FreshRSS is a self-hosted RSS feed aggregator. Prior to version 1.26.2, when the server is using HTTP auth via reverse proxy, its possible to impersonate any user either via the Remote-User header or the X-WebAuth-User header by making specially crafted requests via the add feed functionality and obtaining the CSRF token via XPath scraping. The attacker has to know the IP address of the proxied FreshRSS instance and the admins username, while also having an account on the instance. An attacker can send specially crafted requests in order to gain unauthorized access to internal services. This can also lead to privilege escalation like in the demonstrated scenario, although users that have setup OIDC are not affected by privilege escalation. Version 1.26.2 contains a patch for the issue.
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.