An unauthenticated server-side request forgery (SSRF) vulnerability in the Thumbnail via-uri endpoint of Halo CMS 2.21 allows a remote attacker to cause the server to issue HTTP requests to attacker-controlled URLs, including internal addresses. The endpoint performs a server-side GET to a user-supplied URI without adequate allow/blocklist validation and returns a 307 redirect that can disclose internal URLs in the Location header.
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.