CSRF PHP Guard

CSRF PHP Guard is a library to implement protection against CSRF (Cross Site Request Forgery) attacks.
Multiple CSRF tokens are allowed.

Warning

This library has not been fully tested and is more a proof of concept than production-proof.

Use at your own risk or make sure to do a detailed code review first.

How to use it

• When generating a form in an HTML page, call CsrfGuard::generate(). This function can take the name of the form as optional parameter and returns a security token.
• Add this token in your form as an hidden parameter: <input type="hidden" name="CSRF" value="<?= token ?>">. NB: your can change the parameter name if you want, the library won't complain.
• When the user validate the form, and before any othen action (esp. database modification), read the submited token and check its validity by calling CsrfGuard::check(). This function takes the value of the submited token as first parameter, plus two optional parameters:
  • the form name (used to generate the token in step 1),
  • the validity timespan of the token (in milliseconds) - if ommited, tokens never timeout.
• If the token is invalid, an exception will be thrown.

Example

Generate the form:

<?php
    const $token = CsrfGuard::generate("subscription_form");
?>

<form action="validate.php" method="post">
    <!-- label, inputs, buttons, etc... -->
    <input type="hidden" name="CSRF" value="<?= token ?>">
    <input type="submit" value="Submit">
</form>

Validate the form upon submition:

<?php
    // Before any business action
    const $submitedToken = $_POST['CSRF'];
    try {
        CsrfGuard::check($submitedToken, "subscription_form");
    } catch (err) {
        // log error and show an error to the user
    }
    
    // Business logic
    // ...
?>

How it works under the hood

When you generate a token, the library also sets a cookie containing:

• the token value - 32 random bytes (generated with random_bytes() PHP primitive),
• the form name (if any),
• the generation time (using time() PHP primitive).

This cookie is signed using sha256 HMAC (with hash_hmac() PHP primitive) to provent modification.

• The key is generated the same way as the token. It is stored in memory and shared between all users
• If you want, you can manage this key yourself by using CsrfGuard::setKey() before any token generation. Just make sure that this key remains confidential.

To verify a token, the library:

• reads the cookie and checks its signature,
• ensure that the form name and token correspond to the given values
• if needed, it also check for token timeout by using a rather simple equation: is cookie.time + timespawn < time() ?

NB: actually, the cookie may contain several structures (token + form name + time) encoded in JSON to allow several tokens to coexist at the same time.

Why ?

When I looked for a CSRF protection library (for an internal project), I didn't find any which was both:

• Robust (I still find it astonishing how many flaws I found just by looking the code of several implementations...),
• Simple and user friendly.

That's why I decided to code this little helper library on my free time.

However, the project was never finished (business needs diappeared) and I never took the time to add proper tests and packages... sad story :(.