WordPress Plugin Security Testing Cheat Sheet

This cheat sheet was compiled by Dewhurst Security to record the knowledge gained when testing WordPress plugins for security issues for our clients. The security documentation provided by WordPress and found online for plugin security is sparse, outdated or unclear. This cheat sheet is intended for Penetration Testers who audit WordPress plugins or developers who wish to audit their own WordPress plugins.

This is a living document, feedback in the form of Issues or Pull Requests is very much welcomed.

Testing Environment

Install easy test WordPress instances with Local by Flywheel.

Proxy WordPress

To proxy WordPress HTTP requests through a proxy, such as Burp Suite, you can configure WordPress to use a proxy. This is useful if a plugin makes backend API requests. You may need to set Burp Suite to listen on all interfaces. You may also need to disable SSL verification within the plugin's source code.

define('WP_PROXY_HOST', '127.0.0.1');
define('WP_PROXY_PORT', '8080');
define('WP_PROXY_BYPASS_HOSTS', 'localhost, www.example.com, *.wordpress.org');

Log HTTP Requests

The Log HTTP Requests WordPress plugin logs any outgoing HTTP requests, which can be useful if a plugin uses a backend API.

DISALLOW_UNFILTERED_HTML

When doing dynamic testing for XSS the following setting in the wp-config.php file may reduce false positive results as it prevents administrative and editor users from being able to embed/execute JavaScript/HTML, which by default they are permitted to do.

define( 'DISALLOW_UNFILTERED_HTML', true );

Cross-Site Scripting (XSS)

Check if the following global PHP variables are echo'd to pages, or stored in the database and echo'd at a later time without first being sanitised or output encoded.

$_GET
$_POST
$_REQUEST
$_SERVER['REQUEST_URI']
$_SERVER['PHP_SELF']
$_SERVER['HTTP_REFERER']
$_COOKIE

(Note: the list of sources above is not extensive nor complete)

Cross-Site Scripting (XSS) Tips

Unsafe API functions

The following functions can cause XSS if not secured as they use the PHP_SELF variable when the second or third paramater is not a URL:

add_query_arg()
remove_query_arg()

Example regex: add_query_arg$['"\s]+[^,]+['"]\s?$

References:

https://blog.sucuri.net/2015/04/security-advisory-xss-vulnerability-affecting-multiple-wordpress-plugins.html https://make.wordpress.org/plugins/2015/04/20/fixing-add_query_arg-and-remove_query_arg-usage/ https://developer.wordpress.org/reference/functions/add_query_arg/ https://developer.wordpress.org/reference/functions/remove_query_arg/

SQL Injection

Unsafe API methods (require sanitising/escaping):

$wpdb->query()
$wpdb->get_var()
$wpdb->get_row()
$wpdb->get_col()
$wpdb->get_results()
$wpdb->replace()

Safe API methods (according to WordPress):

$wpdb->insert()
$wpdb->update()
$wpdb->delete()

Safe code, prepared statement:

<?php $sql = $wpdb->prepare( 'query' , value_parameter[, value_parameter ... ] ); ?>

Note: Before WordPress 3.5 $wpdb->prepare could be used insecurely as you could just pass the query without using placeholders, like in the following example:

$wpdb->query( $wpdb->prepare( "INSERT INTO table (user, pass) VALUES ('$user', '$pass')" ) );

SQL Injection Tips

Unsafe escaping ('securing') API methods:

esc_sql() function does not adequately protect against SQL Injection https://codex.wordpress.org/Function_Reference/esc_sql
escape() same as above
esc_like() same as above
like_escape() same as above

Displaying/hiding SQL errors:

<?php $wpdb->show_errors(); ?> 
<?php $wpdb->hide_errors(); ?> 
<?php $wpdb->print_error(); ?>

File Download

file()
readfile()
file_get_contents()

File Inclusion

include()
require()
include_once()
require_once()
fread()

File Manipulation

unlink() delete arbitrary files

File Upload

sanitize_file_name() can create valid PHP files, turns test.(php) into test.php

PHP Object Injection

unserialize() any raw user input passed to this function it is probably exploitable
maybe_unserialize() any raw user input passed to this function it is probably exploitable

PHP Object Injection Tips

Use this simple Burp Suite extention along with the PHP Object Injection WordPress Plugin created by White Fir Design.

Command Execution

system()
exec()
passthru()
shell_exec()

PHP Code Execution

eval()
assert()
preg_replace() dangerous "e" flag deprecated since PHP >= 5.5.0 and removed in PHP >= 7.0.0.
php://input reads raw data from the request body, can lead to RCE if used in eval

Authorisation

is_admin() does not check if the user is authenticated as administrator, only checks if page displayed is in the admin section, can lead to auth bypass if misused.
is_user_admin() same as above
current_user_can() used for checking authorisation. This is what should be used to check authorisation.
add_action( 'wp_ajax_nopriv_ permits non-authenticated users to use the AJAX function (https://codex.wordpress.org/Plugin_API/Action_Reference/wp_ajax_(action)).

Open Redirect

wp_redirect() function can be used to redirect to user supplied URLs. If user input is not sanitised or validated this could lead to Open Redirect vulnerabilities.

Cross-Site Request Forgery (CSRF)

wp_nonce_field() adds CSRF token to forms
wp_nonce_url() adds CSRF token to URL
wp_verify_nonce() checks the CSRF token validity server side
check_admin_referer() checks the CSRF token validity server side and came from admin screen

SSL/TLS

CURLOPT_SSL_VERIFYHOST if set to 0 then does not check name in host certificate
CURLOPT_SSL_VERIFYPEER if set to FALSE then does not check if the certificate (inc chain), is trusted. A Man-in-the-Middle (MitM) attacker could use a self-signed certificate.
Check if HTTP is used to communicate with backend servers or APIs. A grep for "http://" should be sufficient.

Priviledge Escalation

update_option() if user input is sent unvalidated, it could allow an attacker to update arbitrary WordPress options.
do_action() if user input is sent unvalidated, it could allow an attacker to update arbitrary WordPress actions.

See: https://www.wordfence.com/blog/2018/11/privilege-escalation-flaw-in-wp-gdpr-compliance-plugin-exploited-in-the-wild/

Automated Static Code Analysis

WordPress-Coding-Standards contains some security rules.

Example:

./vendor/bin/phpcs --standard=WordPress --sniffs=WordPress.CSRF.NonceVerification,WordPress.DB.PreparedSQL,WordPress.DB.PreparedSQLPlaceholders,WordPress.DB.RestrictedClasses,WordPress.DB.RestrictedFunctions,WordPress.Security.NonceVerification,WordPress.Security.PluginMenuSlug,WordPress.Security.SafeRedirect,WordPress.Security.ValidatedSanitizedInput,WordPress.Security.EscapeOutputSniff,WordPress.WP.PreparedSQL,WordPress.XSS.EscapeOutput -p -d memory_limit=256M --colors /path/to/plugin/

See: https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards

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