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.