GHSA-72H4-MXFC-JX37

Vulnerability from github – Published: 2026-04-25 23:30 – Updated: 2026-05-12 13:28
VLAI
Summary
Heimdall: Case-sensitive host matching may lead to policy bypass
Details

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.
Severity
7.8 (High)
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N
Show details on source website
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{
  "affected": [
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/dadrus/heimdall"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.17.14"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-42273"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-178",
      "CWE-436"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-25T23:30:18Z",
    "nvd_published_at": "2026-05-08T04:16:22Z",
    "severity": "HIGH"
  },
  "details": "### Summary\n\nHeimdall 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.\n\n**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.\n\n### Details\n\nThis vulnerability can potentially be exploited by an adversary if rule matching relies on the request host.\n\nFor example, consider the following rule:\n\n```yaml\nid: rule-1\nmatch:\n  hosts:\n    - type: exact\n      value: admin.example.com\nexecute: # configured to require authentication and authorization\n  # ...\n```\n\nIf 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:\n\n* If no default rule is configured, the request will result in an error (`404 Not Found`)\n* 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.\n\n### Impact\n\nBypass of access control policies enforced by heimdall may lead to the following consequences:\n\n* Access to or modification of data that should be restricted\n* Invocation of functionality that is expected to require authentication or authorization\n* In certain configurations, escalation of privileges depending on the exposed functionality\n\n### Workarounds\n\n* Normalize request hosts to lowercase in the layers in front of heimdall.\n* Do not configure a permissive default rule. Respectively, do not make use of the `--insecure` or the `--insecure-skip-secure-default-rule-enforcement` flags.\n* When using `regex` type for host matching, expressions shall be defined in a case-insensitive manner (e.g. `(?i)^admin\\.example\\.com$`)\n* Include the ID of the rule expected to be executed in the JWT issued by heimdall and check that value in the consuming project\u0027s service.",
  "id": "GHSA-72h4-mxfc-jx37",
  "modified": "2026-05-12T13:28:54Z",
  "published": "2026-04-25T23:30:18Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/dadrus/heimdall/security/advisories/GHSA-72h4-mxfc-jx37"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-42273"
    },
    {
      "type": "WEB",
      "url": "https://github.com/dadrus/heimdall/pull/3208"
    },
    {
      "type": "WEB",
      "url": "https://github.com/dadrus/heimdall/commit/3d05e56a9e7ef0355f17482b4322054af4e85943"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/dadrus/heimdall"
    },
    {
      "type": "WEB",
      "url": "https://github.com/dadrus/heimdall/releases/tag/v0.17.14"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Heimdall: Case-sensitive host matching may lead to policy bypass"
}



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