How can I throttle user login attempts in PHP

Question

I was just reading this post The definitive guide to form-based website authentication on Preventing Rapid-Fire Login Attempts.

Best practice #1: A short time delay that increases with the number of failed attempts, like:

1 failed attempt = no delay
2 failed attempts = 2 sec delay
3 failed attempts = 4 sec delay
4 failed attempts = 8 sec delay
5 failed attempts = 16 sec delay
etc.

DoS attacking this scheme would be very impractical, but on the other hand, potentially devastating, since the delay increases exponentially.

I am curious how I could implement something like this for my login system in PHP?

Answer 1

You cannot simply prevent DoS attacks by chaining throttling down to a single IP or username. Hell, you can't even really prevent rapid-fire login attempts using this method.

Why? Because the attack can span multiple IPs and user accounts for the sake of bypassing your throttling attempts.

I have seen posted elsewhere that ideally you should be tracking all failed login attempts across the site and associating them to a timestamp, perhaps:

CREATE TABLE failed_logins (
    id INT(11) UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
    username VARCHAR(16) NOT NULL,
    ip_address INT(11) UNSIGNED NOT NULL,
    attempted DATETIME NOT NULL,
    INDEX `attempted_idx` (`attempted`)
) engine=InnoDB charset=UTF8;

A quick note on the ip_address field: You can store the data and retrieve the data, respectively, with INET_ATON() and INET_NTOA() which essentially equate to converting an ip address to and from an unsigned integer.

# example of insertion
INSERT INTO failed_logins SET username = 'example', ip_address = INET_ATON('192.168.0.1'), attempted = CURRENT_TIMESTAMP;
# example of selection
SELECT id, username, INET_NTOA(ip_address) AS ip_address, attempted;

Decide on certain delay thresholds based on the overall number of failed logins in a given amount of time (15 minutes in this example). You should base this on statistical data pulled from your failed_logins table as it will change over time based on the number of users and how many of them can recall (and type) their password.

---

> 10 failed attempts = 1 second
> 20 failed attempts = 2 seconds
> 30 failed attempts = reCaptcha

---

Query the table on every failed login attempt to find the number of failed logins for a given period of time, say 15 minutes:

---

SELECT COUNT(1) AS failed FROM failed_logins WHERE attempted > DATE_SUB(NOW(), INTERVAL 15 minute);

---

If the number of attempts over the given period of time is over your limit, either enforce throttling or force all user's to use a captcha (i.e. reCaptcha) until the number of failed attempts over the given time period is less than the threshold.

// array of throttling
$throttle = array(10 => 1, 20 => 2, 30 => 'recaptcha');

// retrieve the latest failed login attempts
$sql = 'SELECT MAX(attempted) AS attempted FROM failed_logins';
$result = mysql_query($sql);
if (mysql_affected_rows($result) > 0) {
    $row = mysql_fetch_assoc($result);

    $latest_attempt = (int) date('U', strtotime($row['attempted']));

    // get the number of failed attempts
    $sql = 'SELECT COUNT(1) AS failed FROM failed_logins WHERE attempted > DATE_SUB(NOW(), INTERVAL 15 minute)';
    $result = mysql_query($sql);
    if (mysql_affected_rows($result) > 0) {
        // get the returned row
        $row = mysql_fetch_assoc($result);
        $failed_attempts = (int) $row['failed'];

        // assume the number of failed attempts was stored in $failed_attempts
        krsort($throttle);
        foreach ($throttle as $attempts => $delay) {
            if ($failed_attempts > $attempts) {
                // we need to throttle based on delay
                if (is_numeric($delay)) {
                    $remaining_delay = time() - $latest_attempt - $delay;
                    // output remaining delay
                    echo 'You must wait ' . $remaining_delay . ' seconds before your next login attempt';
                } else {
                    // code to display recaptcha on login form goes here
                }
                break;
            }
        }        
    }
}

Using reCaptcha at a certain threshold would ensure that an attack from multiple fronts would be stopped and normal site users would not experience a significant delay for legitimate failed login attempts.

Answer 2

You have three basic approaches: store session information, store cookie information or store IP information.

If you use session information the end user (attacker) could forcibly invoke new sessions, bypass your tactic, and then login again with no delay. Sessions are pretty simple to implement, simply store the last known login time of the user in a session variable, match it against the current time, and make sure the delay has been long enough.

If you use cookies, the attacker can simply reject the cookies, all in all, this really isn't something viable.

If you track IP addresses you'll need to store login attempts from an IP address somehow, preferably in a database. When a user attempts to log on, simply update your recorded list of IPs. You should purge this table at a reasonable interval, dumping IP addresses that haven't been active in some time. The pitfall (there's always a pitfall), is that some users may end up sharing an IP address, and in boundary conditions your delays may affect users inadvertantly. Since you're tracking failed logins, and only failed logins, this shouldn't cause too much pain.

