A shock absorber comprising: a tubular housing having a front end and a rear end; a bearing assembly sealingly mounted on the front end of the tubular housing; a piston assembly comprising a piston mounted for reciprocal sliding movement in the tubular housing, said piston defining within the tubular housing a working chamber and an accumulator chamber disposed on the rear side and on the front side of the piston, respectively, and a stem connected to the piston and extending out of the front end of the tubular housing through the bearing assembly; a first fluid pathway (34p, 42, 34g; 34p, 142, 34g; 146d) and a second fluid pathway (46, T, 34f; 46, T, 34f, 34g) parallel disposed for connecting the working chamber to the accumulator chamber, wherein said first fluid pathway establishes permanent fluid communication between the working chamber and the accumulator chamber; and valve means (44, 34q; 144, 34q) for selectively establishing fluid communication between the working chamber and the accumulator chamber; wherein said valve means comprise a ring-shaped obturating element (44; 144) arranged around the piston and slidable along an axial length of the piston; and said second fluid pathway comprises a first pathway section (34f; 34f, 34g) formed through said piston and a second pathway section (46, T) formed between the piston and the obturating element (44; 144) so that said second fluid pathway is able to selectively assume an open configuration, in which said second fluid pathway establishes a fluid communication between the working chamber and the accumulator chamber, and a closed configuration, in which said second fluid pathway is blocked, said second fluid pathway having in the open configuration a lower fluid resistance than the first fluid pathway, said shock absorber being characterised in that said obturating element is at least partially of sufficiently deformable material so that, when the second fluid pathway is in closed configuration and pressure in the working chamber is greater than a predetermined level p 0, said obturating element is deformed against the wall of said tubular housing so as to open said first pathway section of the second fluid pathway. a tubular housing having a front end and a rear end; a bearing assembly sealingly mounted on the front end of the tubular housing; a piston assembly comprising a piston mounted for reciprocal sliding movement in the tubular housing, said piston defining within the tubular housing a working chamber and an accumulator chamber disposed on the rear side and on the front side of the piston, respectively, and a stem connected to the piston and extending out of the front end of the tubular housing through the bearing assembly; a first fluid pathway (34p, 42, 34g; 34p, 142, 34g; 146d) and a second fluid pathway (46, T, 34f; 46, T, 34f, 34g) parallel disposed for connecting the working chamber to the accumulator chamber, wherein said first fluid pathway establishes permanent fluid communication between the working chamber and the accumulator chamber; and valve means (44, 34q; 144, 34q) for selectively establishing fluid communication between the working chamber and the accumulator chamber; wherein said valve means comprise a ring-shaped obturating element (44; 144) arranged around the piston and slidable along an axial length of the piston; and said second fluid pathway comprises a first pathway section (34f; 34f, 34g) formed through said piston and a second pathway section (46, T) formed between the piston and the obturating element (44; 144) so that said second fluid pathway is able to selectively assume an open configuration, in which said second fluid pathway establishes a fluid communication between the working chamber and the accumulator chamber, and a closed configuration, in which said second fluid pathway is blocked, said second fluid pathway having in the open configuration a lower fluid resistance than the first fluid pathway, said shock absorber being characterised in that said obturating element is at least partially of sufficiently deformable material so that, when the second fluid pathway is in closed configuration and pressure in the working chamber is greater than a predetermined level p 0, said obturating element is deformed against the wall of said tubular housing so as to open said first pathway section of the second fluid pathway.