The Real3D Flipbook Lite plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'lightboxtext' shortcode attribute (and other unsanitized attributes handled by on_shortcode()) in versions up to, and including, 5.1.1. This is due to insufficient input sanitization and output escaping in the on_shortcode() and print_global_options() functions: shortcode attribute values are copied verbatim into $this->flipbook_options and then emitted via wp_json_encode() inside a <script type="application/json"> block without the JSON_HEX_TAG flag, allowing a literal </script> byte sequence in the attribute value to break out of the JSON script context. Because WordPress's shortcode_parse_atts() applies stripcslashes() to attribute values, an attacker can encode the breakout tag as \x3c/script\x3e\x3cscript\x3e…\x3c/script\x3e, which survives the wp_kses_post save-time filter applied to Contributor content (the escape bytes are safe text characters, not HTML tags) and is decoded to real angle brackets at render time. This makes it possible for authenticated attackers, with Contributor-level access and above, to inject arbitrary web scripts in pages that will execute whenever a user (typically an Editor or Administrator previewing/moderating the pending post) accesses an injected page.