Heap-based buffer overflow in the IMA_SECURE_DecryptData1 function in ImaSystem.dll for Citrix MetaFrame XP 1.0 and 2.0, and Presentation Server 3.0 and 4.0, allows remote attackers to execute arbitrary code via requests to the Independent Management Architecture (IMA) service (ImaSrv.exe) with invalid size values that trigger the overflow during decryption.