CVE-2026-67298: FreeRDP 3.28.0 Heap Buffer Overflow via RAIL orderLength Underflow

FreeRDP versions 3.28.0 and earlier contain a heap buffer overflow in the server-side RAIL channel handler (railserverhandlemessages() in channels/rail/server/railmain.c). When processing a RAIL PDU header, the code subtracts RAILPDUHEADERLENGTH from the peer-controlled orderLength field without first verifying orderLength is at least the header length. For orderLength values 0..3 this causes an unsigned integer underflow to a very large size, which bypasses the StreamEnsureRemainingCapacity() capacity check (due to pointer arithmetic wraparound) and is then passed to WTSVirtualChannelRead(), resulting in an out-of-bounds heap write. A malicious or compromised RDP client can exploit this to corrupt the heap and crash the server. Fixed in FreeRDP 3.29.0.