Sitecom WL-153 router firmware before 1.38 allows remote attackers to bypass access restrictions and conduct unauthorized operations via a UPnP request with a modified InternalClient parameter, which is not validated, as demonstrated by using AddPortMapping to forward arbitrary traffic.
http://www.vupen.com/english/advisories/2006/1912
http://www.securityview.org/how-does-the-upnp-flaw-works.html
http://www.securityview.org/dutch-student-finds-a-bug-in-upnp.html