What is the best way to implement "remember me" for a website? [closed]

I want my website to have a checkbox that users can click so that they will not have to log in each time they visit my website. I know I will need to store a cookie on their computer to implement this, but what should be contained in that cookie?

Also, are there common mistakes to watch out for to keep this cookie from presenting a security vulnerability, which could be avoided while still giving the 'remember me' functionality?

Improved Persistent Login Cookie Best Practice

You could use this strategy described here as best practice (2006) or an updated strategy described here (2015):

1. When the user successfully logs in with Remember Me checked, a login cookie is issued in addition to the standard session management cookie.
2. The login cookie contains a series identifier and a token. The series and token are unguessable random numbers from a suitably large space. Both are stored together in a database table, the token is hashed (sha256 is fine).
3. When a non-logged-in user visits the site and presents a login cookie, the series identifier is looked up in the database.
   1. If the series identifier is present and the hash of the token matches the hash for that series identifier, the user is considered authenticated. A new token is generated, a new hash for the token is stored over the old record, and a new login cookie is issued to the user (it's okay to re-use the series identifier).
   2. If the series is present but the token does not match, a theft is assumed. The user receives a strongly worded warning and all of the user's remembered sessions are deleted.
   3. If the username and series are not present, the login cookie is ignored.

This approach provides defense-in-depth. If someone manages to leak the database table, it does not give an attacker an open door for impersonating users.

Store their UserId and a RememberMeToken. When they login with remember me checked generate a new RememberMeToken (which invalidate any other machines which are marked are remember me).

When they return look them up by the remember me token and make sure the UserId matches.

I would store a user ID and a token. When the user comes back to the site, compare those two pieces of information against something persistent like a database entry.

As for security, just don't put anything in there that will allow someone to modify the cookie to gain extra benefits. For example, don't store their user groups or their password. Anything that can be modified that would circumvent your security should not be stored in the cookie.

Investigating persistent sessions myself I have found that it's simply not worth the security risk. Use it if you absolutely have to, but you should consider such a session only weakly authenticated and force a new login for anything that could be of value to an attacker.

The reason being of course is that your cookies containing your persistent session are so easily stolen.

4 ways to steal your cookies (from a comment by Jens Roland on the page @splattne based his answer on):

1. By intercepting it over an unsecure line (packet sniffing / session hijacking)
2. By directly accessing the user's browser (via either malware or physical access to the box)
3. By reading it from the server database (probably SQL Injection, but could be anything)
4. By an XSS hack (or similar client-side exploit)