GHSA-jrpc-7vxp-69p6: http4k `reverseProxy()` Defaulted To Substring (`Contains`) Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-jrpc-7vxp-69p6",
  "modified" : "2026-09-24T14:57:44Z",
  "published" : "2026-06-19T21:15:59Z",
  "aliases" : [ ],
  "summary" : "http4k: `reverseProxy()` defaulted to substring (`Contains`) matching on `Host`; tightened to `Exact`",
  "details" : "### Impact\n\n`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.\n\nIf, 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.\n\nThe `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.\n\n**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.\n\n### Patches\n\n| Line | Fixed in | Edition |\n|------|----------|---------|\n| v6.x (Community) | **6.49.0.0** | Community |\n| v5.x (LTS) | **5.42.0.0** | Enterprise — contact [enterprise@http4k.org](mailto:enterprise@http4k.org) (if `reverseProxy()` is present in your v5.x line) |\n| v4.x (LTS) | **4.51.0.0** | Enterprise — contact [enterprise@http4k.org](mailto:enterprise@http4k.org) (if `reverseProxy()` is present in your v4.x line) |\n\nThe fix changes the default matcher to `Exact`. Existing callers that genuinely need substring matching (e.g. AWS subdomain dispatch) must explicitly pass `matcher = Contains`.\n\n### Workarounds\n\nFor 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.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "org.http4k:http4k-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "6.0.0.0"
      }, {
        "fixed" : "6.49.0.0"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "org.http4k:http4k-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "5.0.0.0"
      }, {
        "fixed" : "5.42.0.0"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "org.http4k:http4k-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.51.0.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/http4k/http4k/security/advisories/GHSA-jrpc-7vxp-69p6"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/http4k/http4k/commit/0121b05537"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/http4k/http4k/commit/54c6385615"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/http4k/http4k"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/http4k/http4k/releases/tag/6.49.0.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-444" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-19T21:15:59Z",
    "nvd_published_at" : null
  }
}