will see the same default settings as for the User Manager. Again, the Administrator group can manage and set default permissions for Menu Manager objects whereas the Manager group cannot.

In short, we can see that the different permissions for the Administrator and Manager groups are set using the Options->Permissions forms on the User Manager and Menu Manager screens.

It is also important to understand that this same Options->Permissions form for setting default permissions is available for all Joomla! objects, including Media Manager, Banners, Contacts, Newsfeeds, Redirect, Search Statistics, Web Links, Extensions, Modules, Plugins, Templates, and Language. So you now have the option to create user groups with fine-tuned sets of back-end permissions.

Front End Permissions

Default permissions for the front end are also set using the Options form. Let's look at Content->Article Manager->Options->Permissions. First, let's look at the permissions for Manager, as shown below.

Screenshot acl tutorial 20110111-11a-en.png

Manager has allowed permission for all actions except Configure. So members of the Manager group can do everything with Articles except open the Options screen.

Now let's look at Administrator, as shown below.

Screenshot acl tutorial 20110111-12a-en.png

Administrator has Allowed for Configure, so Administrators can edit this Options screen.

Both groups can create, delete, edit, and change the state of articles.

Now, let's look at the groups Publisher, Editor, and Author and see how their permissions are set.

Authors only have Create and Edit Own permissions, as shown below.

Screenshot acl tutorial 20110112-07-en.png

This means that Authors can create articles and can edit articles they have created. They may not delete articles, change the published state of articles, or edit articles created by others.

Editors have the same permissions as Authors with the addition of permission for the Edit action, as shown below.

Screenshot acl tutorial 20110112-08-en.png

So Editors can edit articles written by anyone.

Publishers can do everything Editors can do plus they have permission for the Edit State action, as shown below.

Screenshot acl tutorial 20110112-09-en.png

So Publishers can change the published state of an article. The possible states include Published, Unpublished, Archived, and Trashed.

All of these groups have Inherit permission for Configure and Access Component. Remember that Author is a child of the Registered group, and Registered does not have any default permissions except for Login. Since Registered does not have permission for Configure and Access Component, and since Author's permission for these actions is "Inherited", then Author does not have these permissions either. This same permission is passed from Author to Editor and from Editor to Publisher. So, by default, none of these groups are allowed to work with articles in the back end.

It is important to remember that these permissions are only default settings for categories and articles and for any child groups that are created. So they can be overridden for child groups, for categories, and for specific articles.

Also, note that there are no Denied permissions for any actions in the default settings. This allows you to add Allowed permissions at any level. Remember, once you have an action set for Denied, this action will be denied at all lower levels in the hierarchy. For example, if you set the Admin Login for Registered to Denied (instead of Inherited), you could not grant Publishers Allowed permissions for this action.

Article Manager & Actions Diagram

The diagram below shows how each action in the permissions form relates to the various options on the Article Manager screen.

Screenshot acl tutorial 20110111-16-en.png
  • Configure allows you to view and change the Options for the component.
  • Access Component allows you to navigate to the Article Manager. Without this permission, no other actions are possible.
  • Create allows you to add new articles.
  • Delete allows you to delete trashed articles. Note that the Delete icon only shows in the toolbar when you have the "Select State" filter set to "Trash".
  • Edit allows you to edit existing articles.
  • Edit State allows to you Publish, Unpublish, Archive, or Trash articles.
  • Edit Own is the same as Edit except that it only applies to articles written by you.

Allowing Guest-Only Access to Menu Items and Modules

Version 1.6 introduces the ability to create a View Access Level that is only for guests of the site (meaning a user who is not logged in). The example below shows how you can set up this new feature.

  1. Create a new user group called Guest. Make it a child of the Public group as shown below.
    Screenshot acl tutorial 20110112-01-en.png
  2. Create a new access level called Guest and grant only the Guest group access to this level, as shown below.
    Screenshot acl tutorial 20110112-02-en.png
  3. Edit the Public access level and add the Guest group to it, as shown below.
    Screenshot acl tutorial 20110112-03.png
  4. Navigate to User Manager→Options→Component and change the Guest User Group from the default value of "Public" to "Guest", as shown below.
Screenshot acl tutorial 20110112-04-en.png

Now, if we assign a menu item, module, or other object to the Guest access level, only non-logged in users will have access. For example, if we create a new menu item with access level of Guest, as shown below,

Screenshot acl tutorial 20110112-05-en.png

this menu item will only be visible to non-logged-in visitors to the site. Login/logout in frontend (for changing data in session) to see the change.

