How the BEAST attack works: Reading encrypted data without decryption

BEAST, or Browser Exploit Against SSL/TLS, was a man-in-the-middle attack that could expose information from an encrypted SSL/TLS 1.0 session. The attack exploited a known cipher suite vulnerability that was considered low-risk until a proof of concept arrived, prompting browser vendors and web server administrators to quickly move to TLS v1.1. This article shows how the BEAST attack worked, how a theoretical vulnerability became practically exploitable, and why modern browsers are no longer vulnerable.