How Magento Generates Form Keys

Published: February 12, 2018

Tags:

Magento

As a follow up to my recent article “How Magento Generates Admin Secret URL Keys” I’ve decided to take a look at yet another mechanism Magento uses to protect against CSRF attacks…form keys. In this post we’ll dig into Magento’s core code to understand how exactly, they are generated in both Magento 1 and Magento 2.

How It Works

Magento 2

NOTE: This below is based on the Magento 2 code base as of version 2.2.2.

The code responsible for generating form keys can be found in Magento\Framework\Data\Form\FormKey::getFormKey(). Here’s the method in its entirely.

/**
 * Retrieve Session Form Key
 *
 * @return string A 16 bit unique key for forms
 */
public function getFormKey()
{
    if (!$this->isPresent()) {
        $this->set($this->mathRandom->getRandomString(16));
    }
    return $this->escaper->escapeHtmlAttr($this->session->getData(self::FORM_KEY));
}

The comment tells us the form key will be a 16 bit unique string. But how specifically is it generated? For that we need to look at Magento\Framework\Math\Random::getRandomString()…

/**
 * Get random string
 *
 * @param int $length
 * @param null|string $chars
 * @return string
 * @throws \Magento\Framework\Exception\LocalizedException
 */
public function getRandomString($length, $chars = null)
{
    $str = '';
    if (null === $chars) {
        $chars = self::CHARS_LOWERS . self::CHARS_UPPERS . self::CHARS_DIGITS;
    }

    if (function_exists('openssl_random_pseudo_bytes')) {
        // use openssl lib if it is installed
        for ($i = 0, $lc = strlen($chars) - 1; $i < $length; $i++) {
            $bytes = openssl_random_pseudo_bytes(PHP_INT_SIZE);
            $hex = bin2hex($bytes); // hex() doubles the length of the string
            $rand = abs(hexdec($hex) % $lc); // random integer from 0 to $lc
            $str .= $chars[$rand]; // random character in $chars
        }
    } elseif ($fp = @fopen('/dev/urandom', 'rb')) {
        // attempt to use /dev/urandom if it exists but openssl isn't available
        for ($i = 0, $lc = strlen($chars) - 1; $i < $length; $i++) {
            $bytes = @fread($fp, PHP_INT_SIZE);
            $hex = bin2hex($bytes); // hex() doubles the length of the string
            $rand = abs(hexdec($hex) % $lc); // random integer from 0 to $lc
            $str .= $chars[$rand]; // random character in $chars
        }
        fclose($fp);
    } else {
        throw new \Magento\Framework\Exception\LocalizedException(
            new \Magento\Framework\Phrase("Please make sure you have 'openssl' extension installed")
        );
    }

    return $str;
}

Randomness is always tricky. We can see that Magento first tries to use openssl_random_pseudo_bytes and falls back OS level generation if not available via /dev/urandom.

Magento 1

NOTE: This below is based on the Magento 1 code base as of version 1.9.3.7.

In Magento 1, form key generation happens in Mage_Core_Model_Session::getFormKey…

/**
 * Retrieve Session Form Key
 *
 * @return string A 16 bit unique key for forms
 */
public function getFormKey()
{
    if (!$this->getData('_form_key')) {
        $this->renewFormKey();
    }
    return $this->getData('_form_key');
}

Again, it’s a unique 16 bit string. But where does it come from this time?

Mage_Core_Model_Session::renewFormKey() simply does the following…

/**
 * Creates new Form key
 */
public function renewFormKey()
{
    $this->setData('_form_key', Mage::helper('core')->getRandomString(16));
}

So we need to check Mage_Core_Helper_Data::getRandomString()…

public function getRandomString($len, $chars = null)
{
    if (is_null($chars)) {
        $chars = self::CHARS_LOWERS . self::CHARS_UPPERS . self::CHARS_DIGITS;
    }
    for ($i = 0, $str = '', $lc = strlen($chars)-1; $i < $len; $i++) {
        $str .= $chars[mt_rand(0, $lc)];
    }
    return $str;
}

We can see here that mt_rand is used. In other words, Magento 1 uses a lesser grade of randomness to generate form keys than exists in Magento 2.