Nonce verification in Wordpress

Introduction

Nonce verification helps protect your WordPress site from unwanted actions by checking if a request is safe and comes from a trusted source.

When processing form submissions to make sure the request is valid.

When handling AJAX requests to confirm they come from your site.

When performing actions like deleting or updating content to prevent attacks.

When creating custom admin pages that accept user input.

When you want to add an extra layer of security to any user action.

Syntax

Wordpress

<?php
if ( ! isset( $_POST['my_nonce_field'] ) || ! wp_verify_nonce( $_POST['my_nonce_field'], 'my_action' ) ) {
    die( 'Security check failed' );
}
// Continue processing safe request
?>

wp_verify_nonce checks if the nonce is valid for the given action.

Always check the nonce before processing sensitive data.

Examples

Check nonce from a form field named 'nonce' for the action 'save_post'.

Wordpress

<?php
if ( isset( $_POST['nonce'] ) && wp_verify_nonce( $_POST['nonce'], 'save_post' ) ) {
    // Safe to save post
} else {
    // Invalid nonce
}
?>

Use check_ajax_referer to verify nonce in AJAX requests.

Wordpress

<?php
add_action( 'wp_ajax_my_action', function() {
    check_ajax_referer( 'my_action', 'security' );
    // Process AJAX request safely
} );
?>

Verify nonce passed via URL query before deleting an item.

Wordpress

<?php
if ( ! isset( $_GET['nonce'] ) || ! wp_verify_nonce( $_GET['nonce'], 'delete_item' ) ) {
    wp_die( 'Invalid nonce' );
}
// Proceed with deletion
?>

Sample Program

This example shows a simple form with a nonce field. When the form is submitted, the nonce is checked to make sure the request is safe before processing the username.

Wordpress

<?php
// Generate nonce field in a form
$nonce = wp_create_nonce( 'submit_form' );
?>
<form method="post">
    <input type="hidden" name="form_nonce" value="<?php echo $nonce; ?>">
    <input type="text" name="username" placeholder="Enter username">
    <button type="submit">Submit</button>
</form>

<?php
if ( $_SERVER['REQUEST_METHOD'] === 'POST' ) {
    if ( ! isset( $_POST['form_nonce'] ) || ! wp_verify_nonce( $_POST['form_nonce'], 'submit_form' ) ) {
        echo 'Security check failed';
        exit;
    }
    $username = sanitize_text_field( $_POST['username'] ?? '' );
    echo "Hello, $username! Your form was submitted safely.";
}
?>

Important Notes

Nonces in WordPress are not true nonces but tokens that expire after 12-24 hours.

Always use wp_create_nonce to generate and wp_verify_nonce to check.

Never trust user input without nonce verification to avoid security risks.

Summary

Nonce verification protects your site from unauthorized actions.

Use wp_create_nonce to create and wp_verify_nonce to check nonces.

Always verify nonces before processing form or AJAX data.

Practice

1. What is the main purpose of using a nonce in WordPress?

easy

A. To create database backups automatically

B. To speed up page loading times

C. To store user preferences

D. To protect against unauthorized actions and CSRF attacks

Solution

1. Step 1: Understand what a nonce is

   A nonce is a security token used to verify requests are intentional and authorized.

2. Step 2: Identify nonce purpose in WordPress

   WordPress uses nonces to protect against CSRF (Cross-Site Request Forgery) attacks by verifying actions.

3. Final Answer:

   To protect against unauthorized actions and CSRF attacks -> Option D

4. Quick Check:

   Nonce = Security token for authorization [OK]

Hint: Nonce = security check token to stop bad requests [OK]

Common Mistakes:

• Thinking nonce speeds up performance
• Confusing nonce with user data storage
• Assuming nonce creates backups

2. Which function is used to verify a nonce in WordPress?

easy

A. wp_create_nonce()

B. wp_verify_nonce()

C. check_nonce()

D. nonce_verify()

Solution

1. Step 1: Recall nonce creation function

   wp_create_nonce() is used to create a nonce, not verify it.

