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WordPress Security & Data Validation

Version: 1.0.0 Target: WordPress 6.7+ | PHP 8.3+ Skill Level: Intermediate to Advanced

Overview

Security is not optional in WordPress development—it's fundamental. This skill teaches the three-layer security model that prevents XSS, CSRF, SQL injection, and other common web vulnerabilities through proper input sanitization, business logic validation, and output escaping.

The Golden Rule: "Sanitize on input, validate for logic, escape on output."

Why This Matters

Every year, thousands of WordPress sites are compromised due to security vulnerabilities in plugins and themes. Most of these attacks exploit one of three weaknesses:

1. XSS (Cross-Site Scripting): Malicious JavaScript injected through unsanitized output
2. CSRF (Cross-Site Request Forgery): Unauthorized actions performed on behalf of authenticated users
3. SQL Injection: Database manipulation through unsanitized database queries

This skill provides complete, production-ready patterns for preventing all three attack vectors.

The Three-Layer Security Model

WordPress security follows a defense-in-depth strategy with three distinct layers:

User Input → [1. SANITIZE] → [2. VALIDATE] → Process → [3. ESCAPE] → Output

Layer 1: Sanitization (Input Cleaning)

Purpose: Remove dangerous characters and normalize data format When: Immediately upon receiving user input Example: sanitize_text_field($_POST['username'])

Layer 2: Validation (Logic Checks)

Purpose: Ensure data meets business requirements When: After sanitization, before processing Example: if (!is_email($email)) { /* error */ }

Layer 3: Escaping (Output Protection)

Purpose: Prevent XSS by encoding special characters When: Every time you output data to browser Example: echo esc_html($user_input);

Critical Distinction:

• Sanitization removes/transforms invalid data (changes the value)
• Validation checks if data is acceptable (returns true/false)
• Escaping makes data safe for display (context-specific encoding)

1. Nonces: CSRF Protection

What Are Nonces?

Nonces (Numbers Used Once) are cryptographic tokens that verify a request originated from your site, not a malicious external source. They prevent Cross-Site Request Forgery (CSRF) attacks.

How CSRF Attacks Work:

<!-- Attacker's malicious site: evil.com -->
<img src="https://yoursite.com/wp-admin/admin.php?action=delete_user&id=1">
<!-- If user is logged into yoursite.com, this executes! -->

How Nonces Prevent CSRF:

<!-- Legitimate request with nonce -->
<form action="admin.php?action=delete_user&id=1" method="POST">
    <?php wp_nonce_field('delete_user_1', 'delete_nonce'); ?>
    <button>Delete User</button>
</form>

<!-- Attacker cannot generate valid nonce (tied to user session) -->

Nonce Implementation Patterns

Pattern 1: Form Nonces (Most Common)

BEFORE (Vulnerable):

// Vulnerable form processing
if (isset($_POST['submit'])) {
    $user_id = absint($_POST['user_id']);
    delete_user($user_id); // ⚠️ CSRF vulnerable!
}

AFTER (Secure):

// Generate nonce in form
<form method="post" action="">
    <?php wp_nonce_field('delete_user_action', 'delete_user_nonce'); ?>
    <input type="hidden" name="user_id" value="42">
    <button type="submit" name="submit">Delete User</button>
</form>

// Verify nonce on submission
if (isset($_POST['submit'])) {
    // Security check #1: Verify nonce
    if (!isset($_POST['delete_user_nonce']) ||
        !wp_verify_nonce($_POST['delete_user_nonce'], 'delete_user_action')) {
        wp_die('Security check failed: Invalid nonce');
    }

    // Security check #2: Capability check
    if (!current_user_can('delete_users')) {
        wp_die('You do not have permission to delete users');
    }

    // Now safe to process
    $user_id = absint($_POST['user_id']);
    wp_delete_user($user_id);
}

Key Functions:

• wp_nonce_field($action, $name) - Generates hidden nonce field
• wp_verify_nonce($nonce, $action) - Verifies nonce validity

Pattern 2: URL Nonces

Use Case: Delete/trash links, admin actions

// Generate nonce URL
$delete_url = wp_nonce_url(
    admin_url('admin.php?action=delete_post&post_id=123'),
    'delete_post_123',  // Action (must be unique)
    'delete_nonce'      // Query parameter name
);

echo '<a href="' . esc_url($delete_url) . '">Delete Post</a>';

// Verify nonce in handler
add_action('admin_action_delete_post', 'handle_delete_post');
function handle_delete_post() {
    // Verify nonce from URL
    if (!isset($_GET['delete_nonce']) ||
        !wp_verify_nonce($_GET['delete_nonce'], 'delete_post_123')) {
        wp_die('Invalid security token');
    }

    // Verify capability
    $post_id = absint($_GET['post_id']);
    if (!current_user_can('delete_post', $post_id)) {
        wp_die('You cannot delete this post');
    }

    // Delete post
    wp_delete_post($post_id, true); // true = force delete

    // Redirect with success message
    wp_redirect(add_query_arg('message', 'deleted', wp_get_referer()));
    exit;
}

Pattern 3: AJAX Nonces

Use Case: Frontend AJAX requests

BEFORE (Vulnerable):

