Struts 2 Bad Practices: Application Map Tampering

A Struts 2.x Action implements a class which allows an attacker to modify the application business logic by binding arbitrary data into the session, application or request server side objects

Apache Struts 2.x included the new Aware interfaces to allow developers to easily inject maps with relevant runtime information into their Actions code. These interfaces include: org.apache.struts2.interceptor.ApplicationtAware, org.apache.struts2.interceptor.SessionAware and org.apache.struts2.interceptor.RequestAware. In order to get any of these data maps injected into their Actions code, developers need to implement the setter specified in the interface (eg: setSession for SessionAware Interface):

public class VulnerableAction extends ActionSupport implements SessionAware {

  protected Map<String, Object> session;

  @Override
  public void setSession(Map<String, Object> session) {
    this.session = session;
  }

On the other hand, Struts 2.x automatically binds the request data coming from the user to the Action's properties through public accessors defined in the Action. As the Aware interfaces require the implementation of the public setter defined in the Aware interface, this setter will also be automatically bound to any request parameter that matches the Aware interface setter name which might allow remote attackers to modify run-time data values via a crafted parameter to an application that implements an affected interface, as demonstrated by the SessionAware, RequestAware, ApplicationAware interfaces.

The following URL will let an attacker overwrite the "roles" attribute in the session map. This can potentially allow the attacker to become an administrator.

http://server/VulnerableAction?session.roles=admin


While these interfaces only require the implementation of the setter accessors, if the corresponding getter is also implemented, the changes to these map collections will be session-scoped persisted, rather than just affect the current request scope.