http4k: `reverseProxy()` defaulted to substring (`Contains`) matching on `Host`; tightened to `Exact`

Impact

reverseProxy() and reverseProxyRouting() matched configured vhosts by substring on the Host header (Contains matcher) by default. The intended use of these functions in http4k is outbound dispatch (e.g. matching AWS service subdomains, per the Contains docstring) and test-time composition of fake backend networks. In either of those contexts the matched Host is set by the calling application, not by an external attacker, so the loose match has no exploit surface.

If, however, reverseProxy() was deployed as a public-facing inbound HTTP handler — which the function technically supports but is not the documented intent — an external attacker could send Host: admin.evil.com and reach a vhost configured as admin, bypassing routing-based authorization.

The Contains matcher's docstring explicitly documented this loose behaviour, but because Contains was the default, callers who never read the matcher docs would still get the loose behaviour.

Who is affected: only deployments using reverseProxy() / reverseProxyRouting() as a public-facing inbound HTTP handler with two or more configured virtual hosts. The intended outbound / test-time usage is unaffected. If you did deploy reverseProxy() inbound and rely on multi-vhost routing for authorization, treat upgrade as urgent.

Patches

The fix changes the default matcher to Exact. Existing callers that genuinely need substring matching (e.g. AWS subdomain dispatch) must explicitly pass matcher = Contains.

Workarounds

For deployments that cannot upgrade immediately: wrap your reverseProxy() with a host-allow-list filter that requires an exact match against expected vhost names before delegating.