Difference: TWikiAccessControl (40 vs. 41)

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TWiki Access Control

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

TWiki Access Control allows you restrict access to single topics and entire webs, by individual user and by user Groups. Access control, combined with TWikiUserAuthentication, lets you easily create and manage an extremely flexible, fine-grained privilege system.

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An Important Control Consideration

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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 great care - the more restrictions, the less Wiki in the mix. Experience shows that unrestricted write access works very well because:
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Your organization will learn that, while fostering an open collaborative environment, soft security (peer review), together with version control (complete audit trail) will take care of any security concern you might have.

Open, free-form editing is the essence of WikiCulture - what makes TWiki different and often more effective than other collaborative environments. For that reason, it is strongly recommended that decisions to restrict read or write access to a web or a topic are made with great 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 focused.
  • In TWiki, content is transparently preserved under revision control:
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  • Avoid creating view-only Users (if you can read it, you should be able to contribute to it).

Permissions settings of the webs on this TWiki site

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Web Sitemap VIEW CHANGE RENAME
  Listed DENY ALLOW DENY ALLOW DENY ALLOW
Preferences Home Main on   TWikiAdminGroup, TWikiUserGroup, TWikiGuest   TWikiAdminGroup,Main.TWikiUserGroup    
Preferences Home TWiki on       TWikiAdminGroup   TWikiAdminGroup
Preferences Home Sandbox on            

Please Note:

  • A blank in the the above table may mean either the corresponding control is absent or commented out or that it has been set to a null value. The two conditions have dramatically different and possibly opposed semantics.
  • TWikiGuest is the guest account - used by unauthenticated users.
  • The TWiki web must not deny view to TWikiGuest; otherwise, people will not be able to register.
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Web Sitemap VIEW CHANGE RENAME
  Listed DENY ALLOW DENY ALLOW DENY ALLOW
Preferences Home Main on   TWikiAdminGroup, TWikiUserGroup, TWikiGuest   TWikiAdminGroup,Main.TWikiUserGroup    
Preferences Home TWiki on       TWikiAdminGroup   TWikiAdminGroup
Preferences Home Sandbox on            

Please Note:

  • A blank in the the above table may mean either the corresponding control is absent or commented out or that it has been set to a null value. The two conditions have dramatically different and possibly opposed semantics.
  • TWikiGuest is the guest account - used by unauthenticated users.
  • The TWiki web must not deny view to TWikiGuest; otherwise, people will not be able to register.
 
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Note: Above table comes from SitePermissions
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Note: Above table comes from SitePermissions
 

Authentication vs. Access Control

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 For example, set this to restrict a topic to be viewable only by the MarketingExecGroup:
  • Set ALLOWTOPICVIEW = Main.MarketingExecGroup
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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.
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See "How TWiki evaluates ALLOW/DENY settings" below for more on how ALLOW and DENY interacts.
 
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Be careful with empty values for any of these.
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ALERT! If the same setting is defined multiple times the last one overrides the previous. They are not OR'ed together.
 
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  • Set ALLOWTOPICVIEW =
    This means the same as not setting it at all. (This was documented wrong in versions 4.0.X, 4.1.0 and 4.1.1)
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Allowing public access to specific topics in a restricted web

 
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  • Set DENYTOPICVIEW =
    Since TWiki 4.0 this means do not deny anyone the right to view this topic. If DENYTOPICVIEW is set to an empty value anyone has access even if ALLOWTOPICVIEW or ALLOWWEBVIEW is defined. This allows to have very restrictive default access rights to an entire web and still allow individual topics to have more open access.
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You may want to completely open up access to a specific topic within a restricted web - allowing access by anybody. There is a special group for that - Main.AllUsersGroup. The following setting allows view access to the topic by anybody even if they are not authenticated.
  • Set ALLOWTOPICVIEW = Main.AllUsersGroup
 
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The same rules apply to ALLOWTOPICCHANGE/DENYTOPICCHANGE and APPLYTOPICRENAME/DENYTOPICRENAME. Setting ALLOWTOPICCHANGE or ALLOWTOPICRENAME to en empty value means the same as not defining it. Setting DENYTOPICCHANGE or DENYTOPICRENAME to an empty value means that anyone can edit or rename the topic.
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Alternatively, you can grant access only to authenticated users by Main.AllAuthUsersGroup. If an unauthenticated user accesses a topic having the following setting, they are asked to authenticate themself.
  • Set ALLOWTOPICVIEW = Main.AllAuthUsersGroup
 
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ALERT! If the same setting is defined multiple times the last one overrides the previous. They are not OR'ed together.
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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.
 
