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Louis Jannett, Vladislav Mladenov, Christian Mainka, Jörg Schwenk

• Single Sign-On (SSO) protocols like OAuth 2.0 and OpenID Connect 1.0 are cornerstones of modern web security, and have received much academic attention. Users sign in at a trusted Identity Provider (IdP) that subsequently allows many Service Providers (SPs) to verify the users' identities. Previous research concentrated on the standardized - called textbook SSO in this paper - authentication flows, which rely on HTTP redirects to transfer identity tokens between the SP and IdP. However, modern web applications like single page apps may not be able to execute the textbook flow because they lose the local state in case of HTTP redirects. By using novel browser technologies, such as postMessage, developers designed and implemented SSO protocols that were neither documented nor analyzed thoroughly. We call them dual-window SSO flows. In this paper, we provide the first comprehensive evaluation of dual-window SSO flows. In particular, we focus on the In-Browser Communication (InBC) used to exchange authentication tokens between SPs and IdPs in iframes and popups. We automate our analysis by developing Distinct - a tool that dynamically analyzes the JavaScript code executing as part of the SSO flow. Distinct translates the flow into a sequence diagram depicting all communicating entities and their exchanged messages, highlights insecure communication channels, and quantifies novel threats in dual-window SSO flows. We found that 56% of the SPs in the Tranco top 1k list support dual-window SSO. Surprisingly, 28% of the SPs implemented dual-window SSO without using official SDKs, leading to identity theft and XSS in 31% of these self-implemented SPs.