Plugin security (nonces, sanitization) in Wordpress - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nonce in WordPress plugin security?

A nonce is a unique token used to verify that a request comes from a trusted source, helping to protect against CSRF attacks.

beginner

Why is sanitization important in WordPress plugins?

Sanitization cleans user input to remove harmful code or characters, preventing security issues like SQL injection or XSS attacks.

beginner

Which WordPress function is commonly used to create a nonce field in forms?

The function wp_nonce_field() creates a hidden nonce field to include in forms for security verification.

intermediate

How do you verify a nonce in WordPress plugin code?

Use check_admin_referer() or check_ajax_referer() functions to verify the nonce before processing the request.

intermediate

Name two WordPress functions used for sanitizing user input.

Examples include sanitize_text_field() for text inputs and esc_url_raw() for URLs to ensure safe data handling.

What does a nonce protect against in WordPress plugins?

A Cross-Site Request Forgery (CSRF)

B SQL Injection

C Brute force login attempts

D File upload errors

Nonces help prevent CSRF by ensuring requests come from trusted sources.

Which function should you use to sanitize a URL input in WordPress?

A sanitize_text_field()

B wp_nonce_field()

C check_admin_referer()

D esc_url_raw()

esc_url_raw() cleans URLs to prevent malicious content and is used for sanitization.

What is the purpose of wp_nonce_field()?

A To create a hidden nonce input in forms

B To sanitize user input

C To verify a nonce

D To escape HTML output

wp_nonce_field() adds a hidden field with a nonce to forms.

Which function verifies a nonce in an AJAX request?

A check_admin_referer()

B wp_nonce_field()

C check_ajax_referer()

D sanitize_text_field()

check_ajax_referer() is used to verify nonces in AJAX calls.

Sanitization in WordPress plugins helps prevent:

A Slow page loading

B Security vulnerabilities like XSS

C Plugin conflicts

D Theme styling issues

Sanitization removes harmful input to prevent attacks like Cross-Site Scripting (XSS).

Explain how nonces work in WordPress plugin security and why they are important.

Think about how you confirm a request is safe before processing it.

Describe the role of sanitization in handling user input within WordPress plugins.

Imagine cleaning dirty water before drinking it.

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