The RuntimeHelpers.InitializeArray method in metadata/icall.c in Mono when Moonlight 2.x before 2.4.1 or 3.x before 3.99.3 is used does not properly restrict data types which allows remote attackers to modify internal read-only data structures and cause a denial of service (plugin crash) or corrupt the internal state of the security manager via a crafted media file as demonstrated by modifying a C# struct.