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Project description

This PAS plugin will reject multiple logins with the same user at the same time. It ensures that only one browser may be logged with the same userid at one time.


The implementation works like this: Suppose that Anna and Karl are two people who share a login annaandkarl in our site. Anna logs in, authenticating for the first time. We generate a cookie with a unique id for Anna and remember the id ourselves. For every subsequent authentication (i.e. for every request), we will make sure that Anna’s browser has the cookie.

Now Karl decides to log in into the site with the same login annaandkarl, the one that Anna uses to surf the site right now. The plugin sees that Karl’s browser doesn’t have our cookie yet, so it generates one with a unique id for Karl’s browser, remembers it and forgets about Anna’s cookie.

What happens when Anna clicks on a link on the site? The plugin sees that Anna has our cookie but that it differs from the cookie value that it remembered (Karl’s browser has that cookie value). Anna is logged out but the plugin and sees the message “Someone else logged in under your name”.


This product, when active, will automatically prevent users from staying logged in if they attempt to open a second session. This works well for all authentication schemes that have a concept of credential reset, but for users logged in with Basic Auth this will not work as expected, and both sessions will interfere with eachother without one being permanently ended. This is a limitation of the Basic Auth technology, so we recommend not using this plugin in combination with Basic Auth sites.

This plugin also provides a revoke_session view on itself, which can be accessed by visiting, which can be used by site administrators to manually invoke the forced logout mechanism.


Add Products.NoDuplicateLogin to the eggs parameter of your plone.recipe.zope2instance section:

recipe = plone.recipe.zope2instance
eggs =

Now run buildout and restart Plone. Once Plone has started, login and browse to Site Setup -> Zope Management Interface -> acl_users and add a No Duplicate Login Plugin from the drop down menu in the upper right.

After that, click on the No Duplicate Login Plugin object in the acl_users folder listing. For each of the Authentication, Credential Update and Reset Credentials objects in the No Duplicate Login Plugin folder listing, click, then move the no_duplicate_login plugin from the Available Plugins display widget on the left to the Active Plugins display widget on the right using the arrow buttons in the middle.

It you will also need to “arrow up” the no_duplicate_login plugin in the Active Plugins display listing for both Authentication and Reset Credentials.

If you are using a policy product to install this as part of your deployment you can perform these actions by including an empty noduplicatelogin.xml file in your profile directory.


2.0 (2014-11-25)

  • Security and performance improvements [matthewwilkes]
  • Remove reimplementation of plone.session [matthewwilkes]
  • Change internal data structures to avoid unnecessary object stores [matthewwilkes]
  • Add tests to test harness [matthewwilkes]
  • Fix pure Zope compatibility [matthewwilkes]

1.0a2 (02/18/2011)

  • Add more installation instructions

1.0a1 (12/17/2010)

  • Add test harness [aclark]
  • Rip out “experimental” session storage, too many ZODB conflicts. [aclark]
  • Plone 4 compat [aclark]
  • Re-package as egg [aclark]

1.0 svn/dev

  • Plone 3 compat [perrito]
  • Original implementation [nouri]

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