Audiobookshelf Vulnerable to Cross-Site-Scripting Reflected via POST Request in /api/upload

Audiobookshelf is a self-hosted audiobook and podcast server. Prior to version 2.21.0, an improper input handling vulnerability in the `/api/upload` endpoint allows an attacker to perform a reflected cross-site scripting (XSS) attack by submitting malicious payloads in the `libraryId` field. The unsanitized input is reflected in the server’s error message, enabling arbitrary JavaScript execution in a victim's browser. This issue has been patched in version 2.21.0.