Nonce verification in Wordpress - Mini Project: Build & Apply

Nonce Verification in WordPress

📖 Scenario: You are building a WordPress plugin that needs to securely handle form submissions. To protect against unauthorized requests, you will use WordPress nonces (numbers used once) to verify that the form submission is valid and comes from your site.

🎯 Goal: Build a simple WordPress plugin that creates a form with a nonce field, and then verifies the nonce when the form is submitted to ensure the request is safe.

📋 What You'll Learn

Create a nonce field with a specific action name

Add a form that submits data via POST

Verify the nonce on form submission using the same action name

Display a success or error message based on nonce verification

💡 Why This Matters

🌍 Real World

Nonce verification is essential in WordPress plugins and themes to protect forms and actions from malicious attacks like CSRF (Cross-Site Request Forgery).

💼 Career

Understanding nonce verification is important for WordPress developers to build secure plugins and themes that protect user data and site integrity.

1

Create a nonce field in the form

Create a function called myplugin_form that outputs a simple HTML form with method post. Inside the form, add a nonce field using wp_nonce_field with the action name myplugin_nonce_action and the nonce name myplugin_nonce.

Wordpress

Hint

Use wp_nonce_field inside your form to create the nonce field with the exact action and name.

2

Add a form submission handler

Create a function called myplugin_handle_form that will process the form submission. Inside it, create a variable $nonce that gets the value from $_POST['myplugin_nonce']. This will be used to verify the nonce later.

Wordpress

';
    wp_nonce_field('myplugin_nonce_action', 'myplugin_nonce');
    echo '';
    echo '';
}

function myplugin_handle_form() {
    // Your code here
}
?>

Hint

Use the null coalescing operator ?? to safely get the nonce from $_POST.

3

Verify the nonce in the form handler

Inside the myplugin_handle_form function, use wp_verify_nonce to check if $nonce is valid for the action myplugin_nonce_action. If the nonce is valid, set a variable $message to 'Nonce verified successfully.'. Otherwise, set $message to 'Nonce verification failed.'.

Wordpress

';
    wp_nonce_field('myplugin_nonce_action', 'myplugin_nonce');
    echo '';
    echo '';
}

function myplugin_handle_form() {
    $nonce = $_POST['myplugin_nonce'] ?? '';
    // Verify nonce and set message
    // Your code here
}
?>

Hint

Use wp_verify_nonce with the nonce and the exact action name to check validity.

4

Display the form and message

Call myplugin_handle_form to process the form submission. Then call myplugin_form to display the form. After the form, if the variable $message is set, echo it inside a <p> tag.

Wordpress

';
    wp_nonce_field('myplugin_nonce_action', 'myplugin_nonce');
    echo '';
    echo '';
}

function myplugin_handle_form() {
    $nonce = $_POST['myplugin_nonce'] ?? '';
    if (wp_verify_nonce($nonce, 'myplugin_nonce_action')) {
        $message = 'Nonce verified successfully.';
    } else {
        $message = 'Nonce verification failed.';
    }
}

myplugin_handle_form();
myplugin_form();
// Your code here
?>

Hint

Call the handler first to get the message, then display the form, then show the message safely.

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