Envira Gallery <= 1.12.4 - Authenticated (Author+) Stored Cross-Site Scripting via 'arrows' Parameter

The Envira Gallery Lite plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the REST API in versions up to and including 1.12.4. This is due to insufficient input sanitization in the update_gallery_data() function and improper output escaping in the gallery_init() function. The sanitize_config_values() function only sanitizes the justified_gallery_theme and justified_row_height parameters, but does not sanitize the arrows parameter. When the arrows value is output in the inline JavaScript configuration, it uses esc_attr() which is designed for HTML attribute contexts, not JavaScript contexts, allowing JavaScript expression injection. This makes it possible for authenticated attackers, with Author-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.