Versions of the package hackney before 1.21.0 are vulnerable to Server-side Request Forgery (SSRF) due to improper parsing of URLs by URI built-in module and hackey. Given the URL http://127.0.0.1?@127.2.2.2/, the URI function will parse and see the host as 127.0.0.1 (which is correct), and hackney will refer the host as 127.2.2.2/. This vulnerability can be exploited when users rely on the URL function for host checking.
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.