OAuth users are advised to scrutinize their sign-in flows for third-party scripts, including error flows, due to potential vulnerabilities that could lead to single-click account takeovers, as detailed in recent research by Frans Rosén from Detectify. Rosén discovered that many popular websites are not adhering to OAuth specification best practices, making them susceptible to complex attack chains involving response-type switching, invalid state, redirect-uri quirks, and third-party JavaScript inclusions. His research identified three main vulnerabilities: weak postMessage-listeners, cross-site scripting on sandbox domains, and out-of-bounds API URL fetching. OAuth, a widely-used open specification for secure access delegation via third-party credentials, has faced numerous security challenges over the past decade, yet remains integral to both consumer and enterprise web applications. With the increasing adoption of OAuth, these vulnerabilities present significant risks for targeted attacks, emphasizing the need for organizations to ensure their OAuth flows are free from third-party scripts. Rosén has responsibly disclosed his findings to affected parties and provided insights to the IETF's working group on OAuth 2.0 security best practices.