A connection assembly for directing a medical fluid, comprising - a connection piece through which a medical fluid can be delivered, - an attachment part which can be attached to the connection piece and which can be connected to a delivery device for delivering a medical fluid through the connection piece, and - a sealing element for sealing a transition between the connection piece (11, 12) and the attachment part in such manner that when the attachment part is attached to the connection piece, the sealing element is held between the attachment part and the connection piece, wherein the sealing element has a sealing head which, when the attachment part is attached to the connection piece, closes an opening of the attachment part against the passage of fluid, and a cylindrical body adjoining the sealing head which can be arranged in the opening at a radial distance from the attachment part, such that there is a space around the body within the opening of the attachment part, into which space the sealing element can deform by means of a radial compensating movement, when a delivery device is attached to the attachment part, and wherein an interior space of the sealing element is free of a body having a point which is intended to support the opening of the sealing element, wherein, the sealing element has - at the transition between the sealing head and the body, a shoulder which projects radially to the insertion direction over the sealing head and which can be brought into contact with an associated contact surface within the opening of the attachment part along the insertion direction, - a flange which is spaced apart from the shoulder along the insertion direction and projects radially outwards relative to the body of the sealing element, and which is held between the attachment part and the connection piece when the attachment part is attached to the connection piece in such manner that the flange rests with its underside on a support surface of the connection piece and its upper side is exposed at least in sections, and - a foot section which, when viewed axially along the insertion direction, adjoins the body below the flange and, when the attachment part is attached to the connection piece, engages in an associated recess in the connection piece. - a connection piece through which a medical fluid can be delivered, - an attachment part which can be attached to the connection piece and which can be connected to a delivery device for delivering a medical fluid through the connection piece, and - a sealing element for sealing a transition between the connection piece (11, 12) and the attachment part in such manner that when the attachment part is attached to the connection piece, the sealing element is held between the attachment part and the connection piece, wherein the sealing element has a sealing head which, when the attachment part is attached to the connection piece, closes an opening of the attachment part against the passage of fluid, and a cylindrical body adjoining the sealing head which can be arranged in the opening at a radial distance from the attachment part, such that there is a space around the body within the opening of the attachment part, into which space the sealing element can deform by means of a radial compensating movement, when a delivery device is attached to the attachment part, and wherein an interior space of the sealing element is free of a body having a point which is intended to support the opening of the sealing element, - at the transition between the sealing head and the body, a shoulder which projects radially to the insertion direction over the sealing head and which can be brought into contact with an associated contact surface within the opening of the attachment part along the insertion direction, - a flange which is spaced apart from the shoulder along the insertion direction and projects radially outwards relative to the body of the sealing element, and which is held between the attachment part and the connection piece when the attachment part is attached to the connection piece in such manner that the flange rests with its underside on a support surface of the connection piece and its upper side is exposed at least in sections, and - a foot section which, when viewed axially along the insertion direction, adjoins the body below the flange and, when the attachment part is attached to the connection piece, engages in an associated recess in the connection piece.