// ⚠️ Vulnerable AJAX request
jQuery.post(ajaxurl, {
    action: 'update_user_meta',
    user_id: 42,
    meta_key: 'favorite_color',
    meta_value: 'blue'
}, function(response) {
    console.log(response);
});

AFTER (Secure):

PHP (Enqueue script with nonce):

add_action('wp_enqueue_scripts', 'enqueue_ajax_script');
function enqueue_ajax_script() {
    wp_enqueue_script('my-ajax-script',
        plugin_dir_url(__FILE__) . 'js/ajax.js',
        ['jquery'],
        '1.0.0',
        true
    );

    // Pass nonce and AJAX URL to JavaScript
    wp_localize_script('my-ajax-script', 'myAjax', [
        'ajaxurl' => admin_url('admin-ajax.php'),
        'nonce' => wp_create_nonce('my_ajax_nonce'), // Generate nonce
    ]);
}

// AJAX handler with nonce verification
add_action('wp_ajax_update_user_meta', 'handle_ajax_update');
function handle_ajax_update() {
    // Verify nonce
    check_ajax_referer('my_ajax_nonce', 'nonce');

    // Verify capability
    if (!current_user_can('edit_users')) {
        wp_send_json_error(['message' => 'Permission denied']);
    }

    // Sanitize input
    $user_id = absint($_POST['user_id']);
    $meta_key = sanitize_key($_POST['meta_key']);
    $meta_value = sanitize_text_field($_POST['meta_value']);

    // Update meta
    update_user_meta($user_id, $meta_key, $meta_value);

    wp_send_json_success(['message' => 'Updated successfully']);
}

JavaScript (Use nonce in AJAX):

jQuery(document).ready(function($) {
    $('#update-button').on('click', function() {
        $.post(myAjax.ajaxurl, {
            action: 'update_user_meta',
            nonce: myAjax.nonce,  // Include nonce
            user_id: 42,
            meta_key: 'favorite_color',
            meta_value: 'blue'
        }, function(response) {
            if (response.success) {
                console.log(response.data.message);
            } else {
                console.error(response.data.message);
            }
        });
    });
});

Key Functions:

• wp_create_nonce($action) - Generate nonce token
• check_ajax_referer($action, $query_arg) - Verify AJAX nonce (dies on failure)
• wp_send_json_success($data) - Send JSON success response
• wp_send_json_error($data) - Send JSON error response

Nonce Best Practices

✅ DO:

• Use unique action names (e.g., delete_post_$post_id, not just delete)
• Always verify nonces BEFORE processing any data
• Combine nonce checks with capability checks
• Use specific nonce functions (check_ajax_referer for AJAX)

❌ DON'T:

• Reuse the same nonce action for multiple operations
• Skip nonce verification for "read-only" operations
• Trust nonce verification alone (always check capabilities too)
• Store nonces in cookies or URLs for long-term use (they expire)

Nonce Lifespan: WordPress nonces expire after 24 hours by default (12 hours in each direction due to time window).

2. Sanitization Functions Reference

Sanitization transforms user input into a safe format by removing or encoding dangerous characters. It's the first line of defense against malicious data.

Core Sanitization Functions

Detailed Examples

Text Sanitization

// Single-line text (removes HTML, line breaks, extra whitespace)
$username = sanitize_text_field($_POST['username']);
// Input: "  John <b>Doe</b>\n"
// Output: "John Doe"

// Multi-line text (preserves line breaks, removes HTML)
$bio = sanitize_textarea_field($_POST['bio']);
// Input: "Line 1\nLine 2<script>alert('xss')</script>"
// Output: "Line 1\nLine 2alert('xss')"

// Email (validates format and removes invalid characters)
$email = sanitize_email($_POST['email']);
// Input: "user@EXAMPLE.com <script>"
// Output: "user@example.com"

// URL (removes dangerous protocols)
$website = esc_url_raw($_POST['website']);
// Input: "javascript:alert('xss')"
// Output: "" (blocked protocol)
// Input: "http://example.com"
// Output: "http://example.com"

Numeric Sanitization

// Positive integers only (absolute value)
$post_id = absint($_POST['post_id']);
// Input: "-5", "42", "123abc"
// Output: 5, 42, 123

// Any integer (preserves negative)
$temperature = intval($_POST['temperature']);
// Input: "-5", "42.7", "99abc"
// Output: -5, 42, 99

// Floating-point numbers
$price = floatval($_POST['price']);
// Input: "19.99", "20.5abc"
// Output: 19.99, 20.5

HTML Sanitization

wp_kses_post() - Allow WordPress Post Editor Tags:

$content = wp_kses_post($_POST['content']);
// Allows: <p>, <a>, <strong>, <em>, <ul>, <ol>, <li>, <blockquote>, <img>, etc.
// Blocks: <script>, <iframe>, <object>, <embed>, <form>

// Input: "<p>Safe content</p><script>alert('xss')</script>"
// Output: "<p>Safe content</p>alert('xss')"

wp_kses() - Custom Allowed Tags:

// Define allowed tags and attributes
$allowed_html = [
    'a' => [
        'href' => true,
        'title' => true,
        'target' => true,
    ],
    'strong' => [],
    'em' => [],
    'br' => [],
];