Using Permission and Group Levels Together

As discussed above, it is possible to define groups in a hierarchy, where each child group inherits action permissions (for example, the create permission) from its parent group. Action permissions are also be inherited from the permission level above. For example, a permission in the Article Manager is inherited from the same permission in the Global Configuration, and a permission in a child Category is inherited from the parent Category permission.

This dual inheritance can be confusing, but it can also be useful. Let's consider an example as follows. We have a school with a group hierarchy of Teachers → History Teachers → Assistant History Teachers. We also have a category hierarchy of Assignments → History Assignments. We want History Teachers and Assistant History Teachers to have the following permissions:

  • both groups can create new articles only in the History Assignments category.
  • only History Teachers (not Assistant History Teachers) can Publish or otherwise have Edit State permission.

This ACL scheme is very easy to implement. The diagram below shows how this would be set up for the Create Action.

Acl example diagram1 20091018-en.png

In the diagram, the Permission Hierarchy is shown down the left side and the Group hierarchy is shown across the top. Permissions are inherited down and to the right, as shown by the arrows. To implement the desired permissions, we leave the Global Configuration blank (Not Set) for all three groups. Similarly, in the Article Manager and Assignments Category, we leave the Create permission to Inherit for all the groups. As shown in the diagram, this means that these groups do not have Create permission for articles in general or for articles in the Assignments group.

To sum up so far, we have not set any special permissions to get to this point. Now, in the History Assignments category permissions screen, we set the Create permission to Allow for the History Teachers group. This setting overrides the Soft (Implicit) Deny that we had by default and gives members of this group permission to create content (articles and child categories) for this category. This Allow setting also is inherited by the Assistant History Teachers group.

Next, we need to grant History Teachers the Edit State permission while denying this permission to Assistant History Teachers. This is done as shown in the diagram below.

Acl example diagram2 20091018-en.png

This configuration is the same as the one above except that this time we set the Edit State permission in the History Assignments category to Deny for the Assistant History Teachers group. This means that Assistant History Teachers will not be able to Publish or Unpublish articles in this category.

Note that this was accomplished by setting just two permissions in the History Assignments category: Allow for the History Teachers group and Deny for the Assistant History Teachers group.

ACL Examples

Here are some examples of how you might set up the ACL for some specific situations.

Back-end Article Administrator


We want to create a group called "Article Administrator" with back-end permissions only for articles and not for any other back-end menu options. Members of this group should be able to use all of the features of the article manager, including setting article permissions.


  1. Create a new group called Article Administrator and make its parent group Public, as shown below.
    Screenshot acl tutorial 20110112-10-en.png
    Because its parent group is Public, it won't have any permissions by default.
  2. In Users → Access Levels, edit the Special Access level to add the new group. That way they can get access to the back end menu items and modules (This assumes that the modules for the admin menu and quickicons have the Special Access level assigned to them, which is the default.)
    Screenshot acl tutorial 20110112-11-en.png
    By default, the back-end menu items and modules are set to Special access, so if you forget to add the new group to the Special access level, you won't see any modules or menu items when you log in as a user of the new group.
  3. In Site → Global Configuration → Permissions, click on the Article Administrator group and change the permissions to Allowed for the following actions: Admin Login, Create, Delete, Edit, Edit State, and Edit Own. The screen below shows what will show before you press Save.
    Screenshot acl tutorial 20110112-12-en.png
    After you save, the Calculated Permissions should show as shown below.
    Screenshot acl tutorial 20110112-13-en.png
    Note that the permission for the Access Component is Inherited, which translates to Not Allowed. This is important. This means that this group will only be able to access components if we give the group "Allowed" permission for Access Component. So we only have to change the one component we want to give them access to and don't have to change any settings for the components where we don't want them to have access. If we had a case where we wanted to give a group access to everything except for one component, we could set the default to Allowed and then set the one component to Denied. Also note that we did not give the group Site Login permission, so users in this group will not be able to log into the front end. (If we wanted to allow that, we would just change the permission to Allowed for Site Login.)
  4. In Article Manager → Options → Permissions, change permissions to Allowed for this group for the Access Component action, as shown below.
    Screenshot acl tutorial 20110112-14-en.png
    All of the other desired permissions are inherited.

That's all you need to do. Members of this group can login to the back end and do everything in Article Manager but can't do anything else in the back end. For example, the screen below shows what a user in the Article Manager will see when they login to the back end.

Screenshot acl tutorial 20110112-15-en.png