WordPress security audit

A defensive checklist-driven review for WP plugin and theme PHP. Goal: catch the boring, repeatable mistakes that ship to production because no one ran through the basics. This is not a substitute for a real security review of cryptography, business logic, or third-party deps.

When to use this skill

Trigger this skill when ANY of the following is true:

• The user asks for a security review, audit, or "is this safe".
• The diff or file under discussion contains: $_GET, $_POST, $_REQUEST, $_COOKIE, $_FILES, $_SERVER, wp_unslash, wp_verify_nonce, check_admin_referer, current_user_can, add_action( 'wp_ajax, add_action( 'admin_post, register_rest_route, $wpdb->, update_option, update_user_meta, wp_redirect, wp_safe_redirect, file_get_contents, move_uploaded_file.
• The user is preparing a plugin for release or wp.org submission.
• The user is reviewing a contributor's PR.

How to run the audit

Work through the Critical checks below in order. For each finding:

1. State the file and line.
2. Name the issue using its conventional WP terminology (e.g. "missing nonce", "unescaped output", "broken access control").
3. Show the offending code (1–3 lines).
4. Show the fix.
5. Mark severity: HIGH (exploitable now), MEDIUM (exploitable under conditions), LOW (hardening / best practice).

Do NOT silently rewrite the file. Produce a report first; only edit if the user asks you to apply fixes.

Critical checks

1. Nonce verification on state-changing requests

Any handler that writes (saves option, updates meta, deletes a post, sends an email, mutates anything) must verify a nonce.

• Forms: wp_nonce_field( 'action_name', '_wpnonce' ) → check_admin_referer( 'action_name' ) in handler.
• AJAX: wp_create_nonce( 'action_name' ) → check_ajax_referer( 'action_name', 'nonce' ).
• REST: rely on cookie auth + the built-in _wpnonce (wp-api nonce) for logged-in routes; for permission_callback use a real capability check.

Common mistake: verifying the nonce inside an if whose else branch still does the write. The nonce check must short-circuit.

2. Capability checks (authorization)

Authentication ≠ authorization. A logged-in subscriber is still a user.

• Admin actions: current_user_can( 'manage_options' ) or a more specific capability (edit_posts, edit_post with object ID, manage_woocommerce etc.).
• Object-level actions MUST pass the object ID: current_user_can( 'edit_post', $post_id ) — the ID-less form is wrong.
• REST permission_callback must NEVER be __return_true for state-changing routes. Returning true unconditionally is the #1 plugin vulnerability pattern on wp.org.

3. Input: unslash → sanitize → validate

WordPress magically slashes superglobals. The pipeline is fixed:

$raw      = isset( $_POST['email'] ) ? wp_unslash( $_POST['email'] ) : '';
$email    = sanitize_email( $raw );
if ( ! is_email( $email ) ) { /* reject */ }

• Missing wp_unslash before sanitization → escaped quotes leak through.
• Wrong sanitizer for the data type. Map by intent:
  • text field: sanitize_text_field
  • textarea: sanitize_textarea_field
  • email: sanitize_email
  • URL: esc_url_raw (storage) / esc_url (output)
  • key/slug: sanitize_key, sanitize_title
  • integer: absint or (int) with range check
  • HTML allowed: wp_kses_post or wp_kses with explicit allowlist
  • file path: sanitize_file_name + realpath containment check
• Never trust $_SERVER['HTTP_*'] headers without sanitizing; they're attacker-controlled.

4. Output escaping (XSS)

Escape at the point of output, in the right context:

• HTML body: esc_html( $x )
• HTML attribute: esc_attr( $x )
• URL in href/src: esc_url( $x )
• Inside <script> JSON: wp_json_encode( $x ), never raw concatenation
• Translated strings with placeholders: escape the template AND the substituted value separately. esc_html__() only escapes the static template; printf( esc_html__( '%s', 'td' ), $name ) is XSS if $name contains markup. Correct form: printf( esc_html__( 'Hello, %s', 'td' ), esc_html( $name ) ).
• Already-HTML content (post content): wp_kses_post( $x )

echo $foo; where $foo came from input or DB without escaping → XSS. This is the most common finding in plugin audits.

