How severe is CVE-2026-42273?

No CVSS score has been assigned to CVE-2026-42273 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-42273?

CVE-2026-42273 affects the following packages: github.com/dadrus/heimdall (Go). Ecosystems affected: Go.

How do I fix CVE-2026-42273?

Update github.com/dadrus/heimdall to 0.17.14 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-42273 is resolved across your whole dependency graph.

How do I detect CVE-2026-42273 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/dadrus/heimdall, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-42273 if there is no patch (or I can't update yet)?

Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Is CVE-2026-42273 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-42273 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-42273?

CVE-2026-42273 has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 23th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-42273?

CVE-2026-42273 is classified as CWE-178 (CWE-178), CWE-436 (CWE-436). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-42273 published, and has it been updated?

CVE-2026-42273 was published on May 8, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹 Go

Not in CISA KEV

CVE-2026-42273 — heimdall

Fix: dadrus/heimdall@3d05e56

CVE-2026-42273 is a CWE-178 vulnerability in github.com/dadrus/heimdall. A fix is available for github.com/dadrus/heimdall — see the affected versions and patch details below.

Heimdall: Case-sensitive host matching may lead to policy bypass

Also known asGHSA-72h4-mxfc-jx37GO-2026-5200

Published

May 8, 2026

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 7, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-42273.

EPSS Exploitation Probability

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs23th percentile — riskier than 23% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

🐹github.com/dadrus/heimdall

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

Summary

Heimdall performs host matching in a case-sensitive manner, while HTTP hostnames are case-insensitive. This discrepancy can result in heimdall failing to match a rule for a request host that differs only in letter casing, potentially causing the request to be classified differently than intended.

Note: The issue can only lead to unintended access if heimdall is configured with an "allow all" default rule. Since v0.16.0, heimdall enforces secure defaults and refuses to start with such a configuration unless this enforcement is explicitly disabled, e.g. via --insecure-skip-secure-default-rule-enforcement or the broader --insecure flag.

Details

This vulnerability can potentially be exploited by an adversary if rule matching relies on the request host.

For example, consider the following rule:

id: rule-1
match:
  hosts:
    - type: exact
      value: admin.example.com
execute: # configured to require authentication and authorization
  # ...

If an adversary now sends a request with the Host header set to Admin.Example.Com, rule-1 will not be matched, and the following will happen instead:

If no default rule is configured, the request will result in an error (404 Not Found)
If a default rule is configured, it will be executed. If the default rule is configured in an overly permissive way (e.g. allowing anonymous access), this results in a policy bypass.

Impact

Bypass of access control policies enforced by heimdall may lead to the following consequences:

Access to or modification of data that should be restricted
Invocation of functionality that is expected to require authentication or authorization
In certain configurations, escalation of privileges depending on the exposed functionality

Workarounds

Normalize request hosts to lowercase in the layers in front of heimdall.
Do not configure a permissive default rule. Respectively, do not make use of the --insecure or the --insecure-skip-secure-default-rule-enforcement flags.
When using regex type for host matching, expressions shall be defined in a case-insensitive manner (e.g. (?i)^admin\.example\.com$)
Include the ID of the rule expected to be executed in the JWT issued by heimdall and check that value in the consuming project's service.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/dadrus/heimdall`	all versions	0.17.14`go get github.com/dadrus/heimdall@v0.17.14`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/dadrus/heimdall, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/dadrus/heimdall to 0.17.14 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-42273 is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Frequently Asked Questions

### Summary Heimdall performs host matching in a case-sensitive manner, while HTTP hostnames are case-insensitive. This discrepancy can result in heimdall failing to match a rule for a request host that differs only in letter casing, potentially causing the request to be classified differently than intended. **Note:** The issue can only lead to unintended access if heimdall is configured with an "allow all" default rule. Since v0.16.0, heimdall enforces secure defaults and refuses to start with such a configuration unless this enforcement is explicitly disabled, e.g. via `--insecure-skip-sec