libcurl's TFTP state machine fails to validate state transitions against the requested transfer direction, accepting opening server responses for both reads and writes interchangeably. A malicious TFTP server can respond to a download request in a way that forces the client into transmit mode, prompting libcurl to consume and transmit data from the handle's persistent "CURLOPT_READDATA" pointer - including data remaining from unrelated prior operations, such as an HTTP POST request executed on a reused handle.