Cron Background Job Security

Cron & background job security

When to use this skill

Use this skill whenever code schedules or runs WordPress cron events:

• Calling wp_schedule_event() or wp_schedule_single_event().
• Writing a callback for a cron hook (add_action( 'my_plugin_daily_sync', ... )).
• Running jobs through wp-cron.php, including externally triggered real cron.
• Storing context (user ids, arguments) for a job to act on later.

Cron callbacks run with no current user. current_user_can() returns false for everything, and wp-cron.php is publicly reachable. Authorization must be captured at schedule time and re-verified inside the callback against stored context, not the absent current user.

Related: see the secrets-credentials-management skill for keeping keys out of cron URLs and the capability-permission-checks skill for normal request authorization.

Core principles (and why they matter)

1. No current_user_can() inside cron callbacks. There is no logged-in user during cron execution. Capture the acting user or capability context when scheduling and re-verify it from stored data.
2. wp-cron.php is public. Anyone can trigger it. The callback itself must enforce authorization, not rely on the trigger being hidden.
3. Do not put secrets in scheduled URLs or args. Cron arguments are stored in the cron option; treat them as persistent but not secret.
4. Sanitize and validate stored args. The job reads args that were stored earlier; validate them again before acting.
5. Use wp_next_scheduled() to prevent duplicates. Without it, reactivations or repeated init hooks can schedule the same event many times.
6. Clean up on deactivation. Use wp_clear_scheduled_hook() so events don't run for a deactivated plugin.

Step-by-step implementation

1. Schedule the event with wp_next_scheduled() guard:

   if ( ! wp_next_scheduled( 'my_plugin_daily_sync' ) ) {
       wp_schedule_event( time(), 'daily', 'my_plugin_daily_sync' );
   }
   

2. If the job needs per-object authorization, store a user id or capability context alongside the args at schedule time (e.g., the user who queued a one-time export).
3. In the callback, re-verify the stored context:
   • Look up the user and check user_can( $user_id, $capability ).
   • Validate object ids with get_post() / get_userdata() and check ownership/permission.
4. Sanitize every stored arg before use.
5. Avoid passing secrets or raw user input as cron args.
6. On plugin deactivation, clear the hook.

Supporting references

Common AI mistakes / anti-patterns

Mistake 1 — Using current_user_can() inside a cron callback

// ❌ Insecure: current_user_can is meaningless during cron; this always fails.
add_action( 'my_plugin_daily_sync', function () {
    if ( ! current_user_can( 'manage_options' ) ) {
        return;
    }
    my_plugin_sync();
} );

// ✅ Secure: the scheduled event itself is the authorization boundary; validate data only.
add_action( 'my_plugin_daily_sync', 'my_plugin_run_daily_sync' );
function my_plugin_run_daily_sync() {
    $settings = get_option( 'my_plugin_sync_settings', array() );
    $endpoint = isset( $settings['endpoint'] ) ? esc_url_raw( $settings['endpoint'] ) : '';
    if ( ! $endpoint ) {
        return;
    }
    // No current_user_can needed — the fact the event was scheduled is the policy.
    my_plugin_sync( $endpoint );
}

Mistake 2 — Passing a secret in the cron URL

// ❌ Insecure: secret lands in the cron option and may appear in logs.
wp_schedule_single_event( time(), 'my_plugin_fetch', array( 'token' => $api_secret ) );

// ✅ Secure: store the secret in a wp-config constant or encrypted option; pass only ids.
wp_schedule_single_event( time(), 'my_plugin_fetch', array( 'feed_id' => $feed_id ) );
// Callback reads the secret from a secure location.

Mistake 3 — Trusting $_GET passed to a cron trigger

// ❌ Insecure: wp-cron.php is public and $_GET is attacker-controllable.
add_action( 'my_plugin_run_export', function () {
    $user_id = (int) $_GET['user_id'];
    my_plugin_export_for_user( $user_id );
} );

// ✅ Secure: capture and validate context at schedule time, then re-verify inside the job.
function my_plugin_queue_export( $user_id ) {
    if ( ! current_user_can( 'manage_options' ) ) {
        return false;
    }
    $user_id = absint( $user_id );
    wp_schedule_single_event( time(), 'my_plugin_run_export', array( 'user_id' => $user_id ) );
}

add_action( 'my_plugin_run_export', 'my_plugin_run_export_callback' );
function my_plugin_run_export_callback( $args ) {
    $user_id = isset( $args['user_id'] ) ? absint( $args['user_id'] ) : 0;
    if ( $user_id <= 0 || ! user_can( $user_id, 'read' ) ) {
        return;
    }
    my_plugin_export_for_user( $user_id );
}

Mistake 4 — Scheduling duplicate events

// ❌ Risky: creates a new event on every init if not guarded.
add_action( 'init', function () {
    wp_schedule_event( time(), 'hourly', 'my_plugin_hourly' );
} );

// ✅ Secure: guard with wp_next_scheduled().
add_action( 'init', function () {
    if ( ! wp_next_scheduled( 'my_plugin_hourly' ) ) {
        wp_schedule_event( time(), 'hourly', 'my_plugin_hourly' );
    }
} );

Mistake 5 — Forgetting to unschedule on deactivation

// ❌ Risky: scheduled hooks outlive the plugin — nothing clears them.
// (No register_deactivation_hook() anywhere in the plugin.)
register_activation_hook( __FILE__, function () {
    if ( ! wp_next_scheduled( 'my_plugin_hourly' ) ) {
        wp_schedule_event( time(), 'hourly', 'my_plugin_hourly' );
    }
} );

// ✅ Secure: clear scheduled hooks on deactivation.
register_deactivation_hook( __FILE__, 'my_plugin_deactivate' );
function my_plugin_deactivate() {
    wp_clear_scheduled_hook( 'my_plugin_hourly' );
    wp_clear_scheduled_hook( 'my_plugin_daily_sync' );
}

Correct code examples

An integration example — schedule, callback with stored-context verification, and deactivation cleanup — is in references/secure-cron-job.php. It calls my_plugin_generate_user_export( $user_id ), a project callback your plugin must provide (the file documents the contract); adapt names and hooks to your plugin.

Checklist

• Cron callbacks do not rely on current_user_can().
• Per-object jobs capture the acting user/context at schedule time and re-verify it in the callback.
• Secrets are not passed as cron arguments or embedded in cron URLs.
• Stored cron args are sanitized and validated before use.
• wp_next_scheduled() prevents duplicate scheduled events.
• Custom recurrence intervals are added via the cron_schedules filter before scheduling.
• Scheduled hooks are cleared on plugin deactivation.
• Failures are logged without exposing secrets or internal paths.

Official references

• wp_schedule_event()
• wp_schedule_single_event()
• wp_next_scheduled()
• wp_clear_scheduled_hook()
• wp_unschedule_event()
• user_can()
• DISABLE_WP_CRON constant
• Cron — Plugin Handbook
• OWASP — Unvalidated Redirects and Forwards