# CVE-2026-42273: Dadrus Heimdall case-sensitive host matching policy bypass

Source: https://junglewise.ai/threats/cve-2026-42273-dadrus-heimdall-case-sensitive-host-matching-policy-bypass  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-08. Updated: 2026-05-12  
License: CC BY 4.0 with a link back

CVE-2026-42273 · Severity: high · CVSS 4 · Published 2026-05-08

Heimdall, an identity-aware proxy used to control access to web services, incorrectly handles uppercase letters in website addresses. An attacker can bypass security rules by simply changing the capitalization of a URL (e.g., using 'Admin.Example.Com' instead of 'admin.example.com'). This could allow unauthorized users to access restricted data or administrative functions if the system is configured with permissive default settings.

Technologies: [Dadrus Heimdall](https://junglewise.ai/threats/technologies/heimdall), [github.com/dadrus/heimdall (Go)](https://junglewise.ai/threats/technologies/github-com-dadrus-heimdall). Vendors: [Dadrus](https://junglewise.ai/threats/vendors/dadrus), [Go](https://junglewise.ai/threats/vendors/go).

## Quick answers

**Is CVE-2026-42273 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-42273 being exploited in the wild.

**How severe is CVE-2026-42273?** High severity, with a CVSS score of 4, published 8 May 2026.

**Which products and versions are affected?** Dadrus Heimdall < 0.17.14; github.com/dadrus/heimdall (Go) < 0.17.14, SEMVER:introduced=0,fixed=0.17.14.

**Is there a fix for CVE-2026-42273?** Yes. github.com/dadrus/heimdall (Go): upgrade to 0.17.14 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-42273 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-42273?** Check whether you run Dadrus Heimdall or github.com/dadrus/heimdall (Go) 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.

## How to fix

- github.com/dadrus/heimdall (Go): upgrade to 0.17.14 or later

  ```sh
  go get github.com/dadrus/heimdall@v0.17.14
  ```


## Technical details

Heimdall (prior to v0.17.14) implements case-sensitive host matching for its access rules. Because the HTTP Host header is technically case-insensitive, an attacker can provide a hostname with mixed casing (e.g., 'Admin.Example.Com') to avoid matching a specific security rule defined for a lowercase host. If the request fails to match the intended rule, Heimdall may fall back to a default rule; if that default rule is overly permissive (such as allowing anonymous access), the attacker achieves a policy bypass. This vulnerability is most critical in configurations where secure default rule enforcement has been explicitly disabled. The issue is resolved in version 0.17.14.

## Affected products

- dadrus heimdall < 0.17.14

## Identifiers

- CVE-2026-42273
- GHSA-72h4-mxfc-jx37
- GO-2026-5200

## Timeline

- 2026-04-21: disclosed
- 2026-04-21: patched: Version 0.17.14 released
- 2026-04-25: advisory
- 2026-05-08: other: NVD published record

## References

- [https://github.com/dadrus/heimdall/security/advisories/GHSA-72h4-mxfc-jx37](https://github.com/dadrus/heimdall/security/advisories/GHSA-72h4-mxfc-jx37)
- [https://github.com/dadrus/heimdall/pull/3208](https://github.com/dadrus/heimdall/pull/3208)
- [https://github.com/dadrus/heimdall/commit/3d05e56a9e7ef0355f17482b4322054af4e85943](https://github.com/dadrus/heimdall/commit/3d05e56a9e7ef0355f17482b4322054af4e85943)
- [https://github.com/dadrus/heimdall/releases/tag/v0.17.14](https://github.com/dadrus/heimdall/releases/tag/v0.17.14)
- [https://api.github.com/repos/dadrus/heimdall/security-advisories/GHSA-72h4-mxfc-jx37](https://api.github.com/repos/dadrus/heimdall/security-advisories/GHSA-72h4-mxfc-jx37)

## Related threats

- [Heimdall IP spoofing via unvalidated forwarding headers](https://junglewise.ai/threats/cve-2026-57210-heimdall-ip-spoofing-via-unvalidated-forwarding-headers)
- [Heimdall Forwarded header injection in proxy mode](https://junglewise.ai/threats/cve-2026-57209-heimdall-forwarded-header-injection-via-unsanitized-host-header)
- [dadrus Heimdall authorization bypass via path normalization mismatch](https://junglewise.ai/threats/cve-2026-42274-dadrus-heimdall-authorization-bypass-via-path-normalization)
- [dadrus Heimdall authorization bypass via case-sensitive URL encoding](https://junglewise.ai/threats/cve-2026-42272-dadrus-heimdall-authorization-bypass-via-case-sensitive-url)
- [GO-2026-4742 - Heimdall: Path received via Envoy gRPC corrupted when containing query string in github.com/dadrus/heimdall](https://junglewise.ai/threats/cve-2026-32811-heimdall-path-received-via-envoy-grpc-corrupted-when-containing)