Device for protecting a motor vehicle against unauthorized opening of a vehicle door in which a locking device is integrated, in which device a locking cylinder is arranged in a system housing, said locking cylinder consisting of a cylinder core which is mounted rotatably in a cylinder liner and has spring-loaded tumblers which are arranged in a radially displaceable manner and, upon locking by the locking device, engage in corresponding channels of the cylinder liner, wherein the rotational movement of the cylinder core about the axis of the locking cylinder is transmitted via an elongate connecting part, which is indirectly or directly operatively connected to the cylinder core, to the locking mechanism of the locking device such that the locking cylinder, and therefore the cylinder core with its elongate connecting part which is operatively connected to the locking mechanism, is substantially unchangeable in its axial position in the event of the action, in particular of a large force suddenly acting on the locking cylinder from the outside, because of a mechanical lock preventing an inwardly directed axial displacement of the locking cylinder, wherein the mechanical lock is provided in the region of the connection of the cylinder core of the locking cylinder to the elongate connecting part, and the elongate component is connected to the cylinder core via a coupling, characterized in that - the coupling includes a housing which receives an insert of the coupling, which insert is provided with a thread and into which the elongate component is screwed, - wherein, in order to form the mechanical lock, the system housing has an axially extending moulding which substantially engages over the region of the connection of the cylinder core of the locking cylinder to the elongate connecting part and, at its free end directed towards the cylinder core, has a constriction which runs in the direction of the axis of the locking cylinder, forms a first stop and by means of which the locking cylinder is restricted in its axial position, - wherein the moulding is formed in the direction of the elongate connecting part, - and therefore, in order to restrict the axial position of the locking cylinder, the constriction, which forms the first stop, of the axially extending moulding of the system housing is operatively connected to a second stop which is connected to the housing of the coupling and which is arranged in the manner of a flange on the outer circumference of the housing of the coupling via which the elongate connecting part is connected to the cylinder core of the locking cylinder. - the coupling includes a housing which receives an insert of the coupling, which insert is provided with a thread and into which the elongate component is screwed, - wherein, in order to form the mechanical lock, the system housing has an axially extending moulding which substantially engages over the region of the connection of the cylinder core of the locking cylinder to the elongate connecting part and, at its free end directed towards the cylinder core, has a constriction which runs in the direction of the axis of the locking cylinder, forms a first stop and by means of which the locking cylinder is restricted in its axial position, - wherein the moulding is formed in the direction of the elongate connecting part, - and therefore, in order to restrict the axial position of the locking cylinder, the constriction, which forms the first stop, of the axially extending moulding of the system housing is operatively connected to a second stop which is connected to the housing of the coupling and which is arranged in the manner of a flange on the outer circumference of the housing of the coupling via which the elongate connecting part is connected to the cylinder core of the locking cylinder.