Late last week a security advisory came out that allows for executing malicious code[1]. Atlassian, which uses similar technology, also issued announcements around the same time[2]. My wild speculation is this was the attack vector.
Sadly, I feel some responsibility for this pretty major security hole. There have been a few like this and they are all rooted in the fact that almost 9 years ago I made the (bad) decision to use OGNL as WebWork's expression language. I did so because it was "powerful" but it opened up all sorts of extra binding trickery I never intended. I haven't been contributing to the project in 5+ years, but this is a good reminder how technology choices tend to stick around a lot longer than you ever imagine :)
[1] http://struts.apache.org/release/2.3.x/docs/s2-016.html [2] https://confluence.atlassian.com/display/BAMBOO/Bamboo+Secur...