{"id":"EEF-CVE-2026-66296","summary":"Reflected XSS in oaskit's default HTML error handler","details":"## Summary\n\nImproper Neutralization of Input During Web Page Generation (XSS) vulnerability in lud oaskit allows reflected cross-site scripting via the default HTML error handler.\n\n`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.\n\nBecause 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[\u003c/code\u003e\u003c/h2\u003e\u003cscript\u003ealert(document.domain)\u003c/script\u003e]=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.\n\nBoth 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 documented usage are affected without any opt-in.\n\nThis issue affects oaskit: from 0.1.0 before 0.14.1.\n\n## Workarounds\n\nDisable HTML error rendering so that validation failures are returned as JSON only:\n\n```elixir\nplug Oaskit.Plugs.ValidateRequest, html_errors: false\n```\n\nThis prevents the vulnerable HTML page from being rendered at all, on any oaskit version. Alternatively, configure a custom `:error_handler` that does not emit HTML built from request-derived values.","aliases":["CVE-2026-66296","GHSA-h7xw-x8wr-xpcc"],"modified":"2026-09-08T08:30:06.879298290Z","published":"2026-08-03T19:04:30.556Z","database_specific":{"capec_ids":["CAPEC-591"],"cpe_ids":["cpe:2.3:a:lud:oaskit:*:*:*:*:*:elixir:*:*"],"cwe_ids":["CWE-79"]},"references":[{"type":"ADVISORY","url":"https://github.com/lud/oaskit/security/advisories/GHSA-h7xw-x8wr-xpcc"},{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-66296.html"},{"type":"FIX","url":"https://github.com/lud/oaskit/commit/b70c6b2eaf0b11bdd0bbb21b8a87dbb3d46918a1"},{"type":"PACKAGE","url":"https://hex.pm/packages/oaskit"}],"affected":[{"package":{"name":"oaskit","ecosystem":"Hex","purl":"pkg:hex/oaskit"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0.1.0"},{"fixed":"0.14.1"}]}],"versions":["0.1.0","0.1.1","0.1.2","0.10.0","0.10.1","0.11.0","0.12.0","0.13.0","0.13.1","0.13.2","0.14.0","0.2.0","0.3.0","0.3.1","0.4.0","0.4.1","0.5.0","0.5.1","0.6.0","0.7.0","0.8.0","0.9.0","0.9.1"],"database_specific":{"source":"https://cna.erlef.org/osv/EEF-CVE-2026-66296.json"}},{"ranges":[{"type":"GIT","repo":"https://github.com/lud/oaskit","events":[{"introduced":"552de8ca0ff60617bea58bcafa451892d61a161c"},{"fixed":"b70c6b2eaf0b11bdd0bbb21b8a87dbb3d46918a1"}]}],"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://cna.erlef.org/osv/EEF-CVE-2026-66296.json"}}],"schema_version":"1.9.0","severity":[{"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"}],"credits":[{"name":"Ludovic Dem","type":"FINDER"},{"name":"Ludovic Dem","type":"REMEDIATION_DEVELOPER"},{"name":"Jonatan Männchen / EEF","type":"ANALYST"}]}