Modern mobile devices come with first class web browsers that rival their desktop counterparts in power and popularity. However, recent publications point out that mobile browsers are particularly susceptible to attacks on web authentication, such as phishing or clickjacking. We analyze those attacks and find that existing countermeasures from desktop computers can not be easily transfered to the mobile world. The attacks’ root cause is a missing trusted UI for security critical requests. Based on this result, we provide our approach, the MobileAuthenticator, that establishes a trusted path to the web application and reliably prohibits the described attacks. With this approach, the user only needs one tool to protect any number of mobile web application accounts. Based on the implementation as an app for iOS and Android respectively, we evaluate the approach and show that the underlying interaction scheme easily integrates into legacy web applications.