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CVE-2026-100724· NVD / CVE Program· CNA VulnCheck

http4k before 6.49.0.0 Host Header Routing Bypass via reverseProxy

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.

MediumCVSS 6.3 · v4.0—No exploit Fix available
Published
Sep 27, 2026
Updated
Sep 30, 2026
EPSS
0.3% · 25th percentile
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Report tools

JSON

Action score

25

Monitor

Low priority for now.

CVSS
25 / 40 · 6.3 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.3%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · VulnCheck
6.3
NVD has not scored this yet; the score shown is the CNA’s.

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Noroxi analysis

No Noroxi analysis for this record yet

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Affected systems

—

Versions reported by the vendor

Affected version ranges reported by the assigning authority (VulnCheck). Independent of NVD's CPE analysis and usually ahead of it.

  • http4k http4k

    • 6.49.0.0, 5.42.0.0, 4.51.0.0not affected
    • before 6.49.0.0affected
    • before 5.42.0.0affected
    • before 4.51.0.0affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Mavenorg.http4k:http4k-corefrom 5.0.0.0 · before 5.42.0.05.42.0.0
Mavenorg.http4k:http4k-corefrom 6.0.0.0 · before 6.49.0.06.49.0.0
Mavenorg.http4k:http4k-corebefore 4.51.0.04.51.0.0

Same product

http4k: all records

Other highest-scoring records for the same primary product.

  • CVE-2024-55875http4k has a potential XXE (XML External Entity Injection) vulnerability
    40Plan
  • CVE-2026-100725http4k before 6.48.0.0 Cookie Scoping Bypass via BasicCookieStorage
    33Monitor
  • CVE-2026-100834http4k before 6.48.0.0 Digest Authentication Replay Protection Bypass
    32Monitor
  • CVE-2026-54148http4k: `DigestAuthProvider.verify` did not bind to request URI
    32Monitor
  • CVE-2026-53659http4k: Unbounded gzip decompression in `ServerFilters.GZip` / `RequestFilters.GunZip` allowed memory-exhaustion DoS
    30Monitor
  • CVE-2026-54147http4k: `DigestAuthProvider.verify` ignored configured algorithm and did not bind to request URI
    26Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
Maven:org.http4k:http4k-core4.51.0.0Package registry (OSV)
Maven:org.http4k:http4k-core5.42.0.0Package registry (OSV)
Maven:org.http4k:http4k-core6.49.0.0Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
Integrity
Availability
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
X

Weakness class (CWE)

CWE-444 · Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

CVSS vector

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

ATT&CK techniques

—

Change log

  1. SSVC exploitation— → none
  2. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records