Bug 2542027 (CVE-2026-100724)

Summary:	CVE-2026-100724 org.http4k/http4k-core: http4k: Routing authorization bypass via loose Host header matching
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security DevOps Team <prodsec-dev>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	CC:	gmalinko, janstey, pdelbell, rstepani
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
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Doc Text:	A flaw was found in http4k. When reverse proxy routing is configured with multiple virtual hosts, incoming HTTP Host headers are evaluated using loose substring matching instead of exact matching. A remote attacker can exploit this vulnerability by submitting a crafted Host header that contains the name of an internal or privileged host, allowing them to bypass routing-based authorization controls and access restricted endpoints.	Story Points:	---
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oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description OSIDB Bzimport 2026-09-27 01:53:49 UTC

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.