CVE-2026-55430: Coder's subdomain workspace app routing trusts unauthenticated X-Forwarded-Host header, enabling cross-app data access

The workspace app proxy resolves the target app from httpapi.RequestHost() which prefers the X-Forwarded-Host header over the real Host header. No middleware strips X-Forwarded-Host before routing and the header is not browser-forbidden so client-side JavaScript can set it on fetch() calls.

Note: Practical exploitation requires subdomain app routing (wildcard hostname) enabled, a victim who visits the attacker’s shared app and a deployment whose upstream proxy does not strip X-Forwarded-Host.