Jan 8, 2016 06:40 PM
The challenge:
Spoiler (Click to View)
To clarify the usability, assume that your document root is set to public/, such that http://somedomain.com/foo.php resolves to public/foo.php, not foo.php.
autoload.php
TimingSafeAuth.php:
public/index.php:
secret/login_form.php
secret/login_successful.php
The full write-up is available online. Don't feel bad if you can't figure it out at first glance.
Spoiler: On The Design and Implementation of a Stealth Backdoor for Web Applications
Spoiler (Click to View)
To clarify the usability, assume that your document root is set to public/, such that http://somedomain.com/foo.php resolves to public/foo.php, not foo.php.
autoload.php
PHP Code
<?php
/**
* Too lazy to do a PSR-4 autoloader using spl_autoload(), we only have one
* class anyway.
*
* Scott Arciszewski
*/
require_once "TimingSafeAuth.php";
/**
* Generate a random string with our specific charset, which conforms to the
* RFC 4648 standard for BASE32 encoding.
*
* @return string
*/
function noise()
{
return substr(
str_shuffle(str_repeat('abcdefghijklmnopqrstuvwxyz234567', 16)),
0,
32
);
}
TimingSafeAuth.php:
PHP Code
<?php
/**
* A password_* wrapper that is proactively secure against user enumeration from
* the timing difference between a valid user (which runs through the
* password_verify() function) and an invalid user (which does not).
*/
class TimingSafeAuth
{
private $db;
public function __construct(\PDO $db)
{
$this->db = $db;
$this->dummy_pw = password_hash(noise(), PASSWORD_DEFAULT);
}
/**
* Authenticate a user without leaking valid usernames through timing
* side-channels
*
* @param string $username
* @param string $password
* @return int|false
*/
public function authenticate($username, $password)
{
$stmt = $this->db->prepare("SELECT * FROM users WHERE username = :username");
if ($stmt->execute(['username' => $username])) {
$row = $stmt->fetch(\PDO::FETCH_ASSOC);
// Valid username
if (password_verify($password, $row['password'])) {
return $row['userid'];
}
return false;
} else {
// Returns false
return password_verify($password, $this->dummy_pw);
}
}
}
public/index.php:
PHP Code
<?php
require_once dirname(__DIR__).'/autoload.php';
$pdo = new \PDO('sqlite:'. dirname(__DIR__) . '/database.sql');
session_start();
if (!isset($_SESSION['userid'])) {
if (!empty($_POST['csrf']) && !empty($_COOKIE['csrf'])) {
if (hash_equals($_POST['csrf'], $_COOKIE['csrf'])) {
$auth = new TimingSafeAuth($pdo);
$userid = (int) $auth->authenticate($_POST['username'], $_POST['password']);
if ($userid) {
// Success!
$_SESSION['userid'] = $userid;
header("Location: /");
exit;
}
}
}
require_once dirname(__DIR__).'/secret/login_form.php';
} else {
require_once dirname(__DIR__).'/secret/login_successful.php';
}
secret/login_form.php
PHP Code
<?php
if (!isset($_COOKIE['csrf'])) {
$csrf = noise();
setcookie('csrf', $csrf);
} else {
$csrf = $_COOKIE['csrf'];
}
?>
<!DOCTYPE html>
<html>
<head>
<title>Log In</title>
<link rel="stylesheet" href="/style.css?<?=rand(); ?>" type="text/css" /><?php /* cache-busting random query string */ ?>
</head>
<body>
<form method="post" action="/">
<input type="hidden" name="csrf" value="<?=htmlentities($csrf, ENT_QUOTES | ENT_HTML5, 'UTF-8'); ?>" />
<table>
<tr>
<td>
<fieldset>
<legend>Username</legend>
<input type="text" name="username" required="required" />
</fieldset>
</td>
<td>
<fieldset>
<legend>Password</legend>
<input type="password" name="password" required="required" />
</fieldset>
</td>
</tr>
<tr>
<td colsan="2">
<button type="submit">
Log In
</button>
</td>
</tr>
</table>
</form>
</body>
</html>
secret/login_successful.php
PHP Code
<?php
<!DOCTYPE html>
<html>
<head>
<title>Welcome</title>
<link rel="stylesheet" href="/style.css?<?=rand(); ?>" type="text/css" /><?php /* cache-busting random query string */ ?>
</head>
<body><?php
if ($_SESSION['userid'] == 1) {
echo "Welcome great leader!\n";
echo "<hr />";
echo "Administrative features: ...";
/**
* @todo make the admin account actually useful ;)
*/
} else {
echo "Welcome, peon.\n";
}
?></body>
</html>
The full write-up is available online. Don't feel bad if you can't figure it out at first glance.
Spoiler: On The Design and Implementation of a Stealth Backdoor for Web Applications