$clean_html = wp_kses($_POST['content'], $allowed_html);

// Input: "<a href='#' onclick='alert(1)'>Link</a><script>Bad</script>"
// Output: "<a href='#'>Link</a>Bad" (onclick removed, script stripped)

Strip All HTML:

$plain_text = wp_strip_all_tags($_POST['content']);
// Input: "<p>Hello <b>World</b></p>"
// Output: "Hello World"

File Upload Sanitization

// Sanitize filename (removes path traversal, special characters)
$safe_filename = sanitize_file_name($_FILES['upload']['name']);
// Input: "../../etc/passwd", "my file!.php"
// Output: "..etcpasswd", "my-file.php"

// Complete file upload example
if (isset($_FILES['user_avatar'])) {
    // Verify nonce first!
    if (!wp_verify_nonce($_POST['upload_nonce'], 'upload_avatar')) {
        wp_die('Security check failed');
    }

    // Sanitize filename
    $filename = sanitize_file_name($_FILES['user_avatar']['name']);

    // Validate file type
    $allowed_types = ['image/jpeg', 'image/png', 'image/gif'];
    $file_type = $_FILES['user_avatar']['type'];

    if (!in_array($file_type, $allowed_types)) {
        wp_die('Invalid file type. Only JPG, PNG, GIF allowed.');
    }

    // Use WordPress upload handler (handles security)
    $upload = wp_handle_upload($_FILES['user_avatar'], [
        'test_form' => false,
        'mimes' => [
            'jpg|jpeg' => 'image/jpeg',
            'png' => 'image/png',
            'gif' => 'image/gif',
        ],
    ]);

    if (isset($upload['error'])) {
        wp_die('Upload failed: ' . $upload['error']);
    }

    // Store uploaded file URL
    $avatar_url = $upload['url'];
    update_user_meta(get_current_user_id(), 'avatar_url', $avatar_url);
}

Array Sanitization

// Sanitize array of text fields
$tags = array_map('sanitize_text_field', $_POST['tags']);
// Input: ['tag1', '<script>tag2</script>', 'tag3']
// Output: ['tag1', 'tag2', 'tag3']

// Sanitize array of integers
$ids = array_map('absint', $_POST['post_ids']);
// Input: ['1', '2abc', '-5']
// Output: [1, 2, 5]

// Sanitize array of emails
$emails = array_map('sanitize_email', $_POST['email_list']);

Custom Sanitization Callbacks

// Register setting with sanitization callback
register_setting('my_plugin_options', 'my_plugin_settings', [
    'type' => 'array',
    'sanitize_callback' => 'my_plugin_sanitize_settings',
]);

function my_plugin_sanitize_settings($input) {
    $sanitized = [];

    // Sanitize API key (alphanumeric only)
    if (isset($input['api_key'])) {
        $sanitized['api_key'] = preg_replace('/[^a-zA-Z0-9]/', '', $input['api_key']);
    }

    // Sanitize boolean checkbox
    $sanitized['enable_feature'] = isset($input['enable_feature']) ? 1 : 0;

    // Sanitize color (hex format)
    if (isset($input['primary_color'])) {
        $color = sanitize_hex_color($input['primary_color']);
        $sanitized['primary_color'] = $color ? $color : '#000000';
    }

    // Sanitize select option (whitelist)
    $allowed_modes = ['mode1', 'mode2', 'mode3'];
    if (isset($input['mode']) && in_array($input['mode'], $allowed_modes)) {
        $sanitized['mode'] = $input['mode'];
    } else {
        $sanitized['mode'] = 'mode1'; // Default
    }

    return $sanitized;
}

3. Validation Patterns

Validation ensures data meets business logic requirements after sanitization. Unlike sanitization (which transforms data), validation returns true/false.

Built-in Validation Functions

Validation Examples

Email Validation

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

// Validate format
if (!is_email($email)) {
    $errors[] = 'Invalid email address format';
}

// Validate uniqueness (for registration)
if (email_exists($email)) {
    $errors[] = 'Email address already registered';
}

Numeric Range Validation

$age = absint($_POST['age']);

// Validate range
if ($age < 18 || $age > 100) {
    $errors[] = 'Age must be between 18 and 100';
}

// Validate positive number
if ($quantity <= 0) {
    $errors[] = 'Quantity must be greater than zero';
}

String Length Validation

$username = sanitize_text_field($_POST['username']);

// Validate minimum length
if (strlen($username) < 3) {
    $errors[] = 'Username must be at least 3 characters';
}

// Validate maximum length
if (strlen($username) > 20) {
    $errors[] = 'Username cannot exceed 20 characters';
}

Required Field Validation

// Check if field exists and is not empty
if (empty($_POST['title']) || trim($_POST['title']) === '') {
    $errors[] = 'Title is required';
}

// Alternative: isset() + non-empty check
if (!isset($_POST['terms']) || $_POST['terms'] !== 'accepted') {
    $errors[] = 'You must accept the terms and conditions';
}

Pattern Matching (Regex)

$phone = sanitize_text_field($_POST['phone']);

