The web is the most wide-spread and de facto distributed platform, with a plethora of valuable applications and services. Building stateful services on the web requires a session mechanism that keeps track of server-side session state, such as authentication data. These sessions are an attractive attacker target, since taking over an authenticated session fully compromises the user’s account. This paper focuses on session fixation, where an attacker forces the user to use the attacker’s session, allowing the attacker to take over the session after authentication.We present Serene, a self-reliant client-side countermeasure that protects the user from session fixation attacks, regardless of the security provisions – or lack thereof – of a web application. By specifically protecting session identifiers from fixation and not interfering with other cookies or parameters, Serene is able to autonomously protect a large majority of web applications, without being disruptive towards legitimate functionality. We experimentally validate these claims with a large scale study of Alexa’s top one million sites, illustrating both Serene’s large coverage (83.43%) and compatibility (95.55%).