Struts support in OpenSymphony XWork before 1.2.3 and 2.x before 2.0.4 as used in WebWork and Apache Struts recursively evaluates all input as an Object-Graph Navigation Language (OGNL) expression when altSyntax is enabled which allows remote attackers to cause a denial of service (infinite loop) or execute arbitrary code via form input beginning with a \%{\ sequence and ending with a \}\ character.