An open redirect is an URL on a web site that can be used by anyone to redirect users to another site. Google and Adobe open redirects are being used by phishing campaigns in order to add legitimacy to the URLs used in the spam emails. Many companies, including Google, do not consider an open redirect a security vulnerability and thus do nothing about them. Some members of the security community argue that the redirectors aid phishing, because users may be inclined to trust the mouse hover tooltip on a link.
Source: https://www.bleepingcomputer.com/news/security/adobe-and-google-open-redirects-abused-by-phishing-campaigns/