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If you want to use .htaccess based user authentication (with the HTTP Basic method) like this:

then you'll need at least the following modules in your Apache config:

• mod_auth_basic
• mod_authn_file
• mod_authz_user

In case of a Debian server you can enable these modules with the following commands:

If you're missing any of these modules and have a .htaccess in a directory with user-authentication enabled, then you'll most probably get HTTP 500 (Internal Server Error) upon accessing that directory through Apache. In the Apache error log you will find messages like these:
"client denied by server configuration"
or "configuration error: couldn't check user. No user file?"
or "configuration error: couldn't check access. No groups file?"