Classic Theme Security Standards

Classic Theme Security Standards

Use this when writing or reviewing PHP in a classic theme. Themes output most of the HTML on a site, so their main security failure mode is unsafe dynamic output.

This skill complements wp-security-audit; it is theme-specific and focused on templates, functions.php, template parts, and theme forms.

When to Use This Skill

• Reviewing header.php, footer.php, index.php, page.php, single.php, archive.php, 404.php, comments.php, or template-parts/*.php.
• Reviewing functions.php or inc/*.php in a theme.
• Theme code reads $_GET, $_POST, $_REQUEST, $_COOKIE, or custom query vars.
• Theme code prints custom fields, options, term/user meta, search values, image URLs, classes, or inline JS.
• Theme code has a custom form, AJAX handler, template_include override, or direct SQL.

Escape on Output

Escape at the last possible moment, based on context.

<a class="card-link" href="<?php echo esc_url( get_permalink() ); ?>">
    <?php echo esc_html( get_the_title() ); ?>
</a>

Do not escape full editor content with esc_html(). For normal post content, use the_content() and let WordPress content filters run. For custom HTML fields, use wp_kses_post() or a narrower allowlist.

Sanitize Input, Validate Meaning

Request data is slashed. Unslash first, then sanitize, then validate.

$raw_layout = isset( $_GET['layout'] )
    ? wp_unslash( $_GET['layout'] )
    : 'grid';

$layout = sanitize_key( $raw_layout );

if ( ! in_array( $layout, array( 'grid', 'list' ), true ) ) {
    $layout = 'grid';
}

Sanitization changes shape; validation decides whether the value is allowed. Do both when values control template branches, queries, file choices, classes, or external URLs.

Forms Need Nonces

Themes should rarely mutate data. If a theme includes a form that changes state, use a nonce and capability check where relevant.

<form method="post">
    <?php wp_nonce_field( 'mytheme_profile_action', 'mytheme_profile_nonce' ); ?>
    <input type="text" name="display_name" value="">
    <button type="submit"><?php esc_html_e( 'Save', 'mytheme' ); ?></button>
</form>

add_action( 'template_redirect', 'mytheme_handle_profile_form' );

function mytheme_handle_profile_form(): void {
    if ( empty( $_POST['mytheme_profile_nonce'] ) ) {
        return;
    }

    $nonce = sanitize_text_field( wp_unslash( $_POST['mytheme_profile_nonce'] ) );
    if ( ! wp_verify_nonce( $nonce, 'mytheme_profile_action' ) ) {
        wp_die( esc_html__( 'Invalid request.', 'mytheme' ), '', array( 'response' => 403 ) );
    }

    if ( ! is_user_logged_in() ) {
        wp_die( esc_html__( 'You must be logged in.', 'mytheme' ), '', array( 'response' => 403 ) );
    }

    if ( ! current_user_can( 'edit_user', get_current_user_id() ) ) {
        wp_die( esc_html__( 'You cannot edit this profile.', 'mytheme' ), '', array( 'response' => 403 ) );
    }

    $display_name = isset( $_POST['display_name'] )
        ? sanitize_text_field( wp_unslash( $_POST['display_name'] ) )
        : '';

    wp_update_user(
        array(
            'ID'           => get_current_user_id(),
            'display_name' => $display_name,
        )
    );
}

If the form is core search or comments, prefer get_search_form(), comment_form(), and core comment APIs instead of custom handlers.

Safe Template Loading

Never include a PHP file based on request input.

// Wrong: request-controlled file path.
include get_template_directory() . '/views/' . $_GET['view'] . '.php';

Use a fixed map or locate_template():

$view = isset( $_GET['view'] ) ? sanitize_key( wp_unslash( $_GET['view'] ) ) : 'grid';

$templates = array(
    'grid' => 'template-parts/archive-grid.php',
    'list' => 'template-parts/archive-list.php',
);

get_template_part(
    str_replace( '.php', '', $templates[ $view ] ?? $templates['grid'] )
);

For full-template overrides, return a controlled path from template_include; do not include-and-exit from template_redirect.

Assets and Inline Data

Do not hardcode <script> or <link> tags in templates. Use enqueues.

wp_enqueue_script(
    'mytheme-main',
    get_theme_file_uri( 'assets/js/main.js' ),
    array(),
    MYTHEME_VERSION,
    array(
        'strategy'  => 'defer',
        'in_footer' => true,
    )
);

For inline configuration:

wp_add_inline_script(
    'mytheme-main',
    'window.MyTheme = ' . wp_json_encode(
        array(
            'homeUrl' => home_url( '/' ),
            'isRtl'   => is_rtl(),
        )
    ) . ';',
    'before'
);

Do not pass <script> tags to wp_add_inline_script(); WordPress strips and warns.

Database and Queries

Prefer WordPress APIs over SQL:

• WP_Query, get_posts(), get_terms(), get_users().
• get_post_meta(), get_term_meta(), get_user_meta().
• update_post_meta() only when the theme truly owns presentation-specific meta.

If SQL is unavoidable, use $wpdb->prepare():

global $wpdb;

$count = (int) $wpdb->get_var(
    $wpdb->prepare(
        "SELECT COUNT(*) FROM {$wpdb->posts} WHERE post_type = %s AND post_status = %s",
        'post',
        'publish'
    )
);

Do not build SQL with concatenated request data.

Coding Standards Baseline

• Use full PHP tags: <?php, never short tags.
• Do not close PHP-only files with ?>.
• Prefix global functions/hooks with the theme slug, or use a unique namespace for non-template code.
• Use lowercase, hyphenated file names.
• Use require_once for mandatory includes.
• Text domain must match the Text Domain header.
• Escape translated strings with the context-specific helper.
• Keep feature code that should survive a theme switch in a plugin.

Theme Review Checklist

• Every echo, printf, attribute interpolation, and custom field output has context-appropriate escaping.
• Request data uses wp_unslash() before sanitization.
• Values controlling templates/classes/query args are allowlisted.
• Mutating forms have nonce checks and permission checks.
• template_include returns only controlled, existing theme paths.
• Assets are enqueued; inline data uses wp_json_encode().
• No direct SQL without $wpdb->prepare().
• header.php has wp_head() and wp_body_open(); footer.php has wp_footer().

Cross-References

• Broader WP security checklist: wp-security-audit
• Secrets/key handling: wp-security-secrets
• Theme structure and asset placement: classic-theme-structure
• Template file selection: classic-template-hierarchy

References

• Official documentation: https://developer.wordpress.org/themes/advanced-topics/security/
• Official documentation: https://developer.wordpress.org/apis/security/
• Official documentation: https://developer.wordpress.org/apis/security/escaping/
• Official documentation: https://developer.wordpress.org/apis/security/sanitizing/
• Official documentation: https://developer.wordpress.org/coding-standards/wordpress-coding-standards/php/
• Verified source paths:
  • wp-includes/template-loader.php
  • wp-includes/template.php
  • wp-includes/general-template.php
  • wp-includes/functions.wp-scripts.php
  • wp-includes/formatting.php
  • wp-content/themes/storefront/404.php
  • wp-content/themes/generatepress/header.php