// Validate phone format (US format: (555) 123-4567)
if (!preg_match('/^\(\d{3}\) \d{3}-\d{4}$/', $phone)) {
    $errors[] = 'Phone must be in format: (555) 123-4567';
}

// Validate alphanumeric only
$product_code = sanitize_text_field($_POST['product_code']);
if (!preg_match('/^[a-zA-Z0-9]+$/', $product_code)) {
    $errors[] = 'Product code must contain only letters and numbers';
}

Multi-Field Validation

function validate_registration_form($data) {
    $errors = [];

    // Email validation
    $email = sanitize_email($data['email']);
    if (!is_email($email)) {
        $errors['email'] = 'Invalid email address';
    } elseif (email_exists($email)) {
        $errors['email'] = 'Email already registered';
    }

    // Username validation
    $username = sanitize_text_field($data['username']);
    if (strlen($username) < 3) {
        $errors['username'] = 'Username too short (minimum 3 characters)';
    } elseif (username_exists($username)) {
        $errors['username'] = 'Username already taken';
    }

    // Password validation
    if (strlen($data['password']) < 8) {
        $errors['password'] = 'Password must be at least 8 characters';
    }

    // Password confirmation
    if ($data['password'] !== $data['password_confirm']) {
        $errors['password_confirm'] = 'Passwords do not match';
    }

    // Age validation
    $age = absint($data['age']);
    if ($age < 18) {
        $errors['age'] = 'You must be 18 or older to register';
    }

    return empty($errors) ? true : $errors;
}

// Usage
$result = validate_registration_form($_POST);
if ($result === true) {
    // Process registration
} else {
    // Display errors
    foreach ($result as $field => $error) {
        echo "<p class='error'>$error</p>";
    }
}

Custom Validation Rules

// Validate URL is from allowed domain
function validate_allowed_domain($url) {
    $allowed_domains = ['example.com', 'wordpress.org'];
    $host = parse_url($url, PHP_URL_HOST);

    return in_array($host, $allowed_domains);
}

// Validate date format and range
function validate_date($date_string) {
    $date = DateTime::createFromFormat('Y-m-d', $date_string);

    if (!$date) {
        return false; // Invalid format
    }

    // Check date is not in the past
    $now = new DateTime();
    if ($date < $now) {
        return false;
    }

    return true;
}

// Validate credit card (Luhn algorithm)
function validate_credit_card($number) {
    $number = preg_replace('/\D/', '', $number); // Remove non-digits

    if (strlen($number) < 13 || strlen($number) > 19) {
        return false;
    }

    $sum = 0;
    $double = false;

    for ($i = strlen($number) - 1; $i >= 0; $i--) {
        $digit = (int) $number[$i];

        if ($double) {
            $digit *= 2;
            if ($digit > 9) {
                $digit -= 9;
            }
        }

        $sum += $digit;
        $double = !$double;
    }

    return ($sum % 10) === 0;
}

4. Output Escaping Reference

Escaping prevents XSS (Cross-Site Scripting) by encoding special characters before output. This is the final security layer.

Core Escaping Functions

Detailed Escaping Examples

HTML Content Escaping

// Escape HTML content (converts special characters to entities)
$user_comment = "<script>alert('XSS')</script>Hello";
echo esc_html($user_comment);
// Output: &lt;script&gt;alert('XSS')&lt;/script&gt;Hello
// Browser displays: <script>alert('XSS')</script>Hello (as text, not code)

// WRONG: No escaping
echo $user_comment; // ⚠️ Executes JavaScript!

HTML Attribute Escaping

// Escape attribute values
$title = 'My "Awesome" Title';
?>
<input type="text"
       value="<?php echo esc_attr($title); ?>"
       placeholder="<?php echo esc_attr($placeholder); ?>">
<!-- Output: value="My &quot;Awesome&quot; Title" -->

<!-- WRONG: No escaping -->
<input value="<?php echo $title; ?>">
<!-- Output: <input value="My "Awesome" Title"> (breaks HTML!) -->

URL Escaping

// Escape URLs (blocks dangerous protocols)
$user_url = "javascript:alert('XSS')";
echo '<a href="' . esc_url($user_url) . '">Link</a>';
// Output: <a href="">Link</a> (javascript: protocol blocked)

// Safe URL
$safe_url = "https://example.com";
echo '<a href="' . esc_url($safe_url) . '">Link</a>';
// Output: <a href="https://example.com">Link</a>

// WRONG: No escaping
echo '<a href="' . $user_url . '">Link</a>'; // ⚠️ XSS vulnerability!

