PHP "Remember Me" security flaw?

Question

I'm in the middle of coding a 'remember me'-equipped login form, and so far the tutorials I've read (partly to make sure I'm doing it right) all say to store the encrypted password in a cookie along with the username. Then, each time PHP checks if the current user is not logged in, check their cookies and look for those values. If the username matches the password, you're in.

To me, this is a gaping security hole. If somebody were to hack the database or somehow get access to the encrypted passwords, they wouldn't even need to crack them. Just set your own cookies and go. Am I correct, or just being paranoid?

My login system uses sessions to keep track of the current user id, and a 1/0 for a quick logged in/logged out check. The user can't edit sessions AFAIK, so this is secure (if it's not, please let me know). I was thinking of just storing the session ID in a cookie, to later resume it, but that's also not secure.

I care a lot about the security of my users, how can I properly protect their information while still maintaining a functioning website?

Answer 1

Usually when the server is requested to remember the user, it issues an additional cookie beside the normal session cookie, and this new string contains the username and a reasonable amount of random characters which make it unguessable. This is obviously saved in the database (for security reasons you may want to treat it like a password and thus salt and hash it), and the next time the user connects with the same browser (assuming the old session expired) the application receives the random string, checks the database, and if there's a match the user is authenticated.

An alternative is moving the session expiration time ahead of some amount.

What are the security holes here? Provided you use a good random generator, the only thing that can happen is that the user accesses the application from a shared browser and doesn't manually logs out when it ends.

The normal rules always apply since you are on an insecure channel: use HTTPS, otherwise whoever sits in between your computer and the server can steal cookies (either the session one or the remember-me one) and act as if it were you.

Bonus, this must-read by Jeff Atwood

Answer 2

Never save passwords in any way (encrypted password which can be used to login is still a password) on client.

Use randomly generated key, safely stored on server. These keys can be revoked later unlike the encrypted passwords which will be valid until changed.

Using php you can generate random key like this md5(uniqid(mt_rand(), true)). For better safety store it salted and hashed in db.

Example table:

login_keys (
  user_id int,
  key char(40), # sha1
  salt char(15)
)

Also note you should enable the HTTP only cookie option.