Apache Struts 2 before 2.2.3.1 evaluates a string as an OGNL expression during the handling of a conversion error, which allows remote attackers to modify run-time data values, and consequently execute arbitrary code, via invalid input to a field.
Improper Input Validation The product receives input or data, but it does
not validate or incorrectly validates that the input has the
properties that are required to process the data safely and
correctly.