JavaScript Escaping

// Escape JavaScript strings
$user_message = "It's \"dangerous\" to trust user input";
?>
<script>
    var message = '<?php echo esc_js($user_message); ?>';
    alert(message);
</script>
<!-- Output: var message = 'It\'s \"dangerous\" to trust user input'; -->

<!-- WRONG: No escaping -->
<script>
    var message = '<?php echo $user_message; ?>'; // ⚠️ Breaks JavaScript!
</script>

Textarea Escaping

// Escape textarea content
$bio = "Line 1\nLine 2 <script>alert('XSS')</script>";
?>
<textarea><?php echo esc_textarea($bio); ?></textarea>
<!-- Output preserves line breaks, escapes HTML -->

<!-- WRONG: Using esc_html() in textarea -->
<textarea><?php echo esc_html($bio); ?></textarea>
<!-- ⚠️ Line breaks converted to &lt;br&gt; (not displayed correctly) -->

Context-Specific Escaping

HTML Context

// Paragraph content
echo '<p>' . esc_html($user_content) . '</p>';

// Link text
echo '<a href="' . esc_url($url) . '">' . esc_html($link_text) . '</a>';

// Image alt text
echo '<img src="' . esc_url($image_url) . '" alt="' . esc_attr($alt_text) . '">';

Attribute Context

// Data attributes
echo '<div data-user-id="' . esc_attr($user_id) . '"
           data-username="' . esc_attr($username) . '"></div>';

// Class names (use sanitize_html_class)
echo '<div class="' . esc_attr(sanitize_html_class($class)) . '"></div>';

// Style attribute (dangerous - avoid if possible)
$safe_color = sanitize_hex_color($user_color); // Validate first
echo '<div style="color: ' . esc_attr($safe_color) . ';"></div>';

JavaScript Context

// Inline JavaScript (avoid if possible, use wp_localize_script instead)
<script>
    var config = {
        username: '<?php echo esc_js($username); ?>',
        apiUrl: '<?php echo esc_js(admin_url('admin-ajax.php')); ?>'
    };
</script>

// BETTER: Use wp_localize_script
wp_localize_script('my-script', 'myConfig', [
    'username' => $username, // Automatically JSON-encoded
    'apiUrl' => admin_url('admin-ajax.php'),
]);

Internationalization + Escaping

// Translate and escape
echo esc_html__('Welcome User', 'my-plugin');

// Translate with variable, then escape
$message = sprintf(
    __('Hello %s, you have %d new messages', 'my-plugin'),
    esc_html($username),
    absint($message_count)
);
echo $message;

// Escape translatable attributes
<input placeholder="<?php echo esc_attr__('Enter your name', 'my-plugin'); ?>">

// Allow HTML in translations (use wp_kses_post)
$welcome_html = __('Welcome to <strong>My Plugin</strong>!', 'my-plugin');
echo wp_kses_post($welcome_html);

Common Escaping Mistakes

❌ WRONG:

// Double-escaping (displays HTML entities to user)
echo esc_html(esc_html($content)); // ⚠️ Displays <script>

// Wrong function for context
echo '<a href="' . esc_html($url) . '">Link</a>'; // ⚠️ Use esc_url()

// No escaping in JavaScript
echo "<script>var x = '$user_input';</script>"; // ⚠️ Use esc_js()

// Escaping before storage (store raw, escape on output)
update_option('setting', esc_html($value)); // ⚠️ Escape on output, not input

✅ CORRECT:

// Escape once, on output
echo esc_html($content);

// Use correct function for context
echo '<a href="' . esc_url($url) . '">' . esc_html($text) . '</a>';

// Escape JavaScript properly
wp_localize_script('script', 'data', ['value' => $user_input]);

// Store raw, escape on output
update_option('setting', $value); // Store raw
echo esc_html(get_option('setting')); // Escape on output

5. Capability Checks (Authorization)

Capability checks ensure users have permission to perform actions. Always combine with nonce verification.

Built-in Capabilities

Capability Check Patterns

Basic Capability Check

// Check if user is logged in
if (!is_user_logged_in()) {
    wp_die('You must be logged in to access this page');
}

// Check if user has capability
if (!current_user_can('manage_options')) {
    wp_die('You do not have permission to manage settings');
}

// Check if user can edit specific post
$post_id = absint($_GET['post_id']);
if (!current_user_can('edit_post', $post_id)) {
    wp_die('You cannot edit this post');
}

Complete Security Example

add_action('admin_post_update_settings', 'handle_settings_update');
function handle_settings_update() {
    // 1. Check if user is logged in
    if (!is_user_logged_in()) {
        wp_die('You must be logged in');
    }

    // 2. Verify nonce
    if (!isset($_POST['settings_nonce']) ||
        !wp_verify_nonce($_POST['settings_nonce'], 'update_settings')) {
        wp_die('Security check failed');
    }

    // 3. Check user capability
    if (!current_user_can('manage_options')) {
        wp_die('You do not have permission to update settings');
    }

    // 4. Sanitize input
    $api_key = sanitize_text_field($_POST['api_key']);
    $enable_feature = isset($_POST['enable_feature']) ? 1 : 0;

    // 5. Validate data
    if (strlen($api_key) < 10) {
        wp_die('API key must be at least 10 characters');
    }

    // 6. Update options
    update_option('my_plugin_api_key', $api_key);
    update_option('my_plugin_enable_feature', $enable_feature);

    // 7. Redirect with success message
    wp_redirect(add_query_arg('message', 'updated', wp_get_referer()));
    exit;
}

Post-Specific Capabilities

// Check if user can edit specific post
$post_id = absint($_POST['post_id']);
if (!current_user_can('edit_post', $post_id)) {
    wp_send_json_error(['message' => 'You cannot edit this post']);
}

