readthedocs before 8.8.1 is vulnerable to Cross-Site Scripting. This vulnerability allowed a malicious user to serve arbitrary HTML files from the main application domain (readthedocs[.]org/readthedocs[.]com) by exploiting a vulnerability in the code that serves downloadable content from a project. Exploiting this would have required the attacker to get a logged-in user to visit the malicious URL, which would have allowed the attacker to take control of the user's session with JavaScript (making requests to the API/site on behalf of the user).