SillyTavern SSO Header Spoofing Authentication Bypass

Unvalidated Remote-User/X-Authentik-Username headers let any network client impersonate any user, including admins, when Authelia/Authentik SSO is enabled

SillyTavern, a self-hosted web UI for local LLM, image-generation, and TTS models, contains a critical authentication bypass in versions prior to 1.18.0. When an admin enables SSO integration with Authelia or Authentik (sso.autheliaAuth/sso.authentikAuth), the app blindly trusts the Remote-User or X-Authentik-Username HTTP headers to auto-login users, with no check that a real reverse proxy set them. Any client that can reach the SillyTavern port directly can forge these headers and log in as any user, including administrators, without a password — full account and data takeover. Fixed in 1.18.0, which adds an IP allowlist for SSO header trust.