The description of the vulnerability states
The Struts 2 DefaultActionMapper supports a method for short-circuit navigation state changes by prefixing parameters with "action:" or "redirect:", followed by a desired navigational target expression. This mechanism was intended to help with attaching navigational information to buttons within forms.
In Struts 2 before 126.96.36.199 the information following "action:", "redirect:" or "redirectAction:" is not properly sanitized. Since said information will be evaluated as OGNL expression against the value stack, this introduces the possibility to inject server side code.
The timing seems to fit, as observed by czhiddy on HackerNews:
Struts 188.8.131.52 was released on 7/16/13, and developer.apple.com went down on 7/18/13 for "maintenance". Plenty of time for an enterprising black hat to notice and exploit the vulnerability.