We present application-based TCP hijacking (ABTH), a new attack on TCP applications that exploits flaws due to the interplay between TCP and application protocols to inject data into an application session without either server or client applications noticing the spoofing attack. Following the injection of a TCP packet, ABTH resynchronizes the TCP stacks of both the server and the client. To evaluate the feasibility and effectiveness of ABTH, we developed a tool that allows impersonating users of Windows Live Messenger in the matter of few seconds. Due to its generic nature, ABTH can be mounted on a variety of modern protocols for TCP-based applications. Countermeasures to thwart and/or limit the effectiveness of ABTH could include strict Ethernet switching and cryptographic protection of messages. However, the former cannot be guaranteed by the application provider and the latter appears to be still prohibitively expensive for such large-scale applications with hundreds of millions of sporadic users as Windows Live Messenger.