Answer 3

The short answer is: Do not do this. You will not protect yourself from brute forcing, you could even make your situation worse.

None of the proposed solutions would work. If you use the IP as any parameter for throttling, the attacker will just span the attack across a huge number of IPs. If you use the session(cookie), the attacker will just drop any cookies. The sum of all you can think of is, that there is absolutely nothing a brute forcing attacker could not overcome.

There is one thing, though - you just rely on the username that tried to log in. So, not looking at all the other parameters you track how often a user tried to log in and throttle. But an attacker wants to harm you. If he recognizes this, he will just also brute force user names.

This will result in almost all of your users being throttled to your maximum value when they try to log in. Your website will be useless. Attacker: success.

You could delay the password check in general for around 200ms - the website user will almost not notice that. But a brute-forcer will. (Again he could span across IPs) However, nothing of all this will protect you from brute forcing or DDoS - as you can not programatically.

The only way to do this is using the infrastructure.

You should use bcrypt instead of MD5 or SHA-x to hash your passwords, this will make decrypting your passwords a LOT harder if someone steals your database (because I guess you are on a shared or managed host)

Sorry for disappointing you, but all the solutions here have a weakness and there is no way to overcome them inside the back-end logic.

Answer 4

The login process needs reduce its speed for both successful and unsuccessful login. The login attempt itself should never be faster than about 1 second. If it is, brute force uses the delay to know that the attempt failed because success is shorter than failure. Then, more combinations can be evaluated per second.

The number of simultaneous login attempts per machine needs to be limited by the load balancer. Finally, you just need to track if the same user or password is re-used by more than one user/password login attempt. Humans cannot type faster than about 200 words per minite. So, successive or simultaneous login attempts faster than 200 words per minite are from a set of machines. These can thus be piped to a black list safely as it is not your customer. Black list times per host do not need to be greater than about 1 second. This will never inconvenience a human, but plays havoc with a brute force attempt whether in serial or parallel.

2 * 10^19 combinations at one combination per second, run in parallel on 4 billion separate IP addresses, will take 158 years to exhaust as a search space. To last one day per user against 4 billion attackers, you need a fully random alphanumeric password 9 places long at a minimum. Consider training users in pass phrases at least 13 places long, 1.7 * 10^20 combinations.

This delay, will motivate the attacker to steal your password hash file rather than brute force your site. Use approved, named, hashing techniques. Banning the entire population of Internet IP for one second, will limit the effect of parallel attacks without a dealy a human would appreciate. Finally, if your system allows more than 1000 failed logon attempts in one second without some response to ban systems, then your security plans have bigger problems to work on. Fix that automated response first of all.

Answer 5

session_start();
$_SESSION['hit'] += 1; // Only Increase on Failed Attempts
$delays = array(1=>0, 2=>2, 3=>4, 4=>8, 5=>16); // Array of # of Attempts => Secs

sleep($delays[$_SESSION['hit']]); // Sleep for that Duration.

or as suggested by Cyro:

sleep(2 ^ (intval($_SESSION['hit']) - 1));

It's a bit rough, but the basic components are there. If you refresh this page, each time you refresh the delay will get longer.

You could also keep the counts in a database, where you check the number of failed attempts by IP. By using it based on IP and keeping the data on your side, you prevent the user from being able to clear their cookies to stop the delay.

Basically, the beginning code would be:

$count = get_attempts(); // Get the Number of Attempts

sleep(2 ^ (intval($count) - 1));

function get_attempts()
{
    $result = mysql_query("SELECT FROM TABLE WHERE IP=\"".$_SERVER['REMOTE_ADDR']."\"");
    if(mysql_num_rows($result) > 0)
    {
        $array = mysql_fetch_assoc($array);
        return $array['Hits'];
    }
    else
    {
        return 0;
    }
}

Answer 6

Store fail attempts in the database by IP. (Since you have a login system, I assume you know well how to do this.)

Obviously, sessions is a tempting method, but someone really dedicated can quite easily realize that they can simply delete their session cookie on failed attempts in order to circumvent the throttle entirely.

On attempt to log in, fetch how many recent (say, last 15 minutes) login attempts there were, and the time of the latest attempt.

$failed_attempts = 3; // for example
$latest_attempt = 1263874972; // again, for example
$delay_in_seconds = pow(2, $failed_attempts); // that's 2 to the $failed_attempts power
$remaining_delay = time() - $latest_attempt - $delay_in_seconds;
if($remaining_delay > 0) {
    echo "Wait $remaining_delay more seconds, silly!";
}

Answer 7

You can use sessions. Anytime the user fails a login, you increase the value storing the number of attempts. You can figure the required delay from the number of attempts, or you can set the actual time the user is allowed to try again in the session as well.

