{"schema":"threatline.record/2","record":{"id":"CVE-2026-50571","aliases":["GHSA-f4hw-w632-m6wh"],"summary":"WebSub: Reflected script execution through intent-verification challenges","details":"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.","categories":["Unclassified"],"product":"websub","vendor":"w3c","ecosystem":null,"affected_ranges":[{"source":"CVE List","source_url":"https://www.cve.org/CVERecord?id=CVE-2026-50571","source_modified_at":"2026-10-27T22:40:54.912Z","fetched_at":"2026-10-28T01:10:53.534015Z","product":"websub","vendor":"w3c","versions":[{"version":"= REC-websub-20180123","status":"affected"}],"defaultStatus":null},{"source":"GitHub Advisory Database","source_url":"https://github.com/w3c/websub/security/advisories/GHSA-f4hw-w632-m6wh","source_modified_at":"2026-06-20T08:53:53Z","fetched_at":"2026-10-28T01:10:53.534015Z","package":{"ecosystem":"","name":"WebSub"},"range":"REC-websub-20180123"}],"fixed_versions":[],"cvss":5.7,"severity":"Medium","kev":{"listed":false},"epss":{"probability":null,"percentile":null,"score_date":null},"published_at":"2026-10-27T22:40:54.912Z","modified_at":"2026-10-27T22:40:54.912Z","provenance":[{"source":"CVE List","url":"https://www.cve.org/CVERecord?id=CVE-2026-50571","fetched_at":"2026-10-28T01:10:53.534015Z","source_modified_at":"2026-10-27T22:40:54.912Z"},{"source":"GitHub Advisory Database","url":"https://github.com/w3c/websub/security/advisories/GHSA-f4hw-w632-m6wh","fetched_at":"2026-10-28T01:10:53.534015Z","source_modified_at":"2026-06-20T08:53:53Z"}],"sources":["CVE List","GitHub Advisory Database"],"references":[{"source":"CVE List","url":"https://github.com/w3c/websub/security/advisories/GHSA-f4hw-w632-m6wh","tags":["x_refsource_CONFIRM"]},{"source":"CVE List","url":"https://github.com/aaronpk/websub.rocks/commit/c9e24b8815c0d9f43350f0408ce2aedf9aa78ede","tags":["x_refsource_MISC"]}],"locations":[],"motoko_assessment":{"status":"requires_architecture_review","automatic_remediation":false},"cve_state":"PUBLISHED","cvss_source":"CVE List","classification":{"method":"product-first software-layer rules","rules_version":"2.1","matched_on":null,"confidence":"unknown","reviewed":false},"change_timestamp":"2026-10-27T22:40:54.912Z","content_hash":"ba2ed5061b5c90cde6dae8b459547a5eb82ed5ac845dab771b1466b88d61bd9a"}}