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

Fix: http4k/http4k@0121b05

GHSA-jrpc-7vxp-69p6 is a CWE-444 vulnerability in org.http4k:http4k-core. A fix is available for org.http4k:http4k-core — see the affected versions and patch details below.

Also known asCVE-2026-100724
Published
Updated
Affected
3 pkgs
Patched
3 / 3
Exploits
None indexed
Exploitation data as of Oct 22, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-jrpc-7vxp-69p6.

EPSS Exploitation Probability

via FIRST.org ↗
0.3%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs27th percentile — riskier than 27% of all scored CVEsHighest risk
0.00%0.28%0.56%0.84%0.3%0.3%Oct 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

3 pkgs affected
☕org.http4k:http4k-core☕org.http4k:http4k-core☕org.http4k:http4k-core

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

Description

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

LineFixed inEdition
v6.x (Community)6.49.0.0Community
v5.x (LTS)5.42.0.0Enterprise — contact [email protected] (if reverseProxy() is present in your v5.x line)
v4.x (LTS)4.51.0.0Enterprise — 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.

Affected Packages

3 total 3 fixed
EcosystemPackageVulnerable rangeFix
☕Mavenorg.http4k:http4k-core≥ 6.0.0.0&&< 6.49.0.06.49.0.0org.http4k:http4k-core:6.49.0.0
☕Mavenorg.http4k:http4k-core≥ 5.0.0.0&&< 5.42.0.05.42.0.0org.http4k:http4k-core:5.42.0.0
☕Mavenorg.http4k:http4k-coreall versions4.51.0.0org.http4k:http4k-core:4.51.0.0

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4k:http4k-core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update org.http4k:http4k-core to 6.49.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-jrpc-7vxp-69p6 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### 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 func
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