Over the last few years, a worryingly number of attacks against SSL/TLS and other secure channels have been discovered. Fortunately, at least from a defenders perspective, these attacks require an adversary capable of observing or manipulating network traffic. This prevented a wide and easy exploitation of these vulnerabilities. In contrast, we introduce HEIST, a set of techniques that allows us to carry out attacks against SSL/TLS purely in the browser. More generally, and surprisingly, with HEIST it becomes possible to exploit certain flaws in network protocols without having to sniff actual traffic.