{"id":"CVE-2026-50571","state":"PUBLISHED","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.","publishedAt":"2026-10-27T22:40:54.912Z","updatedAt":"2026-10-28T16:12:40.129Z","severity":{"score":5.7,"label":"Medium","version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:N","source":"GitHub_M"},"assigner":"GitHub_M","products":["w3c websub"],"affected":[{"vendor":"w3c","product":"websub","versions":[{"version":"= REC-websub-20180123","status":"affected"}]}],"references":["https://github.com/w3c/websub/security/advisories/GHSA-f4hw-w632-m6wh","https://github.com/aaronpk/websub.rocks/commit/c9e24b8815c0d9f43350f0408ce2aedf9aa78ede"],"weaknesses":["CWE-79"],"ingestedAt":"2026-10-29T20:12:16.098Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/50xxx/CVE-2026-50571.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-50571","state":"PUBLISHED","datePublished":"2026-10-27T22:40:54.912Z","dateUpdated":"2026-10-28T16:12:40.129Z","assignerShortName":"GitHub_M","assignerOrgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateReserved":"2026-06-04T21:34:34.427Z"},"containers":{"cna":{"title":"WebSub: Reflected script execution through intent-verification challenges","descriptions":[{"lang":"en","value":"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."}],"affected":[{"vendor":"w3c","product":"websub","versions":[{"version":"= REC-websub-20180123","status":"affected"}]}],"references":[{"url":"https://github.com/w3c/websub/security/advisories/GHSA-f4hw-w632-m6wh","name":"https://github.com/w3c/websub/security/advisories/GHSA-f4hw-w632-m6wh","tags":["x_refsource_CONFIRM"]},{"url":"https://github.com/aaronpk/websub.rocks/commit/c9e24b8815c0d9f43350f0408ce2aedf9aa78ede","name":"https://github.com/aaronpk/websub.rocks/commit/c9e24b8815c0d9f43350f0408ce2aedf9aa78ede","tags":["x_refsource_MISC"]}],"metrics":[{"cvssV3_1":{"attackComplexity":"LOW","attackVector":"NETWORK","availabilityImpact":"NONE","baseScore":5.7,"baseSeverity":"MEDIUM","confidentialityImpact":"NONE","integrityImpact":"HIGH","privilegesRequired":"LOW","scope":"UNCHANGED","userInteraction":"REQUIRED","vectorString":"CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:N","version":"3.1"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-79","lang":"en","description":"CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')","type":"CWE"}]}],"providerMetadata":{"shortName":"GitHub_M","orgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateUpdated":"2026-10-27T22:40:54.912Z"},"source":{"advisory":"GHSA-f4hw-w632-m6wh","discovery":"UNKNOWN"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-10-28T14:35:09.199744Z","id":"CVE-2026-50571","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-10-28T16:12:40.129Z"}}]}}}