Apache MyFaces 1.1.7 and 1.2.8, as used in IBM WebSphere Application Server and other applications, does not properly handle an unencrypted view state, which allows remote attackers to conduct cross-site scripting (XSS) attacks or execute arbitrary Expression Language (EL) statements via vectors that involve modifying the serialized view object.
https://www.trustwave.com/spiderlabs/advisories/TWSL2010-001.txt
http://www.blackhat.com/presentations/bh-dc-10/Byrne_David/BlackHat-DC-2010-Byrne-SGUI-slides.pdf