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

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, github.com/dadrus/heimdall (Go). Vendors: Dadrus, 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
    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

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.