5. SQL: always prepare

// WRONG
$wpdb->get_results( "SELECT * FROM x WHERE id = $id" );

// RIGHT
$wpdb->get_results( $wpdb->prepare( "SELECT * FROM x WHERE id = %d", $id ) );

• %d integers, %f floats, %s strings. Table/column names CANNOT be parameterized — validate against an allowlist before interpolating.
• LIKE needs $wpdb->esc_like() BEFORE prepare(): $like = '%' . $wpdb->esc_like( $term ) . '%';
• Prefer WP_Query / get_posts / get_users over raw SQL where possible.

6. AJAX endpoints

Two hooks, two meanings — confuse them and you ship a vulnerability:

• wp_ajax_{action} — fires only for logged-in users.
• wp_ajax_nopriv_{action} — fires for logged-out users.

Rules:

• Register nopriv ONLY if the feature is genuinely meant for guests (e.g. public search, login form). Never copy-paste both registrations "to be safe".
• Both handlers still need check_ajax_referer().
• The nopriv handler must NOT perform actions that only logged-in users should do (saving prefs, accessing other users' data, etc.).
• End with wp_send_json_success / wp_send_json_error, not echo + die.

7. admin-post and form handlers

admin_post_{action} and admin_post_nopriv_{action} follow the same rules as AJAX. Plus: redirect with wp_safe_redirect() + exit;. Never redirect with wp_redirect( $_GET['redirect_to'] ) without validating against an allowlist — that's an open-redirect.

8. REST API routes

register_rest_route( 'myplugin/v1', '/save', [
    'methods'             => 'POST',
    'callback'            => 'myplugin_save',
    'permission_callback' => function () {
        return current_user_can( 'manage_options' );
    },
    'args' => [
        'id' => [
            'required'          => true,
            'validate_callback' => 'is_numeric',
            'sanitize_callback' => 'absint',
        ],
    ],
] );

Findings to flag:

• permission_callback missing, or __return_true on a non-public route.
• No args schema — input is unsanitized.
• Returning raw DB rows including sensitive columns (user_pass, user_activation_key, private meta).

9. File operations

• Uploads: validate MIME via wp_check_filetype_and_ext(), store via wp_handle_upload(), never trust the client-provided extension or MIME.
• Path joins with user input: after building, realpath() and check the result starts with the intended base dir. Otherwise: path traversal.
• Never include / require a path containing user input.

10. Secrets and information disclosure

• No API keys or DB credentials in the plugin source. Use options or constants in wp-config.php.
• WP_DEBUG_DISPLAY should be off in prod; flag any var_dump, print_r, error_log( $sensitive ) left in handlers.
• Don't leak stack traces, full SQL, or user enumeration via error messages ("user not found" vs "wrong password" — pick one).

11. Redirects

• Use wp_safe_redirect() for any URL that may be influenced by input.
• Always exit; after a redirect — execution continues otherwise.

12. Cron and background jobs

• wp_schedule_event callbacks run with no current user. If the job performs privileged work, do not trust any "stored intent" without re-validating; treat persisted user input as untrusted.

What this skill does NOT cover

• Cryptographic correctness (key derivation, signing schemes).
• Business-logic flaws (race conditions, IDOR beyond capability checks).
• Third-party library CVEs — run composer audit separately.
• Frontend JS XSS — different skill.
• Server / hosting hardening (file perms, disable_functions, etc.).
• Object injection, SSRF, CSRF on GET, mass assignment, file include, mail/zip injection, timing comparison, TOCTOU races — these are covered by wp-security-deep. Run it after this one.
• Hardcoded credentials, weak randomness for tokens, password storage, cookie flags, secrets in logs — covered by wp-security-secrets. Run it whenever auth or third-party integrations are in scope.

State this scope explicitly in the report. If the reviewed code clearly falls into one of the deeper categories above, recommend the appropriate skill in the report's footer.

Report format

# Security audit: <plugin name>
Scope: <files reviewed>
Date: <YYYY-MM-DD>

## HIGH
1. <file>:<line> — <issue>
   <code>
   Fix: <code>

## MEDIUM
...

## LOW / Hardening
...

## Out of scope
- <thing not checked>

References

• Detailed examples of each finding type, before/after: reference.md
• Real-world snippets with the fix applied: examples/
• WordPress core: Plugin Security Handbook
• Capability map: Roles and Capabilities