The vulnerabilities are caused due to missing input validation in "clusterframe.jsp" and "webserverlist.jsp", which are accessible via the JMC interface. This can be exploited to execute arbitrary script code in a user's browser session by tricking the user into visiting a malicious website or clicking a specially crafted link.
To find out more, see the Secunia Advisory below:
Macromedia JRun JMC Interface Cross-Site Scripting Vulnerabilities (Secunia Advisories, November 28, 2003)