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.