Reflected XSS in oaskit's default HTML error handler

{
  "id": "67c6cc0df67245d01ee2af5ed109a6abaff5ccf3cc99278251283bd816714c8f",
  "url": "https://osv.dev/vulnerability/CVE-2026-66296",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Reflected XSS in oaskit's default HTML error handler",
    "summary": "Improper Neutralization of Input During Web Page Generation (XSS) vulnerability in lud oaskit allows reflected cross-site scripting via the default HTML error handler. Oaskit.ErrorHandler.Default.format_reason/4 and Oaskit.ErrorHandler.Default.reason_to_html/1 in lib/oaskit/error_handler/default.ex render request-validation failures as an HTML page whenever the request's Accept header contains html, interpolating request-controlled strings into that page without HTML escaping. The unescaped values are object keys taken from a request body or from an object or deepObject query parameter, which appear in the JSON Schema error's instance path when a schema rejects them (for example under additionalProperties: false), and the raw Content-Type header, reflected in unsupported-media-type errors when it fails to parse. Because browsers send Accept: text/html on ordinary top-level navigation, a crafted GET link is sufficient to trigger the error page; no form submission, custom Content-Type, or attacker-controlled script on the victim's side is required. A payload such as filter[]=x terminates the enclosing markup and the injected script executes in the origin of the application using oaskit, giving it access to that origin's cookies, session, and same-origin responses. Both HTML error rendering and the vulnerable handler are enabled by default: Oaskit.Plugs.ValidateRequest defaults :html_errors to true and :error_handler to Oaskit.ErrorHandler.Default, so applications following the ",
    "evidence": "osv:CVE-2026-66296",
    "products": [],
    "severity": "medium",
    "structured": {
      "id": "CVE-2026-66296",
      "aliases": [
        "CVE-2026-66296",
        "EEF-CVE-2026-66296",
        "GHSA-h7xw-x8wr-xpcc"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/lud/oaskit",
              "type": "GIT",
              "events": [
                {
                  "introduced": "3ca707778b796488d8054c29d31043bc1e3f66e2"
                },
                {
                  "fixed": "37f1b7df9d9fd9d0bc3d69ad4759f3c71b174e3b"
                },
                {
                  "fixed": "b70c6b2eaf0b11bdd0bbb21b8a87dbb3d46918a1"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "cpe": "cpe:2.3:a:lud:oaskit:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "0.1.0"
                      },
                      {
                        "fixed": "0.14.1"
                      }
                    ],
                    "source": "CPE_RANGE"
                  },
                  {
                    "range": [
                      {
                        "fixed": "b70c6b2eaf0b11bdd0bbb21b8a87dbb3d46918a1"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v0.14.0",
            "v0.13.2",
            "v0.13.1",
            "v0.13.0",
            "v0.12.0",
            "v0.11.0",
            "v0.10.1",
            "v0.10.0",
            "v0.9.1",
            "v0.9.0",
            "v0.8.0",
            "v0.7.0",
            "v0.6.0",
            "v0.5.1",
            "v0.5.0",
            "v0.4.1",
            "v0.4.0",
            "v0.3.1",
            "v0.3.0",
            "v0.2.0",
            "v0.1.2",
            "v0.1.1",
            "v0.1.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-66296.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:40.547590777Z",
      "provider": "osv",
      "references": [
        "https://cna.erlef.org/cves/CVE-2026-66296.html",
        "https://github.com",
        "https://osv.dev/vulnerability/EEF-CVE-2026-66296",
        "https://repo.hex.pm",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/66xxx/CVE-2026-66296.json",
        "https://github.com/lud/oaskit/security/advisories/GHSA-h7xw-x8wr-xpcc",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-66296",
        "https://github.com/lud/oaskit/commit/b70c6b2eaf0b11bdd0bbb21b8a87dbb3d46918a1",
        "https://github.com/lud/oaskit"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-79"
        ],
        "cna_assigner": "EEF",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/66xxx/CVE-2026-66296.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-66296",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 508246,
  "published_at": "2026-08-03T19:04:30.556+00:00"
}