CVE-2026-42273: Heimdall Case-sensitive Host Matching May Lead Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-72h4-mxfc-jx37",
  "modified" : "2026-05-12T13:28:54Z",
  "published" : "2026-04-25T23:30:18Z",
  "aliases" : [ "CVE-2026-42273" ],
  "summary" : "Heimdall: Case-sensitive host matching may lead to policy bypass",
  "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's service.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/dadrus/heimdall"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.17.14"
      } ]
    } ]
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-178", "CWE-436" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-25T23:30:18Z",
    "nvd_published_at" : "2026-05-08T04:16:22Z"
  }
}