# http4k auth bypass via loose Host matching in reverseProxy

Source: https://junglewise.ai/threats/http4k-auth-bypass-via-loose-host-matching-in-reverseproxy-bcd046a3  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-19. Updated: 2026-06-19  
License: CC BY 4.0 with a link back

Severity: medium · CVSS 6.3 · Published 2026-06-19

A vulnerability in the http4k toolkit's reverse proxy component could allow unauthorized access to internal services. By default, the system used loose matching for web addresses, meaning an attacker could bypass security rules by using a specially crafted address that merely contains a legitimate name. This could lead to unauthorized access to administrative interfaces or sensitive internal data if the proxy is used to handle public internet traffic.

Technologies: [Http4k-Core](https://junglewise.ai/threats/technologies/http4k-core). Vendors: [Http4k](https://junglewise.ai/threats/vendors/http4k).

## Quick answers

**Is This vulnerability exploited in the wild?** No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.

**How severe is This vulnerability?** Medium severity, with a CVSS score of 6.3, published 19 June 2026.

**Which products and versions are affected?** Http4k-Core < 4.51.0.0, >= 5.0.0.0 < 5.42.0.0, >= 6.0.0.0 < 6.49.0.0.

**Is there a fix for This vulnerability?** A patch was published on 2026-06-19: Patched in versions 6.49.0.0, 5.42.0.0, and 4.51.0.0 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is This vulnerability attacked?** It is attacked over the network, in a software library.

**Am I affected?** Check whether you run Http4k-Core at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The `reverseProxy()` and `reverseProxyRouting()` functions in `http4k-core` defaulted to a `Contains` matcher for the HTTP `Host` header. When deployed as a public-facing inbound handler with multiple virtual hosts, an attacker can provide a `Host` header like `admin.attacker.com` to match a vhost configured as `admin`. This results in an inconsistent interpretation of HTTP requests (CWE-444), allowing for authorization bypass. The vulnerability is exploitable over the network without authentication if the application relies on these functions for inbound routing security. The fix changes the default matcher to `Exact`.

## Affected products

- http4k http4k-core < 4.51.0.0, >= 5.0.0.0 < 5.42.0.0, >= 6.0.0.0 < 6.49.0.0

## Timeline

- 2026-06-16: disclosed
- 2026-06-19: advisory
- 2026-06-19: patched: Patched in versions 6.49.0.0, 5.42.0.0, and 4.51.0.0

## References

- [https://github.com/http4k/http4k/security/advisories/GHSA-jrpc-7vxp-69p6](https://github.com/http4k/http4k/security/advisories/GHSA-jrpc-7vxp-69p6)
- [https://github.com/http4k/http4k/commit/0121b05537](https://github.com/http4k/http4k/commit/0121b05537)
- [https://github.com/http4k/http4k/commit/54c6385615](https://github.com/http4k/http4k/commit/54c6385615)
- [https://github.com/http4k/http4k/releases/tag/6.49.0.0](https://github.com/http4k/http4k/releases/tag/6.49.0.0)
- [https://api.github.com/repos/http4k/http4k/security-advisories/GHSA-jrpc-7vxp-69p6](https://api.github.com/repos/http4k/http4k/security-advisories/GHSA-jrpc-7vxp-69p6)

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