Apache NiFi 1.10.0 through 1.26.0 and 2.0.0-M1 through 2.0.0-M3 support a description field in the Parameter Context configuration that is vulnerable to cross-site scripting. An authenticated user, authorized to configure a Parameter Context, can enter arbitrary JavaScript code, which the client browser will execute within the session context of the authenticated user. Upgrading to Apache NiFi 1.27.0 or 2.0.0-M4 is the recommended mitigation.