http4k HTTP reverseProxy/reverseProxyRouting Host improper authorization

Summaryinfo

A vulnerability was found in http4k. It has been classified as critical. This vulnerability affects the function reverseProxy/reverseProxyRouting of the component HTTP Handler. The manipulation of the argument Host leads to improper authorization. This vulnerability is referenced as CVE-2026-100724. Remote exploitation of the attack is possible. No exploit is available.

Detailsinfo

A vulnerability was found in http4k (unknown version) and classified as critical. This issue affects the function reverseProxy/reverseProxyRouting of the component HTTP Handler. The manipulation of the argument Host with an unknown input leads to a improper authorization vulnerability. Using CWE to declare the problem leads to CWE-285. The product does not perform or incorrectly performs an authorization check when an actor attempts to access a resource or perform an action. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-100724 since 09/26/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 09/27/2026). MITRE ATT&CK project uses the attack technique T1548.002 for this issue.

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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Name

• http4k

Website

• Product: https://github.com/http4k/http4k/

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