I think you should create a custom Users Permssions provider.
Though I don’t see any examples or documentation of that online…
You can try to register a new provider with this service:
strapi.plugin('users-permissions').service('providers-registry').register('ldap', ldapProvider);
The second parameter is the provider callback function.
As an example you could look at the native reddit provider:
You’ll also need to initialize the grant config.
For reddit it’s done like this:
Once you have that done you can enable the provider through the admin panel.
Then finally you could follow the official docs on how to implement a provider in your front-end