// Check if user can delete specific post
if (!current_user_can('delete_post', $post_id)) {
    wp_send_json_error(['message' => 'You cannot delete this post']);
}

// Check if user can publish posts
if (!current_user_can('publish_posts')) {
    wp_send_json_error(['message' => 'You cannot publish posts']);
}

Custom Capabilities

// Register custom role with custom capability
add_action('init', 'register_custom_role');
function register_custom_role() {
    add_role('store_manager', 'Store Manager', [
        'read' => true,
        'edit_posts' => true,
        'manage_products' => true, // Custom capability
    ]);
}

// Add custom capability to existing role
$role = get_role('editor');
$role->add_cap('manage_products');

// Check custom capability
if (current_user_can('manage_products')) {
    // Allow product management
}

6. SQL Injection Prevention

CRITICAL: Never trust user input in SQL queries. Always use $wpdb->prepare().

The Problem: SQL Injection

BEFORE (Vulnerable):

global $wpdb;

// ⚠️ CRITICAL VULNERABILITY - SQL INJECTION!
$user_id = $_GET['user_id'];
$results = $wpdb->get_results(
    "SELECT * FROM {$wpdb->posts} WHERE post_author = $user_id"
);

// Attacker can inject SQL:
// ?user_id=1 OR 1=1 -- (returns all posts)
// ?user_id=1; DROP TABLE wp_posts; -- (deletes table!)

AFTER (Secure):

global $wpdb;

// ✅ SECURE - Using prepared statements
$user_id = absint($_GET['user_id']); // Sanitize first
$results = $wpdb->get_results(
    $wpdb->prepare(
        "SELECT * FROM {$wpdb->posts} WHERE post_author = %d",
        $user_id
    )
);

Prepared Statement Placeholders

Complete Examples

SELECT Query

global $wpdb;

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

// Prepared statement (prevents SQL injection)
$user = $wpdb->get_row(
    $wpdb->prepare(
        "SELECT * FROM {$wpdb->users} WHERE user_email = %s",
        $email
    )
);

if ($user) {
    echo "User found: " . esc_html($user->user_login);
}

INSERT Query

global $wpdb;

// Use wpdb->insert() (automatically prepares)
$result = $wpdb->insert(
    $wpdb->prefix . 'my_table',
    [
        'title' => sanitize_text_field($_POST['title']),
        'content' => wp_kses_post($_POST['content']),
        'user_id' => absint($_POST['user_id']),
        'price' => floatval($_POST['price']),
        'created_at' => current_time('mysql'),
    ],
    ['%s', '%s', '%d', '%f', '%s'] // Format specifiers
);

if ($result === false) {
    wp_die('Database insert failed: ' . $wpdb->last_error);
}

$inserted_id = $wpdb->insert_id;

UPDATE Query

global $wpdb;

$wpdb->update(
    $wpdb->prefix . 'my_table',
    [
        'title' => sanitize_text_field($_POST['title']), // New values
        'updated_at' => current_time('mysql'),
    ],
    ['id' => absint($_POST['id'])], // WHERE condition
    ['%s', '%s'], // Format for new values
    ['%d']        // Format for WHERE condition
);

DELETE Query

global $wpdb;

$wpdb->delete(
    $wpdb->prefix . 'my_table',
    ['id' => absint($_POST['id'])],
    ['%d']
);

Complex WHERE Clause

global $wpdb;

$status = sanitize_text_field($_POST['status']);
$min_price = floatval($_POST['min_price']);

// Multiple placeholders
$results = $wpdb->get_results(
    $wpdb->prepare(
        "SELECT * FROM {$wpdb->prefix}products
         WHERE status = %s AND price >= %f
         ORDER BY created_at DESC
         LIMIT %d",
        $status,
        $min_price,
        10 // LIMIT value
    )
);

Common SQL Injection Mistakes

❌ WRONG:

// String concatenation (vulnerable!)
$sql = "SELECT * FROM table WHERE name = '" . $_POST['name'] . "'";

// Using esc_sql() (deprecated and insufficient)
$sql = "SELECT * FROM table WHERE name = '" . esc_sql($_POST['name']) . "'";

// Not using placeholders
$wpdb->query("DELETE FROM table WHERE id = $id"); // ⚠️ Vulnerable

✅ CORRECT:

// Always use $wpdb->prepare()
$wpdb->get_results($wpdb->prepare(
    "SELECT * FROM table WHERE name = %s",
    $_POST['name']
));

// Use wpdb methods (insert, update, delete)
$wpdb->insert('table', ['name' => $_POST['name']], ['%s']);

7. Common Vulnerabilities & Attack Scenarios

XSS (Cross-Site Scripting)

Attack Scenario:

// Vulnerable code
echo "Welcome, " . $_GET['username'];
// Attacker visits: ?username=<script>alert(document.cookie)</script>
// Browser executes JavaScript, stealing session cookies

Prevention:

// Escape output
echo "Welcome, " . esc_html($_GET['username']);
// Output: Welcome, <script>alert(document.cookie)</script>

CSRF (Cross-Site Request Forgery)

Attack Scenario:

<!-- Attacker's site (evil.com) -->
<img src="https://yoursite.com/wp-admin/admin.php?action=delete_all_posts">
<!-- If admin is logged in, this executes without their knowledge! -->

Prevention:

// Require nonce verification
if (!wp_verify_nonce($_GET['nonce'], 'delete_all_posts')) {
    wp_die('Invalid security token');
}

SQL Injection

Attack Scenario:

// Vulnerable code
$wpdb->query("DELETE FROM posts WHERE id = " . $_GET['id']);
// Attacker visits: ?id=1 OR 1=1
// Deletes ALL posts!

