CVE-2026-100724: HTTP4K

Medium severity, CVSS 5.4. EPSS: 0.3% chance of exploitation in the next 30 days.

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.

Affected products

• HTTP4K HTTP4K: before 6.49.0.0 (fixed in 6.49.0.0); before 5.42.0.0 (fixed in 5.42.0.0); before 4.51.0.0 (fixed in 4.51.0.0)

Published 2026-09-27. Last modified 2026-09-30.