I have a website that allows viewing and modifying of a MySQL database using PERL CGI scripts and I haven't a clue how to do the security.

Yikes! That statement scares me! My first reaction to that statement is if you don't have a clue then learn about it before you create the website.

If I had to solve this problem though, here is one approach I'd consider.

Consider a "roles based" security system. What I mean by that is this:

• Anonyomous user. IE a user not logged in.
• Logged in user
  • Database(s) allowed to access
  • Table(s) allowed to access

There are many ways to implement a roles based system. The approach that I quite often use is to create a set of tables (outside of what users anonymous and otherwise have access to) such as:

create table users (
     user_id         integer not null primary key,
     login           text,
     passwd          text 
);

create table role_types (
     role_type_id   integer not null primary key,
     role_type      text 
);

create table role_map (
       role_mapping_id   integer not null primary key,
       role_type_id      integer not null
                         references role_types(role_type_id)
                         on delete cascade,
       user_id           integer not null
                         references users(user_id) 
                         on delete cascade
);
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Using that schema you now have a way to map users to one or more roles that user can have. Keep your business logic of how those roles map to access in your CGI script (for now..)

That's the first half of it. You now have a table that can be searched to authenticate users, you have a table that defines roles, and you have a mapping table that maps users to their roles. The next step is to track sessions and who owns the session:

create table sessions (
      session_entry_id    integer not null primary key,
      session_id           text,
      session_hack         timestamp
); 
--  Note, session_id is what is used to track the session
--  and session_entry_id is used for FK mapping between
--  tables

create table session_ownership (
     session_ownership_id integer not null primary key,
     session_id           integer not null
                          references
                          sessions(session_entry_id)
                          on delete cascade,
     user_id              integer not null
                          references users(user_id)
                          on delete cascade
);

create table session_vars (
     session_var_id       integer not null primary key,
     var_key              text not null,
     var_value            text
);
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Note: the primary keys are of type integer, but they could be other types. In most cases I'd use either an auto_increment or a sequence depending on if I were coding this for MySQL or PostgreSQL.

In the schema I've shown, you have the ability to persist values for sessions between forms within a session instead of using hidden values. Your session_id should be a string of random (unpredictable) characters of sufficient length to make guessing if not impossible at least too difficult to be worth it. I'm mot going into algorithms for that in this post.

Lastly, in your CGI code you need to provide logic to once in a while (or even every time it is accessed) of visiting the values in the sessions table and deleting rows of sessions that have timed out. Meaning you need to decide how long a session can be idle before you delete the session from the table. In your CGI logic you also need to visit this table between accesses to the CGI to make sure a session is still valid. If it isn't you bounce the user back to the login screen and make them log in again. Optionally you can tell them their session has expired if your nice. :-)

Now.... mapping your roles to the database(s) and/or table(s) they are allowed to access can either be done with hard coded logic (not the best way IMHO) or via more join tables within your schema. I leave that up to your creativity to figure out from here...