Apache Struts 2 before 2.3.14.3 allows remote attackers to execute arbitrary OGNL code via a request with a crafted value that contains both "${}" and "%{}" sequences, which causes the OGNL code to be evaluated twice.
Improper Control of Generation of Code The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.