We describe a new class of tools for protecting computer systems from security attacks. Their distinguished feature is the principle they are based on. Host or network protection is not achieved by strengthening their defenses but by weakening the enemy's offensive capabilities. A prototype tool has been implemented that demonstrates that such an approach is feasible and effective. We show that some of the most popular DoS attacks are effectively blocked with limited impact on the sender's performance. Measurements of the implemented prototype show that controlling the outgoing traffic does not affect performance at the sender machine, when traffic is not hostile. If traffic is hostile, the limited slow down experienced at the source is the price to pay to make the Internet a safer place for all its users. The limited performance impact and the efficacy in attack prevention make tools like the one presented a new component of security architectures. Furthermore, such a type of tools represents an effective way to address security problems that are still unsolved or for which only partial solutions are available, such as the liability problem, intranet security, security tools performance and the use of distributed tools for intrusion.