You should implement a session timeout of your own. Both options mentioned by others (session.gc_maxlifetime and session.cookie_lifetime) are not reliable. I'll explain the reasons for that.
First:
session.gc_maxlifetime
session.gc_maxlifetime specifies the number of seconds after which data will be seen as 'garbage' and cleaned up. Garbage collection occurs during session start.
But the garbage collector is only started with a probability of session.gc_probability divided by session.gc_divisor. And using the default values for those options (1 and 100 respectively), the chance is only at 1%.
Well, you could simply adjust these values so that the garbage collector is started more often. But when the garbage collector is started, it will check the validity for every registered session. And that is cost-intensive.
Furthermore, when using PHP's default session.save_handler files, the session data is stored in files in a path specified in session.save_path. With that session handler, the age of the session data is calculated on the file's last modification date and not the last access date:
Note: If you are using the default file-based session handler, your filesystem must keep track of access times (atime). Windows FAT does not so you will have to come up with another way to handle garbage collecting your session if you are stuck with a FAT filesystem or any other filesystem where atime tracking is not available. Since PHP 4.2.3 it has used mtime (modified date) instead of atime. So, you won't have problems with filesystems where atime tracking is not available.
So it additionally might occur that a session data file is deleted while the session itself is still considered as valid because the session data was not updated recently.
And second:
session.cookie_lifetime
session.cookie_lifetime specifies the lifetime of the cookie in seconds which is sent to the browser. […]
Yes, that's right. This only affects the cookie lifetime and the session itself may still be valid. But it's the server's task to invalidate a session, not the client. So this doesn't help anything. In fact, having session.cookie_lifetime set to 0
would make the session’s cookie a real session cookie that is only valid until the browser is closed.
Conclusion / best solution:
The best solution is to implement a session timeout of your own. Use a simple time stamp that denotes the time of the last activity (i.e. request) and update it with every request:
if (isset($_SESSION['LAST_ACTIVITY']) && (time() - $_SESSION['LAST_ACTIVITY'] > 1800)) {
// last request was more than 30 minutes ago
session_unset(); // unset $_SESSION variable for the run-time
session_destroy(); // destroy session data in storage
}
$_SESSION['LAST_ACTIVITY'] = time(); // update last activity time stamp
Updating the session data with every request also changes the session file's modification date so that the session is not removed by the garbage collector prematurely.
You can also use an additional time stamp to regenerate the session ID periodically to avoid attacks on sessions like session fixation:
if (!isset($_SESSION['CREATED'])) {
$_SESSION['CREATED'] = time();
} else if (time() - $_SESSION['CREATED'] > 1800) {
// session started more than 30 minutes ago
session_regenerate_id(true); // change session ID for the current session and invalidate old session ID
$_SESSION['CREATED'] = time(); // update creation time
}
Notes:
session.gc_maxlifetime
should be at least equal to the lifetime of this custom expiration handler (1800 in this example);
- if you want to expire the session after 30 minutes of activity instead of after 30 minutes since start, you'll also need to use
setcookie
with an expire of time()+60*30
to keep the session cookie active.
OK, let me put this bluntly: if you're putting user data, or anything derived from user data into a cookie for this purpose, you're doing something wrong.
There. I said it. Now we can move on to the actual answer.
What's wrong with hashing user data, you ask? Well, it comes down to exposure surface and security through obscurity.
Imagine for a second that you're an attacker. You see a cryptographic cookie set for the remember-me on your session. It's 32 characters wide. Gee. That may be an MD5...
Let's also imagine for a second that they know the algorithm that you used. For example:
md5(salt+username+ip+salt)
Now, all an attacker needs to do is brute force the "salt" (which isn't really a salt, but more on that later), and he can now generate all the fake tokens he wants with any username for his IP address! But brute-forcing a salt is hard, right? Absolutely. But modern day GPUs are exceedingly good at it. And unless you use sufficient randomness in it (make it large enough), it's going to fall quickly, and with it the keys to your castle.
In short, the only thing protecting you is the salt, which isn't really protecting you as much as you think.
But Wait!
All of that was predicated that the attacker knows the algorithm! If it's secret and confusing, then you're safe, right? WRONG. That line of thinking has a name: Security Through Obscurity, which should NEVER be relied upon.
The Better Way
The better way is to never let a user's information leave the server, except for the id.
When the user logs in, generate a large (128 to 256 bit) random token. Add that to a database table which maps the token to the userid, and then send it to the client in the cookie.
What if the attacker guesses the random token of another user?
