A hydraulic actuator , particularly of the shock absorbing and/or damping type, comprising a piston that is accommodated so that it can slide hermetically in a hollow cylinder so as to divide the internal volume of said hollow cylinder into two chambers mutually separated by the head of said piston , said two chambers being connectable separately, respectively by means of at least one delivery duct and at least one discharge duct , to a first circuit adapted to feed under pressure a first fluid into one of said two chambers with consequent emptying of the other of said two chambers for the extension or compression movement of said piston with respect to said hollow cylinder , wherein the hydraulic actuator comprises a first movable element that is accommodated so that it can slide hermetically in a longitudinal cavity defined inside a stem of said piston so as to divide said longitudinal cavity into two portions with the first of said two portions connected to one of said two chambers and with the second of said two portions connectable to a second circuit adapted to feed under pressure a second fluid into said second portion , said second fluid having a coefficient of compressibility and a nominal pressure greater than those of said first fluid so as to act as a shock absorber and/or damper in case of sudden peaks of pressure in said chamber connected to said first portion of said longitudinal cavity defined inside said stem of said piston , characterized in that the hydraulic actuator further comprises at least one safety valve of the double-acting type with limited control on the delivery and a maximum-pressure valve on the return, which is connected hydraulically to said at least one delivery duct and to said at least one discharge duct , so as to be able to vary the shock absorbing and/or damping action of said hydraulic actuator wherein said at least one safety valve comprises a valve body that defines a first feed or discharge duct provided with a first normally-closed one-way valve and with a first calibrated port that is parallel to said first one-way valve and a second feed or discharge duct , which are connected to a main chamber defined inside said valve body , said first feed or discharge duct being connected to said delivery duct by means of a second normally-closed one-way valve which is accommodated inside said main chamber and allows a flow from said first feed or discharge duct toward said delivery duct , said second feed or discharge duct being connected to said discharge duct by means of a third normally-closed one-way valve and a fourth normally-closed one-way valve , which are accommodated inside said main chamber in sequence to each other and allow a flow from said second feed or discharge duct toward said discharge duct , wherein between said first feed or discharge duct and said second feed or discharge duct there is provided a pilot slider which is accomodated between said second one-way valve and of said third one-way valve so that it can slide hermetically in said main chamber , wherein between said third one-way valve and said fourth one-way valve there is provided a second calibrated port and also a maximum-pressure valve accommodated in said valve body and wherein the maximum-pressure valve is connected to said third one-way valve and to said fourth one-way valve so as to bypass said second calibrated port . <EOS>