Mattermost Team Edition

Mattermost is a team-based instant messaging application.

See the official Mattermost website for a complete presentation.

Mattermost follows an Open Core development model. The freely available Team edition contains all the basic chat features, but lack the integration capabilities found in the Enterprise edition.

The Enterprise edition provides SAML integration out of the box, and you can configure it just like any other SAML service in LemonLDAP::NG

The Team edition, however, only provides SSO integration with Gitlab.

However, it is possible to configure LemonLDAP::NG to behave exactly like a Gitlab Oauth2 server, allowing Mattermost Team Edition to be integrated with LemonLDAP::NG without having to use a Gitlab server.

The following configuration requires your user database to expose a unique numeric identifier for every user.

Configuring Mattermost through the System Console will not allow you to set the correct URLs. You need to edit the Mattermost configuration file, and avoid changing Gitlab integration settings in the System Console

Set the following settings in /opt/mattermost/config/config.json

    "GitLabSettings": {
        "Enable": true,
        "Secret": "CHOOSE_A_CLIENT_SECRET",
        "Id": "CHOOSE_A_CLIENT_ID",
        "Scope": "",
        "AuthEndpoint": "",
        "TokenEndpoint": "",
        "UserApiEndpoint": ""

Mattermost does not use OpenID Connect to communicate with Gitlab, but uses plain OAuth2 instead. Because of that, LemonLDAP::NG will not receive the scope= parameter and will display an error on the portal when trying to authenticate.

In order to fix this, we can add a fake OAuth2 authorize URL on the LemonLDAP::NG server that will automatically add this scope= parametrer, before sending the request to the correct OIDC URL

Here is an example configuration for Nginx, add it in your Portal virtualhost before any other rewrite rule:

    rewrite ^/oauth2/gitlab_(authorize.*)$$1?scope=openid%20gitlab ;

And if you are using Apache

RewriteRule "^/oauth2/gitlab_authorize(.*)$" "$1scope=openid gitlab" [QSA,NE]

We now have to configure LemonLDAP::NG to recognize Mattermost as a valid OAuth2 relaying party and send it the information it needs to recognize a user.

Add a new OpenID Connect relaying party with the following parameters:

  • Client ID: the same you set in Mattermost configuration
  • Client Secret: the same you set in Mattermost configuration
  • Add a new scope in "Extra claims"
    • Key: gitlab
    • Value: id username name email
  • Add the following exported attributes
    • username: set it to the session attribute containing the user login
    • name: session attribute containing the user's full name
    • email: session attribute containing the user's email
    • id: session attribute containing the user's numeric ID
Mattermost absolutely needs to receive a numerical value in the id claim. If you are using a LDAP server, you could use the uidNumber LDAP attribute. If you use something else, you will have to find a trick to assign a unique numeric ID to each Mattermost user.

The id attribute has to be different for each user, since this is the field Mattermost will use internally to map Gitlab identities to Mattermost accouts.

If you see a HTTP code 500 when going back to mattermost, with a panic() in (*GitLabUser).IsValid(…) , it probably means that you are not exporting the correct attributes, but it can also mean that id is exported as a JSON string.

If this case, it can help to create a macro, for example uidNumber_n, with a value of $uidNumber + 0 to force conversion to a numeric value. You must then export it as the id field in the Relaying Party configuration.