Prevention:

// Use prepared statements
$wpdb->query($wpdb->prepare(
    "DELETE FROM posts WHERE id = %d",
    absint($_GET['id'])
));

File Upload Attack

Attack Scenario:

// Vulnerable code
move_uploaded_file($_FILES['file']['tmp_name'], 'uploads/' . $_FILES['file']['name']);
// Attacker uploads: malicious.php
// Executes: https://yoursite.com/uploads/malicious.php

Prevention:

// Validate file type and use wp_handle_upload()
$allowed_types = ['image/jpeg', 'image/png'];
if (!in_array($_FILES['file']['type'], $allowed_types)) {
    wp_die('Invalid file type');
}

$upload = wp_handle_upload($_FILES['file'], ['test_form' => false]);

Path Traversal

Attack Scenario:

// Vulnerable code
include($_GET['template'] . '.php');
// Attacker visits: ?template=../../../../etc/passwd

Prevention:

// Whitelist allowed templates
$allowed_templates = ['template1', 'template2'];
$template = sanitize_file_name($_GET['template']);

if (in_array($template, $allowed_templates)) {
    include($template . '.php');
}

8. Complete Security Implementation Example

<?php
/**
 * Secure Form Handling Example
 * Demonstrates all security layers: nonces, sanitization, validation, escaping
 */

// 1. Display Form (with nonce)
function display_user_profile_form() {
    $user_id = get_current_user_id();
    $user_data = get_user_meta($user_id, 'profile_data', true);

    ?>
    <form method="post" action="<?php echo esc_url(admin_url('admin-post.php')); ?>">
        <input type="hidden" name="action" value="update_user_profile">
        <?php wp_nonce_field('update_profile_' . $user_id, 'profile_nonce'); ?>

        <label>
            Display Name:
            <input type="text"
                   name="display_name"
                   value="<?php echo esc_attr($user_data['display_name'] ?? ''); ?>"
                   required>
        </label>

        <label>
            Email:
            <input type="email"
                   name="email"
                   value="<?php echo esc_attr($user_data['email'] ?? ''); ?>"
                   required>
        </label>

        <label>
            Bio:
            <textarea name="bio"><?php echo esc_textarea($user_data['bio'] ?? ''); ?></textarea>
        </label>

        <label>
            Website:
            <input type="url"
                   name="website"
                   value="<?php echo esc_attr($user_data['website'] ?? ''); ?>">
        </label>

        <button type="submit">Update Profile</button>
    </form>
    <?php
}

// 2. Process Form (with full security)
add_action('admin_post_update_user_profile', 'handle_profile_update');
function handle_profile_update() {
    // SECURITY LAYER 1: Authentication
    if (!is_user_logged_in()) {
        wp_die('You must be logged in to update your profile');
    }

    $user_id = get_current_user_id();

    // SECURITY LAYER 2: Nonce Verification
    if (!isset($_POST['profile_nonce']) ||
        !wp_verify_nonce($_POST['profile_nonce'], 'update_profile_' . $user_id)) {
        wp_die('Security check failed: Invalid nonce');
    }

    // SECURITY LAYER 3: Capability Check
    if (!current_user_can('edit_user', $user_id)) {
        wp_die('You do not have permission to update this profile');
    }

    // SECURITY LAYER 4: Sanitization
    $display_name = sanitize_text_field($_POST['display_name']);
    $email = sanitize_email($_POST['email']);
    $bio = sanitize_textarea_field($_POST['bio']);
    $website = esc_url_raw($_POST['website']);

    // SECURITY LAYER 5: Validation
    $errors = [];

    if (empty($display_name) || strlen($display_name) < 3) {
        $errors[] = 'Display name must be at least 3 characters';
    }

    if (!is_email($email)) {
        $errors[] = 'Invalid email address';
    }

    if (!empty($website) && !filter_var($website, FILTER_VALIDATE_URL)) {
        $errors[] = 'Invalid website URL';
    }

    if (!empty($errors)) {
        wp_die(implode('<br>', array_map('esc_html', $errors)));
    }

    // SECURITY LAYER 6: Process Data
    $profile_data = [
        'display_name' => $display_name,
        'email' => $email,
        'bio' => $bio,
        'website' => $website,
    ];

    update_user_meta($user_id, 'profile_data', $profile_data);

    // SECURITY LAYER 7: Safe Redirect
    wp_redirect(add_query_arg('message', 'profile_updated', wp_get_referer()));
    exit;
}

// 3. Display Success Message (with escaping)
add_action('admin_notices', 'show_profile_update_notice');
function show_profile_update_notice() {
    if (isset($_GET['message']) && $_GET['message'] === 'profile_updated') {
        echo '<div class="notice notice-success is-dismissible">';
        echo '<p>' . esc_html__('Profile updated successfully!', 'my-plugin') . '</p>';
        echo '</div>';
    }
}

