CVE-2026-42272: Heimdall Case-sensitive Handling Of URL-encoded Slashes Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-43jv-5j4x-qv67",
  "modified" : "2026-05-12T13:28:19Z",
  "published" : "2026-04-25T23:29:40Z",
  "aliases" : [ "CVE-2026-42272" ],
  "summary" : "Heimdall: Case-sensitive handling of URL-encoded slashes may lead to inconsistent path interpretation",
  "details" : "### Summary\n\nHeimdall handles URL-encoded slashes (`%2F`) in a case-sensitive manner, while percent-encoding is defined to be case-insensitive. As a result, the lowercase equivalent (`%2f`) is not recognized and therefore not processed as expected when `allow_encoded_slashes` is set to `off` (the default setting).\n\nThis discrepancy can lead to differences in how request paths are interpreted by heimdall and upstream components, which may result in authorization bypass.\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\nConsider the following rule configuration:\n\n```yaml\nid: rule-1\nmatch:\n  routes:\n    - path: /admin/**\nexecute: # configured to require authentication and authorization\n  # ...\n```\n\nIf an adversary sends a request such as `/admin%2fsecret`, neither is the above rule matched, nor is the request rejected (as would be expected when `allow_encoded_slashes` is set to `off`). Instead, the default rule (if configured) will be executed.\n\nIf the configured default rule is overly permissive (e.g. allowing anonymous access), and the upstream service interprets `%2f` as a path separator, the request may ultimately be processed as `/admin/secret`.\n\nThis results in the request being authorized based on a different path than the one processed by the upstream service, leading to authorization 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\n### Workarounds\n\n* Developers should not use the `--insecure` or the `--insecure-skip-secure-default-rule-enforcement` flags and configure their default rule to implement \"deny by default\".\n* Reject HTTP paths containing encoded slashes in the layers in front of heimdall. Some proxies, like e.g., Traefik, do that by default.\n* Include the ID of the rule expected to be executed in the JWT issued by heimdall and verify that value in the 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-43jv-5j4x-qv67"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-42272"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/dadrus/heimdall/pull/3207"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/dadrus/heimdall/commit/8b0de6aba23a047cfee3081df878271bb17f4351"
  }, {
    "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" : "2026-04-25T23:29:40Z",
    "nvd_published_at" : "2026-05-08T04:16:22Z"
  }
}