CVE-2026-100724 http4k before 6.49.0.0 Host Header Routing Bypass via reverseProxy

What is CVE-2026-100724?

CVE-2026-100724 is a medium-severity authentication bypass vulnerability in http4k, published on 27 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

http4k (Maven package org.http4k:http4k-core) before 6.49.0.0, 5.42.0.0 and 4.51.0.0 uses substring (Contains) matching on the Host header by default in reverseProxy() and reverseProxyRouting() when dispatching to configured virtual hosts. If these functions are deployed as a public-facing inbound HTTP handler with two or more configured virtual hosts, a remote attacker can supply a Host header that merely contains a configured vhost name (for example Host: admin.evil.com for a vhost configured as "admin") and be routed to that vhost, bypassing routing-based authorization. The intended outbound-dispatch and test-time uses, where the Host value is set by the calling application, are not affected.

Weakness: CWE-444

Which versions are affected by CVE-2026-100724?

6 open source packages ship an affected version, including org.http4k:http4k-core.

How do I fix CVE-2026-100724?

Upgrade org.http4k:http4k-core to a fixed version for your release line: 4.51.0.0, 5.42.0.0 or 6.49.0.0.

Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.

Is CVE-2026-100724 being exploited?

There is no known exploitation of CVE-2026-100724 in the wild and no public exploit code.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• GitHub advisory
• How this page is made

Also known as GHSA-9hc9-gcfq-xg98, GHSA-jrpc-7vxp-69p6.