A cylinder lock having a first cylinder half, which in use, is mounted on the indoor side and a second cylinder half, which in use, is mounted on the outdoor side of a door, is disclosed. The cylinder lock further comprises a notch cut out on the outdoor end of the second cylinder half for forming a breakable tip portion on the second cylinder half. A rod extends along the second cylinder half and enters, partly at its first end, in the first cylinder half. The rod is secured, at its second end, to the tip portion of the second cylinder half. The cylinder lock further comprises a volume formed on the first cylinder half for accommodating an alarm circuitry including at least an electronic circuit board, a magnetic switch and a magnet.