A multifunction valve for a high-pressure hydrogen supply system of a fuel cell vehicle system, comprising a valve body and a functional device (10,1010), wherein: - the valve body has at least one inner compartment (4,104) having a sealing section having a lateral surface; and - the functional device (10,1010) comprises: a) a stem (12,1012) at least partially accommodated in the inner compartment (4,104), said stem (12,1012) comprising a guide segment provided with a first portion having a first diameter, a second end portion having a second diameter and exposed to high pressure, wherein the first diameter is greater than the second diameter, and a junction portion which joins the first portion to the second portion, having a frustoconical lateral surface converging towards the second portion and having a predetermined opening angle; b) a sealing assembly (18,1018) comprising: i) a primary O-ring fitted with interference onto the second portion of the guide segment and compressed against the lateral surface of the sealing section so as to obtain a pressurized gas seal; characterized in that the sealing assembly furthermore comprises ii) an anti-extrusion ring, placed by the side of the primary O-ring and in contact therewith, fitted onto the junction portion of the guide segment and compressed against the lateral surface of the sealing section, said anti-extrusion ring consisting of an annular unit body, a frustoconical inner lateral surface in contact with the lateral surface of the junction portion of the guide segment having a predetermined opening angle. - the valve body has at least one inner compartment (4,104) having a sealing section having a lateral surface; and - the functional device (10,1010) comprises: a) a stem (12,1012) at least partially accommodated in the inner compartment (4,104), said stem (12,1012) comprising a guide segment provided with a first portion having a first diameter, a second end portion having a second diameter and exposed to high pressure, wherein the first diameter is greater than the second diameter, and a junction portion which joins the first portion to the second portion, having a frustoconical lateral surface converging towards the second portion and having a predetermined opening angle; b) a sealing assembly (18,1018) comprising: i) a primary O-ring fitted with interference onto the second portion of the guide segment and compressed against the lateral surface of the sealing section so as to obtain a pressurized gas seal; characterized in that the sealing assembly furthermore comprises ii) an anti-extrusion ring, placed by the side of the primary O-ring and in contact therewith, fitted onto the junction portion of the guide segment and compressed against the lateral surface of the sealing section, said anti-extrusion ring consisting of an annular unit body, a frustoconical inner lateral surface in contact with the lateral surface of the junction portion of the guide segment having a predetermined opening angle. a) a stem (12,1012) at least partially accommodated in the inner compartment (4,104), said stem (12,1012) comprising a guide segment provided with a first portion having a first diameter, a second end portion having a second diameter and exposed to high pressure, wherein the first diameter is greater than the second diameter, and a junction portion which joins the first portion to the second portion, having a frustoconical lateral surface converging towards the second portion and having a predetermined opening angle; b) a sealing assembly (18,1018) comprising: i) a primary O-ring fitted with interference onto the second portion of the guide segment and compressed against the lateral surface of the sealing section so as to obtain a pressurized gas seal; characterized in that the sealing assembly furthermore comprises ii) an anti-extrusion ring, placed by the side of the primary O-ring and in contact therewith, fitted onto the junction portion of the guide segment and compressed against the lateral surface of the sealing section, said anti-extrusion ring consisting of an annular unit body, a frustoconical inner lateral surface in contact with the lateral surface of the junction portion of the guide segment having a predetermined opening angle. i) a primary O-ring fitted with interference onto the second portion of the guide segment and compressed against the lateral surface of the sealing section so as to obtain a pressurized gas seal; characterized in that the sealing assembly furthermore comprises ii) an anti-extrusion ring, placed by the side of the primary O-ring and in contact therewith, fitted onto the junction portion of the guide segment and compressed against the lateral surface of the sealing section, said anti-extrusion ring consisting of an annular unit body, a frustoconical inner lateral surface in contact with the lateral surface of the junction portion of the guide segment having a predetermined opening angle.