Cross-site scripting in Inilabs School Express

iNiLabs School Express (SMS Express) 6.2 is affected by a Stored Cross-Site Scripting (XSS) vulnerability in the content-management features available to authenticated admin users. The vulnerability resides in POSTed editor parameters submitted to the /posts/edit/{id} endpoint (and similarly in Notice and Pages editors). Due to insufficient input sanitization and output encoding, attackers can inject HTML/JS payloads. The payload is saved and later rendered unsanitized, resulting in JavaScript execution in other users' browsers when they access the affected content. This issue allows an authenticated attacker to execute arbitrary JavaScript in the context of another user, potentially leading to session hijacking, privilege escalation, data exfiltration, or administrative account takeover. The application does not enforce a restrictive Content Security Policy (CSP) or adequate filtering to prevent such attacks.