{"id":"CVE-2026-42272","aliases":["GHSA-43jv-5j4x-qv67","GO-2026-5108"],"url":"https://o3.security/vulnerability/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.","published":"2026-05-08T03:40:17.541Z","modified":"2026-08-12T03:51:20.232141123Z","cvss":null,"epss":{"score":0.00396,"percentile":0.33464,"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/8b0de6aba23a047cfee3081df878271bb17f4351","label":"dadrus/heimdall@8b0de6a"},"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-2026-42272.json"},{"type":"ADVISORY","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":"FIX","url":"https://github.com/dadrus/heimdall/commit/8b0de6aba23a047cfee3081df878271bb17f4351"},{"type":"FIX","url":"https://github.com/dadrus/heimdall/pull/3207"},{"type":"PACKAGE","url":"https://github.com/dadrus/heimdall"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:20.232141123Z"}}