Well, let's do some math here. We're generating a 128 bit random token. That means that there are:
possibilities = 2^128
possibilities = 3.4 * 10^38
Now, to show how absurdly large that number is, let's imagine every server on the internet (let's say 50,000,000 today) trying to brute-force that number at a rate of 1,000,000,000 per second each. In reality your servers would melt under such load, but let's play this out.
guesses_per_second = servers * guesses
guesses_per_second = 50,000,000 * 1,000,000,000
guesses_per_second = 50,000,000,000,000,000
So 50 quadrillion guesses per second. That's fast! Right?
time_to_guess = possibilities / guesses_per_second
time_to_guess = 3.4e38 / 50,000,000,000,000,000
time_to_guess = 6,800,000,000,000,000,000,000
So 6.8 sextillion seconds...
Let's try to bring that down to more friendly numbers.
215,626,585,489,599 years
Or even better:
47917 times the age of the universe
Yes, that's 47917 times the age of the universe...
Basically, it's not going to be cracked.
So to sum up:
The better approach that I recommend is to store the cookie with three parts.
function onLogin($user) {
$token = GenerateRandomToken(); // generate a token, should be 128 - 256 bit
storeTokenForUser($user, $token);
$cookie = $user . ':' . $token;
$mac = hash_hmac('sha256', $cookie, SECRET_KEY);
$cookie .= ':' . $mac;
setcookie('rememberme', $cookie);
}
Then, to validate:
function rememberMe() {
$cookie = isset($_COOKIE['rememberme']) ? $_COOKIE['rememberme'] : '';
if ($cookie) {
list ($user, $token, $mac) = explode(':', $cookie);
if (!hash_equals(hash_hmac('sha256', $user . ':' . $token, SECRET_KEY), $mac)) {
return false;
}
$usertoken = fetchTokenByUserName($user);
if (hash_equals($usertoken, $token)) {
logUserIn($user);
}
}
}
Note: Do not use the token or combination of user and token to lookup a record in your database. Always be sure to fetch a record based on the user and use a timing-safe comparison function to compare the fetched token afterwards. More about timing attacks.
Now, it's very important that the SECRET_KEY
be a cryptographic secret (generated by something like /dev/urandom
and/or derived from a high-entropy input). Also, GenerateRandomToken()
needs to be a strong random source (mt_rand()
is not nearly strong enough. Use a library, such as RandomLib or random_compat, or mcrypt_create_iv()
with DEV_URANDOM
)...
The hash_equals()
is to prevent timing attacks.
If you use a PHP version below PHP 5.6 the function hash_equals()
is not supported. In this case you can replace hash_equals()
with the timingSafeCompare function:
/**
* A timing safe equals comparison
*
* To prevent leaking length information, it is important
* that user input is always used as the second parameter.
*
* @param string $safe The internal (safe) value to be checked
* @param string $user The user submitted (unsafe) value
*
* @return boolean True if the two strings are identical.
*/
function timingSafeCompare($safe, $user) {
if (function_exists('hash_equals')) {
return hash_equals($safe, $user); // PHP 5.6
}
// Prevent issues if string length is 0
$safe .= chr(0);
$user .= chr(0);
// mbstring.func_overload can make strlen() return invalid numbers
// when operating on raw binary strings; force an 8bit charset here:
if (function_exists('mb_strlen')) {
$safeLen = mb_strlen($safe, '8bit');
$userLen = mb_strlen($user, '8bit');
} else {
$safeLen = strlen($safe);
$userLen = strlen($user);
}
// Set the result to the difference between the lengths
$result = $safeLen - $userLen;
// Note that we ALWAYS iterate over the user-supplied length
// This is to prevent leaking length information
for ($i = 0; $i < $userLen; $i++) {
// Using % here is a trick to prevent notices
// It's safe, since if the lengths are different
// $result is already non-0
$result |= (ord($safe[$i % $safeLen]) ^ ord($user[$i]));
}
// They are only identical strings if $result is exactly 0...
return $result === 0;
}
Best Answer
Unfortunately, there is no effective way to unmistakably identify a request that originates from an attacker in opposite to a genuine request. Because most properties that counter measures check like the IP address or user agent characteristics are either not reliable (IP address might change among multiple requests) or can be forged easily (e. g. User-Agent request header) and thus can yield unwanted false positives (i. e. genuine user switched IP address) or false negatives (i. e. attacker was able to successfully forge request with same User-Agent).
That’s why the best method to prevent session hijacking is to make sure an attacker cannot find out another user’s session ID. This means you should design your application and its session management that (1) an attacker cannot guess a valid session ID by using enough entropy, and (2) that there is no other way for an attacker to obtain a valid session ID by known attacks/vulerabilities like sniffing the network communication, Cross-Site Scripting, leakage through Referer, etc.
That said, you should:
HttpOnly
andSecure
attributes to forbid access via JavaScript (in case of XSS vulnerabilities) and to forbid transmission via insecure channel (see session.cookie_httponly and session.cookie_secure)Besides that, you should also regenerate the session ID while invalidating the old one (see
session_regenerate_id
function) after certain session state changes (e. g. confirmation of authenticity after login or change of authorization/privileges) and you can additionally do this periodically to reduce the time span for a successful session hijacking attack.