What type of information should be saved in a Cookie (PHP)

Question

Im making a login/logout class that logs users in, sets cookies based on user's choice. The user enters their email/password and it checks the database, email/password combo exists a session is created, and a cookie is set (with the users id) and the user is redirected... I then have a function that logs users in by taking the user id saved in that cookie, checking whether that user id exists and then saving the users data in a session yet again... i was wondering if anybody see's anything potentialy wrong/unsafe about this.

Short Example, im sure you guys can get the gist of it...

function login($email, $password, $remember){
  // Check the database for email/password combo
  if(/*user exists*/){ // if the user exists
    $_SESSION = /*User data*/ // save the users data in a session
    if($remember){
      setcookie('user_id', /*User id*/); // save the user id in a cookie
    }
    header("location: index.php");// redirect
  }
}

function Check_Cookie(){
  if(isset($_COOKIE['user_id'])){
    return $this->Log_In_ID($_COOKIE['user_id']);
  }else{
    return false
  }
}

function Log_In_ID($id){
  //Check the database if the user id exists
  if(/*user exists*/){ // if the user exists
    $_SESSION = /*User data*/ // save the users data in a session
    header("location: index.php");// redirect
  }else{
    return false;
  }
}

Its not a detailed example of what im trying to ask, but im sure you can get the gist of it... Does anybody see anything potentially wrong with this. If you guys have any recommendations id love to hear them...also, do you guys use oop to log users in, or any other ways.

Answer 1

If your user ID is a sequential number, this is pretty insecure as anyone can just change their cookie to another reasonable-looking number based on their own (e.g. if mine is 1274, I could try some other numbers in that range) and immediately spoof that user.

You would be better off assigning a temporary ID associated with that user, like a GUID. Since GUIDs are astronomically unique and practically collision-proof, they're also virtually impossible to guess or predict from outside the system.

When the user logs in, you create a new GUID and store that with the user:

UserID        TokenID                                        Expires
1274          {3F2504E0-4F89-11D3-9A0C-0305E82C3301}         9/25/2009 12:00:00

When a user returns, look up their user ID by the token, make sure the token hasn't expired and log them in. Then change their token. This secures you against the following:

• An attacker cannot guess another user's token and spoof them
• Token expiration cannot be circumvented by ignoring the cookie's expiration date
• Since tokens change constantly, even if an attacker does manage to gain access to a user's cookie, the window of opportunity to take over is very small.