We identify logical web application flaws which can be exploited by TLS truncation attacks to desynchronize the user- and server-perspective of an application's state. It follows immediately that servers may make false assumptions about users, hence, the flaw constitutes a security vulnerability. Moreover, in the context of authentication systems, we exploit the vulnerability to launch the following practical attacks: we exploit the Helios electronic voting system to cast votes on behalf of honest voters, take full control of Microsoft Live accounts, and gain temporary access to Google accounts.