{"id":"CVE-2026-50571","revision":null,"records":[{"revision":"6e395602b3f5482e92c7ce226520d592","observedAt":"2026-10-28T17:46:48.824+00:00","statement":{"source":"cve-program","id":"CVE-2026-50571","aliases":[],"title":"WebSub: Reflected script execution through intent-verification challenges","description":"WebSub is a W3C recommendation produced by the Social Web Working Group. In recommendation REC-websub-20180123, the intent-verification protocol requires a subscriber to echo the `hub.challenge` value, also exposed as `hub_challenge` by some frameworks, but does not require safe response media typing or disabled MIME sniffing. An attacker can initiate a subscription to obtain a callback URL and redirect a victim browser to a crafted verification request containing HTML or JavaScript. Subscribers that return the challenge as `text/html` can execute the reflected content in the subscriber origin, using a victim's existing session to read private data or perform actions, exposing `localStorage` or `sessionStorage` and browser permissions granted to that origin. An alternative uses the verification response as an attacker-controlled same-origin script resource to bypass `script-src 'self'`; this can remain possible with `application/octet-stream` when MIME sniffing is allowed. Safe media typing and `X-Content-Type-Options: nosniff` are both needed to address the described scenarios. WebSub Rocks demonstrated reflection, but the advisory does not establish practical impact on that demonstration site.","state":0,"publishedAt":"2026-10-27T22:40:54.912+00:00","modifiedAt":"2026-10-28T16:12:40.129+00:00","affected":[{"ecosystem":"Vendor product","name":"websub","ranges":[],"versions":[],"rawVersionEvidence":"{\n                    \"vendor\": \"w3c\",\n                    \"product\": \"websub\",\n                    \"versions\": [\n                        {\n                            \"version\": \"= REC-websub-20180123\",\n                            \"status\": \"affected\"\n                        }\n                    ]\n                }"}],"metrics":[{"system":"CVSS 3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:N","score":5.7,"severity":"MEDIUM"}],"references":["https://github.com/w3c/websub/security/advisories/GHSA-f4hw-w632-m6wh","https://github.com/aaronpk/websub.rocks/commit/c9e24b8815c0d9f43350f0408ce2aedf9aa78ede"],"sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-50571","license":"CVE Program Terms of Use","key":"cve-program:CVE-2026-50571","identifiers":["CVE-2026-50571"]},"retained":false,"historicalAffected":[]}],"coverage":[{"source":"cisa-kev:global","scope":"Complete CISA KEV catalog; all vendors. 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