The sensitivity attack is considered as a serious threat to the security of spread-spectrum-based schemes, since it provides a practical method of removing watermarks with minimum attacking distortion. This paper is intended as a tutorial on this problem, presenting an overview of previous research and introducing a new method based on a general formulation. This new method does not require any knowledge about the detection function nor any other system parameter, but just the binary output of the detector, being suitable for attacking most known watermarking methods. Finally, the soundness of this new approach is tested by attacking several of those methods.