A flaw was found in libnbd. A client did not always correctly verify the NBD server's certificate when using TLS to connect to an NBD server. This would allow a man-in-the-middle attack on NBD traffic. After investigation, several separate related problems were found.
Reference: https://lists.libguestfs.org/archives/list/guestfs@lists.libguestfs.org/message/LHR3BW6RJ7K4BJBQIYV3GTZLSY27VZO2