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ALERT! The setting to an empty has caused confusion and great debate and it has been decided that the empty setting syntax will be replaced by something which is easier to understand in a later version of TWiki. A method to upgrade will be provided. Please read the release notes carefully when you upgrade.
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As mentioned in the following section, meaning of an empty value set to DENYTOPICVIEW, DENYTOPICCHANGE, and DENYTOPICRENAME has been changed in TWiki 6.0. To keep those TWiki topics having empty DENYTOPICOPERAION accessible by everybody, those need to be replaced with
  • Set ALLOWTOPICOPERATION = Main.AllUsersGroup
 
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See "How TWiki evaluates ALLOW/DENY settings" below for more on how ALLOW and DENY interacts.
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For that, tools/eliminate_emptydenytopic is provided. After upgrading from pre 6.0 to post 6.0, you need to run it.

Empty values in access control variables

Setting an empty value to an access control variable is the same as not setting at all:

  • Set ALLOWTOPICVIEW =

ALERT! Since TWiki 4.0 and prior to TWiki 6.0 setting DENYTOPICVIEW, DENYTOPICCHANGE, or DENYTOPICRENAME to an empty value meant "do not deny anyone regardless of the corresponding ALLOWTOPICX", which is no longer the case. Back then, setting an empty value to DENYTOPICX was the only way to open up a topic to everybody in a restricted web. Now that we have AllUsersGroup and AllAuthUsersGroup, there is no need for that behaviour, which caused a lot of confusion and debate.

 
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    • access is PERMITTED.
  1. If DENYTOPIC is set to a list of wikinames
    • people in the list will be DENIED.
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  1. If DENYTOPIC is set to empty ( i.e. Set DENYTOPIC = )
    • access is PERMITTED i.e no-one is denied access to this topic.
      ALERT! Attention: Use this with caution. This is deprecated and will likely change in the next release.
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  1. If DENYTOPIC is set to empty ( i.e. Set DENYTOPIC = )
    • the access control setting is ignored.
      ALERT! Attention: The spec changed in TWiki-6.0; access was permitted in earlier TWiki releases.
 
  1. If ALLOWTOPIC is set
    1. people in the list are PERMITTED
    2. everyone else is DENIED
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    • everyone else will be DENIED
  1. If you got this far, access is PERMITTED

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Allowing web creation by user mapping manager

There are cases where DENYROOTCHANGE, ALLOWROOTCHANGE, DENYWEBCHANGE, and ALLOWWEBCHANGE, and DENYWEBCHANGE are not capable enough to implement web creation permission you want. To cope with such cases, when a new web is created, the canCreateWeb($cUID, $web) method of the user mapping manager is called if the method exists. If it returns true, TWiki goes ahead and create the web without checking access control variables. Please read AllowWebCreateByUserMappingManager for more details.

User masquerading

There are cases where it's handy to access TWiki on behalf of somebody else retaining a trace of your real identity rather than completely becoming a different user. We call it user masquerading. TWiki provides a framework to implement that. Please read UserMasquerading for more information.

This is an advanced feature and not many TWiki sites are using, but there is a part in the following section mentioning it, it's mentioned here.

Dynamic access control

This feature is regarded experimental. There are pitfalls and vulnerability. Before using this feature, please read this entire section through carefully.

You may want to restrict access dynamically -- based on topic name, a form field value, or some combination of factors. To cope with such situations, the dynamic access control mechanism is provided. If you set DYNAMIC_ACCESS_CONTROL 'on' at WebPreferences of the web, TWiki variables in access control variables mentioned above are expanded.

Example 1 - restriction based on topic name

Let's assume you need to restrict changes only to the CroniesGroup members excep with topics whose name ends with Public, which need be changed by anybody. That is achieve by the following settings on WebPrefences.

   * Set DYNAMIC_ACCESS_CONTROL = on
   * Set ALLOWWEBCHANGE = %IF{"'%CALCULATE{$SUBSTRING(%TOPIC%, -6, 6)}%' = 'Public'" then="%WIKINAME%" else="CroniesGroup"}%

Example 2 - restriction based on form field

Let's assume:

  • a web storing requests on topics whose name starts with ReqEnt
  • Each request topic has a form field "Requstor", which has the wiki name of the requestor
  • Users can view only requests they created
  • The members of the SupportGroup mail group can view all requests
That is achieve by the following settings on WebPrefences.

   * Set DYNAMIC_ACCESS_CONTROL = on
   * Set ALLOWWEBVIEW = %IF{"'%CALCULATE{$SUBSTRING(%TOPIC%, 1, 6)}%' = 'ReqEnt' and '%FORMFIELD{Requestor}%' != '%WIKINAME%'" then="SupportGroup" else="%WIKINAME%"}%

Specifically the following access control variables are subject to TWiki variable expansion in their values.

  • DENYTOPIC* (e.g. DENYTOPICVIEW, DENYTOPICCHANGE)
  • ALLOWTOIPC*
  • DENYWEB*
  • ALLOWWEB*
DENYROOT* and ALLOWROOT* are not subject to variable expansion. Because there has been no good use cases presented.

