TWiki Access Control

Restricting read and write access to topics and webs, by Users and groups

TWikiAccessControl allows you restrict access to single topics and entire webs, by individual user and by user Groups, in three areas: view; edit & attach; and rename/move/delete. Access control, combined with TWikiUserAuthentication, lets you easily create and manage an extremely flexible, fine-grained privilege system.

An Important Control Consideration

Open, freeform editing is the essence of WikiCulture - what makes TWiki different and often more effective than other collaboration tools. For that reason, it is strongly recommended that decisions to restrict read or write access to a web or a topic are made with care - the more restrictions, the less Wiki in the mix. Experience shows that unrestricted write access works very well because:

• Peer influence is enough to ensure that only relevant content is posted.

• Peer editing - the ability for anyone to rearrange all content on a page - keeps topics focussed.

• In TWiki, content is transparently preserved under revision control:
  • Edits can be undone by the TWikiAdminGroup (the default administrators group; see #ManagingGroups).
  • Users are encouraged to edit and refactor (condense a long topic), since there's a safety net.

As a collaboration guideline:

• Create broad-based Groups (for more and varied input), and...
• Avoid creating view-only Users (if you can read it, you should be able to contribute to it).

Authentication vs. Access Control

Authentication: Identifies who a user is based on a login procedure. See TWikiUserAuthentication.

Access control: Restrict access to content based on users and groups once a user is identified.

Users and Groups

Access control is based on the familiar concept of Users and Groups. Users are defined by their WikiNames. They can then be organized in unlimited combinations by inclusion in one or more user Groups. For convenience, Groups can also be included in other Groups.

Managing Users

A user can create an account in TWikiRegistration. The following actions are performed:

• WikiName and encrypted password are recorded in .htpasswd if authentication is enabled.
• A confirmation e-mail is sent to the user.
• A user home page with the WikiName of the user is created in the SSECHome web.
• The user is added to the TWikiUsers topic.

Users can be authenticated using Basic Authentication (htaccess) or SSL (secure server). In either case, TWikiUserAuthentication is required in order to track user identities, and use User and Group access control.

The default visitor name is TWikiGuest. This is the non-authenticated user.

Managing Groups

Groups are defined by group topics created in the SSECHome web, like the TWikiAdminGroup. To create a new group:

1. Edit TWikiGroups by entering a new topic with a name that ends in Group. Example:
   • SomeGroup
2. Set Preferences for two Variables in the new group topic:
   • Set GROUP = < list of Users and/or Groups >
   • Set ALLOWTOPICCHANGE = < list of Users and/or Groups >
   • The GROUP variable is a comma-separated list of Users and/or other Groups. Example:
     • Set GROUP = SSECHome.SomeUser, SSECHome.OtherUser, SSECHome.SomeGroup
   • ALLOWTOPICCHANGE defines who is allowed to change the group topic; it is a comma delimited list of Users and Groups. You typically want to restrict that to the members of the group itself, so it should contain the name of the topic. (This prevents Users not in the Group from editing the topic to give themselves or others access. For example, for the TWikiAdminGroup topic write:
   • Set ALLOWTOPICCHANGE = SSECHome.TWikiAdminGroup
3. These are summarised on SitePermissions

The SuperAdminGroup

By mistyping a user or group name in the ALLOWTOPICCHANGE setting, it's possible to lock a topic so that no-one can edit it from a browser. To avoid this, you can create superusers:

• Set the SuperAdminGroup configuration setting to the wikiname of a group of Users who are always allowed to edit/view topics e.g. "TWikiAdminGroup".

Restricting Access

You can define who is allowed to read or write to a web or a topic. Note that some plugins may not respect access permissions.

• Restricting VIEW blocks viewing and searching of content.
• Restricting CHANGE blocks creating new topics, changing topics or attaching files.
• Restricting RENAME controls who is allowed to rename, move or delete a topic.
  • To rename, move or delete a topic, the user also also needs VIEW and CHANGE permission. They also need CHANGE access to change references in any referring topics (though the rename can proceed without this access), and CHANGE access to the target topic.

Controlling access to a Web

You can define restrictions of who is allowed to view a TWiki web. You can restrict access to certain webs to selected Users and Groups, by:

• authenticating all webs and restricting selected webs: Topic access in all webs is authenticated, and selected webs have restricted access.
• authenticating and restricting selected webs only: Provide unrestricted viewing access to open webs, with authentication and restriction only on selected webs.

• You can define these settings in the WebPreferences topic, preferable towards the end of the topic:
  • Set DENYWEBVIEW = < comma-delimited list of Users and Groups >
  • Set ALLOWWEBVIEW = < comma-delimited list of Users and Groups >
  • Set DENYWEBCHANGE = < comma-delimited list of Users and Groups >
  • Set ALLOWWEBCHANGE = < comma-delimited list of Users and Groups >
  • Set DENYWEBRENAME = < comma-delimited list of Users and Groups >
  • Set ALLOWWEBRENAME = < comma-delimited list of Users and Groups >

Controlling access to a Topic

• You can define these settings in the WebPreferences topic, preferable towards the end of the topic:
  • Set DENYTOPICVIEW = < comma-delimited list of Users and Groups >
  • Set ALLOWTOPICVIEW = < comma-delimited list of Users and Groups >
  • Set DENYTOPICCHANGE = < comma-delimited list of Users and Groups >
  • Set ALLOWTOPICCHANGE = < comma-delimited list of Users and Groups >
  • Set DENYTOPICRENAME = < comma-delimited list of Users and Groups >
  • Set ALLOWTOPICRENAME = < comma-delimited list of Users and Groups >

Remember when opening up access to specific topics within a restricted web that other topics in the web - for example, the WebLeftBar - may also be accessed when viewing the topics. The message you get when you are denied access should tell you what topic you were not permitted to access.

