{"id":"CVE-2026-50571","aliases":[],"title":"WebSub: Reflected script execution through intent-verification challenges","titleSource":"GitHub_M","descriptionSource":"GitHub_M","products":[{"vendor":"w3c","name":"websub","platforms":[],"versions":[{"version":"= REC-websub-20180123","status":"affected"}],"source":"GitHub_M","url":"https://www.cve.org/CVERecord?id=CVE-2026-50571"}],"remediation":[],"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site.","severity":"medium","score":5.7,"published":"2026-10-27T22:40:54.912Z","modified":"2026-10-28T16:12:40.129Z","withdrawn":false,"knownExploited":false,"affected":[],"references":["https://github.com/w3c/websub/security/advisories/GHSA-f4hw-w632-m6wh","https://github.com/aaronpk/websub.rocks/commit/c9e24b8815c0d9f43350f0408ce2aedf9aa78ede"],"sources":[{"name":"CVE","id":"CVE-2026-50571","modified":"2026-10-28T16:12:40.129Z","url":"https://www.cve.org/CVERecord?id=CVE-2026-50571"}],"assessments":[{"source":"GitHub_M","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:N","score":5.7}],"sourceAffected":[],"kev":{"status":"not-listed","checkedAt":"2026-10-28T21:40:30.742Z"},"coverage":[{"source":"NVD","status":"not-checked"},{"source":"GHSA","status":"not-checked"},{"source":"EUVD","status":"not-checked"},{"source":"EPSS","status":"not-checked"}],"evidence":[]}