Dynamic access control in accessing a different web's topic

Let's assume WebA has the following lines on WebPreferences.

   * Set DYNAMIC_ACCESS_CONTROL = on
   * Set MEMBERS = JaneSmith, JoeSchmoe
   * Set ALLOWWEBVIEW = %MEMBERS%
This is not a good way to use dynamic access control but it does restrict accessonly to those listed in MEMBERS. However, access control doesn't work as expected when WebA.TopicB is accessed from WebC.TopicD by %INCLUDE{WebA.TopicB}% or other variables. This is because %MEMBERS% is defined in WebA and may have a different value in other webs.

You may think the following lines cheat the access control on WebA but actually not.

   * Set MEMBERS = %WIKINAME%
%INCLUDE{WebA.TopicB}%
This is because when a topic (e.g. WebC.TopicD) is accessed from browser and the topic refers to another topic in a different web (e.g. WebA.TopicB) and the different web employs dynamic access control, access to another topic is defined being on the safer side.

Topic level dynamic access control

On a topic, it's possible to use a variable defined on the topic for topic level access restriction. E.g.

   * Set MEMBERS = JaneSmith, JoeSchmoe
   * Set ALLOWTOPICVIEW = %MEMBERS%
[This is not a good way to use dynamic access control

Dynamic access control and user masquerading

Your user mapping handler may be providing the UserMasquerading feature. In that case, you expect dynamic access control to just work when user masquerading is in effect. Otherwise, you cannot test if your dynamic access control configuration is working as expected on your own.

Dynamic access control does work as expected even if user masquerading is in effect. For that, the following things are happening under the hood.

Let's think about Example 2 mentioned above. When you masquerading as SomebodyElse, you need to be able to see SomebodyElse's requests only. In the access control setting, a form field value is compared with %WIKINAME%. While user masquerading is in effect, your wiki name is YourNameOnBehalfOfSomebodyElse. It cannot match the form field value.

To make dynamic access control work under these circumstances, variable expansion for dynamic access control is skewed as follows. Specifically, the following variables are expanded to the value of SomeboyElse's rather than YourNameOnBehalfOfSomebodyElse's.

  • WIKINAME
  • USERNAME
  • WIKIUSERNAME

Vulnerability

It's possible to redefine function tags such as %IF{...}% in your personal preferences. In a future release of TWiki, a way to prevent function tags from being overridden by personal preferences will be provied. But until then, you need to be well aware of this vulnerability.

Disabling dynamic access control

You may not be comfortable with dynamic access control because it may slow things down. Or you may not want to be bothered by questions raised by users about it. If so, you can disable it by setting DYNAMIC_ACCESS_CONTROL 'off' and then finalizing at the local site level. (cf. TWikiVariables#Setting_Preferences_Variables)

 

Access control and INCLUDE

ALLOWTOPICVIEW and ALLOWTOPICCHANGE only applies to the topic in which the settings are defined. If a topic A includes another topic B, topic A does not inherit the access rights of the included topic B.

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  • If the included topic B has ALLOWTOPICCHANGE set to block editing for a user, it does not prevent editing the including topic A.
  • If the included topic B has ALLOWTOPICVIEW set to block view for a user, the user can still view topic A but he cannot see the included topic B. He will see a message No permission to view B
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Customizing "access denied" message

When access is denied, a page as follows is displayed:

access-denied.png

You may want to customize the passage annotated in the red rectangle. For example, with a web restricting access, you may want to show the link to an access request form.

You can achieve that by setting TOPIC_ACCESS_CONTACT varialbe on WebPreferences. e.g.

   * Set TOPIC_ACCESS_CONTACT = If you need to access this site, please apply [[Main.AccessForm][here]]
Please note that setting it on a topic other than WebPreferences does not take effect. This is a limitation of the current implementation.

Custom user/group notations

You can have custom user/group notations such as USER:userid and LDAPGROUP:group-name and use them for access control. For example:

   * Set ALLOWWEBCHANGE = UID:buzz, LDAPGROUP:foo-bar
In a large organization, TWiki may need to depend on user and group data provided by its infrastructure. Custom user/group notations are handy in such situations though it's not trivial to implement. Please read here for details.
 

Access Control quick recipes

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Related Topics: AdminDocumentationCategory, TWikiUserAuthentication, TWiki:TWiki.TWikiAccessControlSupplement
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Related Topics: AdminDocumentationCategory, TWikiUserAuthentication, AllowWebCreateByUserMappingManager, UserMasquerading, CustomUserGroupNotations, TWiki:TWiki.TWikiAccessControlSupplement
  -- Contributors: TWiki:Main.PeterThoeny, TWiki:Main.MikeMannix, TWiki:Main.CrawfordCurrie
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META FILEATTACHMENT attachment="access-denied.png" attr="" comment="" date="1366771092" name="access-denied.png" path="access-denied.png" size="32745" user="TWikiContributor" version="1.1"
 
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