Plugin security (nonces, sanitization) in Wordpress

Introduction

Plugin security helps keep your WordPress site safe from bad actions. Nonces and sanitization stop hackers from doing harmful things.

When creating forms that change site data, like settings or posts.

When accepting user input to avoid harmful code or scripts.

When verifying that a request comes from a real user, not a hacker.

When saving or displaying data that users submit.

When building plugins that interact with the database or site files.

Syntax

Wordpress

<?php
// Create a nonce field in a form
wp_nonce_field('action_name', 'nonce_name');

// Check nonce on form submission
if (!isset($_POST['nonce_name']) || !wp_verify_nonce($_POST['nonce_name'], 'action_name')) {
    die('Security check failed');
}

// Sanitize text input
$clean_text = sanitize_text_field($_POST['user_input']);

// Sanitize URL input
$clean_url = esc_url_raw($_POST['user_url']);

Nonces are special tokens to verify requests are safe.

Sanitization cleans user input to remove harmful code.

Examples

This adds a hidden field with a nonce to your form for security.

Wordpress

<?php
// Add nonce field to form
wp_nonce_field('save_settings', 'my_nonce');

This checks the nonce to make sure the form submission is valid.

Wordpress

<?php
// Verify nonce when form is submitted
if (!isset($_POST['my_nonce']) || !wp_verify_nonce($_POST['my_nonce'], 'save_settings')) {
    die('Invalid request');
}

This cleans the text input to remove unwanted tags or scripts.

Wordpress

<?php
// Sanitize a text input
$name = sanitize_text_field($_POST['name']);

This cleans a URL input to make sure it is safe and valid.

Wordpress

<?php
// Sanitize a URL input
$url = esc_url_raw($_POST['website']);

Sample Program

This plugin creates a simple form with a nonce for security. It checks the nonce when the form is submitted and sanitizes the user input before showing it back safely.

Wordpress

<?php
/* Plugin Name: Simple Secure Form */

// Show form with nonce
function ssf_show_form() {
    echo '<form method="POST">';
    wp_nonce_field('ssf_action', 'ssf_nonce');
    echo '<label for="ssf_name">Name:</label>';
    echo '<input type="text" id="ssf_name" name="ssf_name">';
    echo '<input type="submit" value="Submit">';
    echo '</form>';
}

// Handle form submission
function ssf_handle_form() {
    if ($_SERVER['REQUEST_METHOD'] === 'POST') {
        if (!isset($_POST['ssf_nonce']) || !wp_verify_nonce($_POST['ssf_nonce'], 'ssf_action')) {
            echo 'Security check failed.';
            return;
        }
        $name = sanitize_text_field($_POST['ssf_name'] ?? '');
        echo 'Hello, ' . esc_html($name) . '! Your form was submitted safely.';
    }
}

// Hook to display form and handle submission
add_shortcode('ssf_form', function() {
    ob_start();
    ssf_handle_form();
    ssf_show_form();
    return ob_get_clean();
});

Important Notes

Always check nonces before processing form data to stop attacks.

Sanitize all user inputs before saving or displaying them.

Use esc_html() or similar functions when showing user data to avoid code running in the browser.

Summary

Nonces protect your plugin from fake requests.

Sanitization cleans user input to keep your site safe.

Always use both together when handling user data in plugins.

Practice

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

easy

A. To speed up the plugin loading time

B. To protect against unauthorized or fake form submissions

C. To style the plugin interface

D. To store user preferences permanently

Solution

1. Step 1: Understand what a nonce is

   A nonce is a unique token used to verify that a request comes from a trusted source.

2. Step 2: Identify nonce purpose in plugins

   Nonces prevent attackers from submitting fake or unauthorized requests to your plugin.

3. Final Answer:

   To protect against unauthorized or fake form submissions -> Option B

4. Quick Check:

   Nonce = Security token for requests [OK]

Hint: Nonces stop fake requests, like a secret handshake [OK]

Common Mistakes:

• Thinking nonce speeds up plugin
• Using nonce for styling
• Confusing nonce with data storage

2. Which of the following is the correct way to check a nonce in a WordPress plugin?

easy

A. check_nonce('my_nonce')

B. nonce_check('my_nonce')

C. wp_verify_nonce($_POST['my_nonce'], 'my_action')

D. verify_nonce('my_nonce')

Solution

1. Step 1: Recall the WordPress function for nonce verification

   The correct function to verify a nonce is wp_verify_nonce().

