http4k before 6.49.0.0 Host Header Routing Bypass via reverseProxy (CVE-2026-100724) | HOL Guard CVE

Medium · CVSS 6.3CVE-2026-100724

http4k before 6.49.0.0 Host Header Routing Bypass via reverseProxy

Answer in brief

CVE-2026-100724 records a Medium severity (CVSS 6.3) vulnerability in http4k before 6.49.0.0 Host Header Routing Bypass via reverseProxy. The current sources do not mark it as known exploited. The current feed maps http4k/http4k (generic), http4k/http4k (generic), http4k/http4k (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Sep 27, 2026Updated Sep 27, 2026Source checked Sep 27, 2026First seen by HOL Sep 27, 2026Material review Sep 27, 2026

Upstream Advisory

Key facts

Risk: Medium · CVSS 6.3
Exploitation: Not marked as known exploited
Affected software: 3 mapped packages or products
Fix availability: Available

Why this deserves its current priority

CVSS is 6.3. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps http4k/http4k (generic), http4k/http4k (generic), http4k/http4k (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
http4k/http4kgeneric	>=0 <6.49.0.0	6.49.0.0
http4k/http4kgeneric	>=0 <5.42.0.0	5.42.0.0
http4k/http4kgeneric	>=0 <4.51.0.0	4.51.0.0

Recommended response

Check inventory. Check lockfiles and deployed manifests for http4k/http4k, http4k/http4k, http4k/http4k.
Review the reported fix. Update http4k/http4k to 6.49.0.0; http4k/http4k to 5.42.0.0; http4k/http4k to 4.51.0.0 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Sep 27, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Sep 27, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Sep 27, 2026

Sources and claim methodology

Upstream source 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.

Quoted source text, attributed separately from HOL analysis.