1. A method of communicating between secured computer systems comprising: supplementing a prepared task file with specified information in a source computer system, transmitting the supplemented task file from the source computer system to a task computer system, transmitting the supplemental task file from the task computer system at least to a second task computer system, transmitting the supplemental task file from the second task computer system to a second source computer system different from the first source computer system, further supplementing the task file within further information in the second source computer system, and transmitting the further supplemented task file from the second source computer system back to the second task computer system, transmitting the further supplemented task file from the second task computer system to a target computer system, verifying validity of the further supplemented task file by the target computer system, and executing at least one task in the target computer system on the basis of the further supplemented task file in the event that verification of the validity of the further supplemented task file was successful, wherein both the source computer system and the target computer system at least temporarily keep specified network ports closed such that addressability of the source computer system or of the target computer system, respectively, via a network for storing or fetching the task file is prevented, but the task computer system and the second task computer system are respectively accessible with at least one listening open network port via network such that the source computer system or the target computer system can access the task computer system or the second task computer system to store the task file in the task computer system or the second task computer system or fetch it from the task computer system or the second task computer system, and transmitting the further supplemented task file from the second task computer system to the target computer system comprises: sending a specified sequence of package data from the second task computer system or the second source computer system to the target computer system, wherein the specified network ports of the target computer system are closed, and the sequence addresses one or multiple network ports of the target computer system in a specified order, verifying the sent sequence for conformance with a predefined sequence in the target computer system, and initiating transfer of the further supplemented task file by the target computer system when the verification of the sent sequence has been positive, wherein the target computer system establishes a connection to the second task computer system and fetches the further supplemented task file from the second task computer system.