A more reliable method would be to store the attempts and new-try-time in the database for that particular ipaddress.

Answer 8

IMHO, defense against DOS attacks is better dealt with at the web server level (or maybe even in the network hardware), not in your PHP code.

Answer 9

I generally create login history and login attempt tables. The attempt table would log username, password, ip address, etc. Query against the table to see if you need to delay. I would recommend blocking completely for attempts greater than 20 in a given time (an hour for example).

Answer 10

As per discussion above, sessions, cookies and IP addresses are not effective - all can be manipulated by the attacker.

If you want to prevent brute force attacks then the only practical solution is to base the number of attempts on the username provided, however note that this allows the attacker to DOS the site by blocking valid users from logging in.

e.g.

$valid=check_auth($_POST['USERNAME'],$_POST['PASSWD']);
$delay=get_delay($_POST['USERNAME'],$valid);

if (!$valid) {
   header("Location: login.php");
   exit;
}
...
function get_delay($username,$authenticated)
{
    $loginfile=SOME_BASE_DIR . md5($username);
    if (@filemtime($loginfile)<time()-8600) {
       // last login was never or over a day ago
       return 0;
    }
    $attempts=(integer)file_get_contents($loginfile);
    $delay=$attempts ? pow(2,$attempts) : 0;
    $next_value=$authenticated ? 0 : $attempts + 1;
    file_put_contents($loginfile, $next_value);
    sleep($delay); // NB this is done regardless if passwd valid
    // you might want to put in your own garbage collection here
 }

Note that as written, this procedure leaks security information - i.e. it will be possible for someone attacking the system to see when a user logs in (the response time for the attackers attempt will drop to 0). You might also tune the algorithm so that the delay is calculated based on the previous delay and the timestamp on the file.

HTH

C.

Answer 11

Cookies or session-based methods are of course useless in this case. The application has to check the IP address or timestamps (or both) of previous login attempts.

An IP check can be bypassed if the attacker has more than one IP to start his/her requests from and can be troublesome if multiple users connect to your server from the same IP. In the latter case, someone failing login for several times would prevent everyone who shares the same IP from logging in with that username for a certain period of time.

A timestamp check has the same problem as above: everyone can prevent everyone else from logging in a particular account just by trying multiple times. Using a captcha instead of a long wait for the last attempt is probably a good workaround.

The only extra things the login system should prevent are race conditions on the attempt checking function. For example, in the following pseudocode

$time = get_latest_attempt_timestamp($username);
$attempts = get_latest_attempt_number($username);

if (is_valid_request($time, $attempts)) {
    do_login($username, $password);
} else {
    increment_attempt_number($username);
    display_error($attempts);
}

What happens if an attacker sends simultaneous requests to the login page? Probably all the requests would run at the same priority, and chances are that no request gets to the increment_attempt_number instruction before the others are past the 2nd line. So every request gets the same $time and $attempts value and is executed. Preventing this kind of security issues can be difficult for complex applications and involves locking and unlocking some tables/rows of the database, of course slowing the application down.

Answer 12

cballuo provided an excellent answer. I just wanted to return the favor by providing an updated version that supports mysqli. I slightly changed up the table/field columns in the sqls and other small things but it should help anyone looking for the mysqli equivalent.

function get_multiple_rows($result) {
  $rows = array();
  while($row = $result->fetch_assoc()) {
    $rows[] = $row;
  }
  return $rows;
}

$throttle = array(10 => 1, 20 => 2, 30 => 5);

$query = "SELECT MAX(time) AS attempted FROM failed_logins";    

if ($result = $mysqli->query($query)) {

    $rows = get_multiple_rows($result);

$result->free();

$latest_attempt = (int) date('U', strtotime($rows[0]['attempted'])); 

$query = "SELECT COUNT(1) AS failed FROM failed_logins WHERE time > DATE_SUB(NOW(), 
INTERVAL 15 minute)";   

if ($result = $mysqli->query($query)) {

$rows = get_multiple_rows($result);

$result->free();

    $failed_attempts = (int) $rows[0]['failed'];

    krsort($throttle);
    foreach ($throttle as $attempts => $delay) {
        if ($failed_attempts > $attempts) {
                echo $failed_attempts;
                $remaining_delay = (time() - $latest_attempt) - $delay;

                if ($remaining_delay < 0) {
                echo 'You must wait ' . abs($remaining_delay) . ' seconds before your next login attempt';
                }                

            break;
        }
     }        
  }
}