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.
References2
Metrics
- CVSS 3.1
-
5.7
MEDIUM
Score source: CNA
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:N - EPSS — exploit probability
- 0% percentile: 14 · as of 3 November 2026
- CWE
- CWE-79
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