It seems that Oracle is taking efforts to make Java more secure. Some time ago our clients started to complain that while installing Image Uploader Java displays the following warning: "Java has discovered application components that could indicate a security concern. Block potentially unsafe components from being run?"
Several times we did attempts to reproduce the problem locally but we failed every time. I tried to dig deeper into this problem and found the Mixing Signed and Unsigned Code article explaining the behavior:
http://download-llnw.oracle.com/javase/6/docs/technotes/guides/jweb/mixed_code.html
As it turned out, starting from Java SE 6 update 19 Oracle has added heuristic to the process of applets installation checking whether an applet contains both signed and unsigned code or not. If it is the case, it warns a user. I am not sure why this problem happened with Image Uploader because it does not have unsigned code. The interesting thing that not all our clients suffered from this problem but only small part of them.
The article mentioned before suggests approach allowing Java developers to prevent displaying this warning dialog. We followed these guidelines and today we posted the updated Image Uploader 6.5.6 to our site. I hope that all things suggested by Oracle is working properly and the updated version of our product will be free of this annoying problem. During the next several weeks we will see the results.