Segue correção se alguem precisar:
<a href="#">
                <security:authentication 
                    property="principal" 
                        var="username" />
                Usuário: ${username.email }
            </a>Erro:
views/produtos/lista.jsp] at line [52]
Description The server encountered an unexpected condition that prevented it from fulfilling the request.
Exception
org.apache.jasper.JasperException: An exception occurred processing JSP page [/WEB-INF/views/produtos/lista.jsp] at line [52]
49:                 <security:authentication 
50:                     property="principal" 
51:                         var="usuario" />
52:                 Usuário: ${usuario.username }
53:             </a>
54:         </li>
55:     </ul> 
            