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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.3 · 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 · score source

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

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…

Published by the source

19 Jun 2026, 21:15 UTC

Last modified by the source

24 Sep 2026, 14:57 UTC

Source last checked

26 Oct 2026, 16:55 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-jrpc-7vxp-69p6

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# Patch Next advisory brief: GHSA-jrpc-7vxp-69p6

Advisory: http4k: `reverseProxy()` defaulted to substring (`Contains`) matching on `Host`; tightened to `Exact`
IDs: GHSA-jrpc-7vxp-69p6
Record: https://patchnext.com/advisories/ghsa-jrpc-7vxp-69p6 (revision 1, source checked 26 Oct 2026, 16:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: org.http4k:http4k-core 6.49.0.0, 5.42.0.0, 4.51.0.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven org.http4k:http4k-core: affected >= 5.0.0.0, < 5.42.0.0; < 4.51.0.0; >= 6.0.0.0, < 6.49.0.0; fixed in 5.42.0.0, 4.51.0.0, 6.49.0.0

## Suggested inspection (read-only)
- Maven org.http4k:http4k-core: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on org.http4k:http4k-core.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-jrpc-7vxp-69p6 (GitHub-reviewed advisory GHSA-jrpc-7vxp-69p6 lists affected ranges: Maven org.http4k:http4k-core >=5.0.0.0 =0 =6.0.0.0 <6.49.0.0)

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