linuxserver Heimdall through 2.8.3 applies its SafeUrlFetcher SSRF protection mechanism only to ItemController; the enhanced-application test and live-stats requests occur via SupportedApps::execute(), a GuzzleHttp client that lacks IP address restrictions. In some realistic installations, the POST /testconfig (and GET /getstats) endpoints are accessible through CSRF, and thus an unauthenticated attacker can force the server to send requests to arbitrary internal hosts and ports (including 169.254.169.254) and read a status/port oracle in addition to partial response data.

LinuxServer.io heimdall 2.6.3-ls307 contains a vulnerability in how it handles user-supplied HTTP headers, specifically X-Forwarded-Host and Referer. An unauthenticated remote attacker can manipulate these headers to perform Host Header Injection and Open Redirect attacks. This allows the loading of external resources from attacker-controlled domains and unintended redirection of users, potentially enabling phishing, UI redress, and session theft. The vulnerability exists due to insufficient validation and trust of untrusted input, affecting the integrity and trustworthiness of the application.

LinuxServer.io Heimdall before 2.7.3 allows XSS via the q parameter.

Heimdall Application Dashboard through 2.5.4 allows reflected and stored XSS via "Application name" to the "Add application" page. The stored XSS will be triggered in the "Application list" page.