GHSA-jrpc-7vxp-69p6 http4k: `reverseProxy()` defaulted to substring (`Contains`) matching on `Host`; tightened to `Exact`

• Maven org.http4k:http4k-core: >= 6.0.0.0, < 6.49.0.0 → fixed in 6.49.0.0 • Maven org.http4k:http4k-core: >= 5.0.0.0, < 5.42.0.0 → fixed in 5.42.0.0 • Maven org.http4k:http4k-core: < 4.51.0.0 → fixed in 4.51.0.0

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 | Line | Fixed in | Edition | |------|----------|---------| | v6.x (Community) | 6.49.0.0 | Community | | v5.x (LTS) | 5.42.0.0 | Enterprise — contact [email protected] (if `reverseProxy()` is present in your v5.x line) | | v4.x (LTS) | 4.51.0.0 | Enterprise — contact [email protected] (if `reverseProxy()` is present in your v4.x line) | 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.