MISP contains a reflected cross-site scripting (XSS) vulnerability in the event REST search export confirmation form. The view template app/View/Events/ajax/eventRestSearchExportConfirmationForm.ctp renders a URL-supplied event ID list into a single-quoted JavaScript string literal using PHP's json_encode() without any hex-encoding flags. By default, json_encode() escapes double quotes and backslashes but does not escape single quotes. Because the JavaScript string is delimited by single quotes, an attacker can inject a single-quote character to terminate the string literal and execute arbitrary JavaScript in the victim's browser session. The vulnerability affects the Default and UiBeta themes, both of which render this view. The Overmind theme's own copy of the form already passed the value through an escaped data attribute and was not affected by this specific sink. Preconditions: the victim must be an authenticated MISP user and must actively open or navigate to the attacker-crafted URL. The attacker does not require an account or any prior access to the MISP instance. Security impact: successful exploitation allows execution of arbitrary JavaScript in the context of the MISP web application, potentially leading to session hijacking, unauthorized actions performed on behalf of the victim, exfiltration of sensitive data visible in the session, or further client-side attacks. The vulnerable component's own confidentiality, integrity, and availability are not directly compromised; the impact is on the underlying user session.