{"id":"CVE-2026-42273","aliases":["GHSA-72h4-mxfc-jx37","GO-2026-5200"],"url":"https://o3.security/vulnerability/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.","published":"2026-05-08T03:42:50.312Z","modified":"2026-08-12T03:51:34.930637935Z","cvss":null,"epss":{"score":0.00301,"percentile":0.22874,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Go","name":"github.com/dadrus/heimdall","fixedVersion":"0.17.14"}],"fix":{"url":"https://github.com/dadrus/heimdall/commit/3d05e56a9e7ef0355f17482b4322054af4e85943","label":"dadrus/heimdall@3d05e56"},"references":[{"type":"WEB","url":"https://github.com/dadrus/heimdall/releases/tag/v0.17.14"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/42xxx/CVE-2026-42273.json"},{"type":"ADVISORY","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":"FIX","url":"https://github.com/dadrus/heimdall/commit/3d05e56a9e7ef0355f17482b4322054af4e85943"},{"type":"FIX","url":"https://github.com/dadrus/heimdall/pull/3208"},{"type":"PACKAGE","url":"https://github.com/dadrus/heimdall"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:34.930637935Z"}}