2. Step 2: Identify the correct verification function

   wp_verify_nonce() is the official WordPress function to check if a nonce is valid.

3. Final Answer:

   wp_verify_nonce() -> Option B

4. Quick Check:

   Verify nonce = wp_verify_nonce() [OK]

Hint: Verify nonce starts with 'wp_verify_' [OK]

Common Mistakes:

• Using wp_create_nonce() to verify
• Assuming check_nonce() is a WordPress function
• Mixing function names

3. What will the following code output if the nonce is invalid?

if ( ! wp_verify_nonce($_POST['my_nonce'], 'my_action') ) {
  echo 'Invalid nonce';
} else {
  echo 'Nonce verified';
}

medium

A. Nonce verified

B. PHP error: undefined index

C. Invalid nonce

D. No output

Solution

1. Step 1: Understand wp_verify_nonce behavior

   wp_verify_nonce returns false if the nonce is invalid or missing.

2. Step 2: Analyze the if condition

   If wp_verify_nonce returns false, the condition !false is true, so 'Invalid nonce' is echoed.

3. Final Answer:

   Invalid nonce -> Option C

4. Quick Check:

   Invalid nonce triggers message 'Invalid nonce' [OK]

Hint: Invalid nonce means wp_verify_nonce returns false [OK]

Common Mistakes:

• Assuming invalid nonce prints 'Nonce verified'
• Ignoring the negation (!) in the if condition
• Expecting no output on invalid nonce

4. Identify the error in this nonce verification code snippet:

if ( wp_verify_nonce($_POST['nonce'], 'save_data') ) {
  // process data
} else {
  wp_die('Security check failed');
}

medium

A. No error, code is correct

B. Missing negation (!) before wp_verify_nonce

C. Using wp_die() instead of wp_redirect()

D. Incorrect nonce action string 'save_data'

Solution

1. Step 1: Understand wp_verify_nonce return values

   wp_verify_nonce() returns truthy integer (1 or 2) if valid or previously used, false if invalid/expired.

2. Step 2: Analyze the code logic

   if (wp_verify_nonce()) processes if truthy (valid), else wp_die() if falsy (invalid). Correct logic.

3. Final Answer:

   No error, code is correct -> Option A

4. Quick Check:

   if (wp_verify_nonce()) process else die [OK]

Hint: if(wp_verify_nonce()) process; else die works [OK]

Common Mistakes:

• Thinking negation (!) is required
• Assuming wp_die() is wrong here
• Changing action string unnecessarily

5. You want to secure an AJAX request in WordPress using a nonce. Which sequence correctly implements nonce creation and verification?

hard

A. Create nonce with wp_create_nonce('ajax_action') in PHP; send it via AJAX; verify with wp_verify_nonce($_POST['nonce'], 'ajax_action')

B. Create nonce with wp_verify_nonce('ajax_action'); send it via AJAX; verify with wp_create_nonce($_POST['nonce'], 'ajax_action')

C. Create nonce with wp_create_nonce('ajax_action') in JavaScript; verify with wp_verify_nonce($_GET['nonce'], 'ajax_action')

D. Create nonce with wp_create_nonce('ajax_action') in PHP; no need to verify on AJAX request

Solution

1. Step 1: Understand nonce creation and usage

   Nonce must be created in PHP using wp_create_nonce with an action name.

2. Step 2: Verify nonce on AJAX request

   Nonce sent via AJAX (usually POST) must be verified in PHP using wp_verify_nonce with the same action.

3. Final Answer:

   Create nonce with wp_create_nonce('ajax_action') in PHP; send it via AJAX; verify with wp_verify_nonce($_POST['nonce'], 'ajax_action') -> Option A

4. Quick Check:

   Nonce create in PHP, verify in PHP with same action [OK]

Hint: Create nonce in PHP, verify in PHP with same action [OK]

Common Mistakes:

• Trying to create nonce in JavaScript
• Mixing up create and verify functions
• Skipping verification on AJAX requests