Controlling access to Attachments

Attachments are referred to directly, and are not normally indirected via TWiki scripts. This means that the above instructions for access control will not apply to attachments. It is possible that someone may inadvertently publicise a URL that they expected to be access-controlled.

The easiest way to apply the same access control rules for attachments as apply to topics is to use the Apache mod_rewrite module, and configure your webserver to redirect accesses to attachments to the TWiki viewfile script. For example,

  ScriptAlias /twiki/bin/ /filesystem/path/to/twiki/bin/
  Alias /twiki/pub/       /filesystem/path/to/twiki/pub/

  RewriteEngine on
  RewriteRule ^/twiki/pub/TWiki/(.*)$ /twiki/pub/TWiki/$1 [L,PT]
  RewriteRule ^/twiki/pub/([^\/]+)/([^\/]+)/([^\/]+)$ /twiki/bin/viewfile/$1/$2?filename=$3 [L,PT]

That way all the controls that apply to the topic also apply to attachments to the topic.

How TWiki evaluates ALLOW/DENY settings

When deciding whether to grant access, TWiki evaluates the following rules in order (read from the top of the list; if the logic arrives at PERMITTED or DENIED that applies immediately and no more rules are applied). You need to read the rules bearing in mind that VIEW, CHANGE and RENAME access may be granted/denied separately.

1. If the user is a super-user
   • access is PERMITTED.
2. If DENYTOPIC is set to a list of wikinames
   • people in the list will be DENIED.
3. If DENYTOPIC is set to empty ( i.e. Set DENYTOPIC = )
   • access is PERMITTED _i.e _ no-one is denied access to this topic
4. If ALLOWTOPIC is set
   1. people in the list are PERMITTED
   2. everyone else is DENIED
      • Note that this means that setting ALLOWTOPIC to empty denies access to everyone except admins (unless DENYTOPIC is also set to empty, as described above)
5. If DENYWEB is set to a list of wikiname
   • people in the list are DENIED access
6. If ALLOWWEB is set to a list of wikinames
   • people in the list will be PERMITTED
   • everyone else will be DENIED
     • Note that setting ALLOWWEB to empty denies access to everyone except admins
7. If you got this far, access is PERMITTED

Access Control quick recipes

Obfusticating Webs

Another way of hiding webs is to keep them hidden by not publishing the URL and by preventing the all webs search option from accessing obfuscated webs. Do so by enabling the NOSEARCHALL variable in WebPreferences:

• Set NOSEARCHALL = on

This setup can be useful to hide a new web until content its ready for deployment.

Obfuscating webs is insecure, as anyone who knows the URL can access the web.

Authenticate all Webs and Restrict Selected Webs

Use the following setup to authenticate users for topic viewing in all webs and to restrict access to selected webs:

1. Restrict view access to selected Users and Groups. Set one or both of these variables in its WebPreferences topic:
   • Set DENYWEBVIEW = < list of Users and Groups >
   • Set ALLOWWEBVIEW = < list of Users and Groups >
   • Note: DENYWEBVIEW is evaluated before ALLOWWEBVIEW. Access is denied if the authenticated person is in the DENYWEBVIEW list, or not in the ALLOWWEBVIEW list. Access is granted in case DENYWEBVIEW and ALLOWWEBVIEW is not defined.
2. Hide the web from an "all webs" search. Enable this restriction with the NOSEARCHALL variable in its WebPreferences topic:
   • Set NOSEARCHALL = on
3. Add view to the list of authenticated scripts in the .htaccess file.

This method only works if the view script is authenticated, which means that all Users have to login, even for read-only access. (An open guest account, like TWikiGuest, can get around this, allowing anyone to login to a common account with, for example, view-only access for public webs.) TWikiInstallationGuide has more on Basic Authentication, using the .htaccess file.

Authenticate and Restrict Selected Webs Only

Use the following setup to provide unrestricted viewing access to open webs, with authentication only on selected webs:

1. Restrict view access to selected Users and Groups. Set one or both of these variables in its WebPreferences topic:
   • Set DENYWEBVIEW = < list of Users and Groups >
   • Set ALLOWWEBVIEW = < list of Users and Groups >
   • Note: DENYWEBVIEW is evaluated before ALLOWWEBVIEW. Access is denied if the authenticated person is in the DENYWEBVIEW list, or not in the ALLOWWEBVIEW list. Access is granted in case DENYWEBVIEW and ALLOWWEBVIEW is not defined.
2. Hide the web from an "all webs" search. Enable this restriction with the NOSEARCHALL variable in its WebPreferences topic:
   • Set NOSEARCHALL = on
3. Enable {RememberUserIPAddress} in http://www.socialscience-ed.org/pages/bin/configure as described in TWikiUserAuthentication. TWiki will now remember the IP address of an authenticated user.
4. Copy the view script to viewauth (or better, create a symbolic link)
5. Add viewauth to the list of authenticated scripts in the .htaccess file. The view script should not be listed in the .htaccess file.

When a user accesses a web where you enabled view restriction, TWiki will redirect from the view script to the viewauth script once (this happens only if the user has never edited a topic). Doing so will ask for authentication. The viewauth script shows the requested topic if the user could log on and if the user is authorized to see that web.

Authenticating webs is not very secure, as there is a way to circumvent the read access restriction. It can be useful in certain situations - for example, to simplify site organization and clutter, by hiding low traffic webs - but is not recommended for securing sensitive content.

Hide Control Settings

To hide access control settings from normal browser viewing, place them in comment markers.

<!--
   * Set DENYTOPICCHANGE = Main.SomeGroup
-->

Related Topics: AdminDocumentationCategory