Due to the shared nature of the wireless channel, a fundamental class of security attacks, the so-called jamming attack, threatens the availability of wireless networks. A jammer can launch an attack by simply disregarding medium access policies and thereby interfering with legitimate transmissions on the wireless channel. This causes packets to be corrupted and hence leads to degradation in the network performance. To address this problem, this paper proposes a flexible application for both detecting and mitigating a constant jammer attack using the help of cross-layer design. We evaluate our application with real-world experiments in a mobile scenario conducted on a military testing ground at the WTD 81 in Greding, Bavaria, Germany and demonstrate that it is able to detect the presence of a constant jammer with sufficient reliability.