The OIDC login route silently fell back to building the redirect URI from
X-Forwarded-Host/X-Forwarded-Proto when APP_URL was not configured. An
attacker could set X-Forwarded-Host: attacker.example.com to redirect the
authorization code to their own server after the user authenticates.
Remove the header-derived fallback entirely. If APP_URL is not set (via env
or the app_url DB setting), the OIDC login endpoint now returns a 500 error
rather than trusting attacker-controlled request headers. Document APP_URL
in .env.example as required for OIDC use.
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