NetX Duo's WebSocket client resets the unmasking cursor to the first "NX_PACKET" each time it advances through a chained packet, while the loop's upper bound belongs to the current packet. With the standard contiguous packet-pool layout, a masked server frame split across two packets therefore drives the XOR loop through the first packet's unused payload area and on through the second packet's "NX_PACKET" control block. The four-byte WebSocket masking key controls the bytes written, so the corruption is attacker-chosen rather than incidental.