2. Step 2: Check the correct usage

   The function takes the nonce value and the action name as parameters, like wp_verify_nonce($_POST['my_nonce'], 'my_action').

3. Final Answer:

   wp_verify_nonce($_POST['my_nonce'], 'my_action') -> Option C

4. Quick Check:

   Verify nonce = wp_verify_nonce() [OK]

Hint: Use wp_verify_nonce() with nonce and action [OK]

Common Mistakes:

• Using non-existent functions like check_nonce
• Passing wrong parameters order
• Not using $_POST or $_GET to get nonce

3. What will be the output of this code snippet in a WordPress plugin?

if ( ! wp_verify_nonce($_POST['nonce'], 'save_data') ) {
  echo 'Invalid nonce';
} else {
  $name = sanitize_text_field($_POST['name']);
  echo 'Hello, ' . $name;
}

Assuming the nonce is valid and $_POST['name'] = '<script>alert(1)</script>John'?

medium

A. Hello, John

B. Hello, John

C. Invalid nonce

D. Hello, alert(1)John

Solution

1. Step 1: Check nonce validity

   The nonce is valid, so the else block runs.

2. Step 2: Understand sanitize_text_field effect

   This function removes HTML tags like <script>, so the input becomes 'alert(1)John' because sanitize_text_field removes tags but leaves the text inside.

3. Step 3: Clarify output

   The output will be 'Hello, alert(1)John' because the script tags are stripped but the text 'alert(1)' remains.

4. Final Answer:

   Hello, alert(1)John -> Option D

5. Quick Check:

   Sanitize removes tags but keeps text inside [OK]

Hint: sanitize_text_field strips HTML tags but keeps inner text [OK]

Common Mistakes:

• Assuming sanitize_text_field allows HTML
• Ignoring nonce check result
• Confusing output with raw input

4. Identify the error in this WordPress plugin code snippet:

if ( ! wp_verify_nonce($_POST['nonce'], 'save_data') ) {
  wp_die('Security check failed');
}
$name = sanitize_text_field($_POST['name']);
update_option('user_name', $name);

medium

A. Not checking if $_POST['name'] is set before sanitizing

B. Missing nonce field in the form

C. Using update_option instead of add_option

D. No error, code is correct

Solution

1. Step 1: Check nonce verification

   The nonce is verified correctly with wp_verify_nonce and wp_die on failure.

2. Step 2: Check input sanitization safety

   The code sanitizes $_POST['name'] without checking if it exists, which can cause a PHP notice or error.

3. Final Answer:

   Not checking if $_POST['name'] is set before sanitizing -> Option A

4. Quick Check:

   Always check input exists before sanitizing [OK]

Hint: Check if input exists before sanitize_text_field [OK]

Common Mistakes:

• Assuming update_option is wrong here
• Ignoring missing input checks
• Thinking nonce check is missing

5. You want to securely save a user email from a form in your WordPress plugin. Which combination of code snippets correctly uses nonce verification and sanitization?

// 1. Verify nonce
if ( ! wp_verify_nonce($_POST['nonce'], 'save_email') ) {
  wp_die('Invalid request');
}

// 2. Sanitize input
$email = ???

// 3. Save email
update_option('user_email', $email);

Choose the correct code for step 2:

hard

A. $email = sanitize_email($_POST['email']);

B. $email = sanitize_text_field($_POST['email']);

C. $email = esc_html($_POST['email']);

D. $email = wp_strip_all_tags($_POST['email']);

Solution

1. Step 1: Confirm nonce verification

   The nonce is verified correctly before processing input.

2. Step 2: Choose proper sanitization for email

   For emails, sanitize_email() is the correct function to clean and validate email format.

3. Step 3: Understand other options

   sanitize_text_field() is for plain text, esc_html() is for output escaping, and wp_strip_all_tags() removes tags but does not validate email.

4. Final Answer:

   $email = sanitize_email($_POST['email']); -> Option A

5. Quick Check:

   Email input = sanitize_email() [OK]

Hint: Use sanitize_email() for email inputs [OK]

Common Mistakes:

• Using sanitize_text_field for emails
• Confusing escaping with sanitizing
• Not verifying nonce before saving