The vulnerability is due to a misconfigured subdomain, security researchers from safety detective and Microsoft bug hunter Sahad Nk identified the bug and reported to Micros oft in June and they are addressed with the November updates
Subdomain Takeover Vulnerability
The subdomains takeover is the process of registering a non-existing domain and gaining control over the domain.
Researchers found that the domain success.office.com was pointing to Azure Web App service through Cname, but the host service is down. So they registered a new Azure web-app in name successcenter-msprod and take-over the subdomain(success.office.com).
Improper OAuth Checks
The Microsoft Outlook, Store, and Sway uses success.office.com as a valid “wreply” for receiving authentication token for successful login with a centralized login system(login.live.com). All the
If the victim clicks on the crafted link it will log the user into the Microsoft system and creates the access tokens. These vulnerable app uses subdomains of *.office.com wildcard and all the subdomains are treated as trusted.
Sahad Nk crafted a malicious URL that passes the access tokens to the subdomain controlled by him if it is the server controlled by an attacker it puts millions of user under risk.
By having the session token attackers can gain access to the victim accounts without having the login credentials.