{"id":"CVE-2026-50571","title":"WebSub: Reflected script execution through intent-verification challenges","summary":"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 …","severity":"medium","cvss":5.7,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:N","cvssSource":"cna","cwe":["CWE-79"],"vendor":"w3c","product":"websub","affected":["websub = REC-websub-20180123"],"ssvc":{"exploitation":"none","automatable":"no","technicalImpact":"partial","timestamp":"2026-10-28T14:35:09.199744Z"},"published":"2026-10-27","updated":"2026-10-28","sourceUpdated":"2026-10-28T16:12:40.129Z","source":"CVEORG","sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-50571","references":[{"url":"https://github.com/w3c/websub/security/advisories/GHSA-f4hw-w632-m6wh","label":"https://github.com/w3c/websub/security/advisories/GHSA-f4hw-w632-m6wh"},{"url":"https://github.com/aaronpk/websub.rocks/commit/c9e24b8815c0d9f43350f0408ce2aedf9aa78ede","label":"https://github.com/aaronpk/websub.rocks/commit/c9e24b8815c0d9f43350f0408ce2aedf9aa78ede"}],"tags":["cve.org"],"epss":0.00233,"epssPercentile":0.14185,"ingestedAt":"2026-10-27T22:58:58.455Z","slug":"CVE-2026-50571","body":"## Overview\n\nWebSub 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.\n\n## Affected\n\n- `websub = REC-websub-20180123`\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"sunlit","depthScore":31,"depthScoreParts":{"impact":31.4,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}