It was reported [1] that Apache Struts2 suffers from a local code execution flaw when processing malformed XSLT files.  This could allow a malicious remote user able to upload an arbitrary file and then view it (such as a graphics file), and execute arbitrary code with the privileges of the struts2 process user.

NOTE: During normal usage, applications that receive untrusted input/files from remote users are expected to properly sanity-check the file and, if nothing else, not immediately make the file uploaded by an untrusted user, available to an untrusted user, without first checking the file.

[1] http://seclists.org/bugtraq/2012/Mar/110