9. Security Checklist

Use this checklist for every WordPress feature you implement:

Input Security (Forms, AJAX, APIs)

• Nonce verification implemented (wp_verify_nonce())
• Capability check performed (current_user_can())
• All input sanitized with appropriate functions
• All input validated for business logic
• File uploads use wp_handle_upload()
• File types whitelisted, not blacklisted

Output Security (Templates, APIs)

• All dynamic content escaped with esc_html(), esc_attr(), etc.
• URLs escaped with esc_url()
• JavaScript variables use wp_localize_script() or esc_js()
• No raw echo $_POST or echo $_GET

Database Security

• All queries use $wpdb->prepare()
• No string concatenation in SQL
• Use $wpdb->insert(), $wpdb->update(), $wpdb->delete()
• Table names use $wpdb->prefix

Session Security

• User authentication checked (is_user_logged_in())
• User roles validated (current_user_can())
• Sensitive operations require re-authentication
• Session data never stored in GET parameters

Code Quality

• No eval(), assert(), or create_function()
• No extract() on user input
• Error messages don't reveal system information
• Debug mode disabled in production (WP_DEBUG = false)

10. Testing Your Security Implementation

Manual Testing Checklist

1. Test Nonce Expiration:

# Generate form with nonce, wait 25 hours, submit
# Expected: "Security check failed" error

2. Test CSRF Protection:

<!-- Create external form pointing to your site -->
<form action="https://yoursite.com/wp-admin/admin-post.php" method="POST">
    <input name="action" value="your_action">
    <button>Submit</button>
</form>
<!-- Expected: Nonce verification fails -->

3. Test XSS Prevention:

Input: <script>alert('XSS')</script>
Expected Output: &lt;script&gt;alert('XSS')&lt;/script&gt; (as text)

4. Test SQL Injection:

Input: 1 OR 1=1
Expected: Treats as literal string, no SQL execution

5. Test Capability Bypass:

// Log in as subscriber (low-privilege user)
// Try to access admin-only features
// Expected: "You do not have permission" error

Automated Security Testing

Install Security Scanner:

# WPScan (CLI tool)
gem install wpscan
wpscan --url https://yoursite.com --enumerate vp

# Sucuri Security Plugin
wp plugin install sucuri-scanner --activate

Run PHP Code Sniffer:

# Check for security issues
vendor/bin/phpcs --standard=WordPress-Extra,WordPress-VIP-Go

11. Related Skills & Resources

Prerequisites

• PHP Fundamentals - Understanding PHP syntax, types, functions
• WordPress Plugin Fundamentals - Hooks, actions, filters, plugin structure

Advanced Topics

• WordPress Testing & QA - Security-focused testing strategies
• WordPress REST API - API endpoint security
• WordPress Performance - Secure caching strategies

External Resources

• WordPress Security Handbook
• Plugin Security Best Practices
• OWASP Top 10
• WordPress Plugin Security Testing

Security Plugins for Testing

• Wordfence Security - Firewall and malware scanner
• Sucuri Security - Security auditing and monitoring
• iThemes Security - Security hardening and monitoring

Quick Reference Card

// ============================================
// NONCES (CSRF Protection)
// ============================================

// Forms
wp_nonce_field('action_name', 'nonce_field_name');
wp_verify_nonce($_POST['nonce_field_name'], 'action_name');

// URLs
wp_nonce_url($url, 'action_name', 'nonce_param');
wp_verify_nonce($_GET['nonce_param'], 'action_name');

// AJAX
wp_create_nonce('ajax_action');
check_ajax_referer('ajax_action', 'nonce');

// ============================================
// SANITIZATION (Input Cleaning)
// ============================================

sanitize_text_field()    // Single-line text
sanitize_textarea_field()// Multi-line text
sanitize_email()         // Email addresses
esc_url_raw()           // URLs (for storage)
sanitize_file_name()    // File names
absint()                // Positive integers
wp_kses_post()          // HTML content

// ============================================
// VALIDATION (Logic Checks)
// ============================================

is_email($email)        // Valid email format
is_numeric($value)      // Numeric value
strlen($str) >= 3       // Minimum length
preg_match($pattern)    // Pattern matching
in_array($value, $allowed) // Whitelist check

// ============================================
// ESCAPING (Output Protection)
// ============================================

esc_html()              // HTML content
esc_attr()              // HTML attributes
esc_url()               // URLs (output)
esc_js()                // JavaScript strings
esc_textarea()          // Textarea content

// ============================================
// CAPABILITIES (Authorization)
// ============================================

is_user_logged_in()
current_user_can('capability')
current_user_can('edit_post', $post_id)

// ============================================
// SQL INJECTION PREVENTION
// ============================================

$wpdb->prepare("SELECT * FROM table WHERE id = %d", $id);
$wpdb->insert($table, $data, $format);
$wpdb->update($table, $data, $where, $format, $where_format);

Remember: Security is not a feature—it's a requirement. Every line of code that handles user input or displays data must follow these principles. When in doubt, sanitize, validate, and escape.