CVE-2026-50571 WebSub: Reflected script execution through intent-verification challenges
- Exploited in the wild
- Not known
- Fix
- None listed
What is CVE-2026-50571?
CVE-2026-50571 is a medium-severity vulnerability in w3c websub, published on 27 Oct 2026.
Official 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.
Weakness: CWE-79
Which versions are affected by CVE-2026-50571?
How do I fix CVE-2026-50571?
No fix has been published for CVE-2026-50571 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-50571 being exploited?
There is no known exploitation of CVE-2026-50571 in the wild and no public exploit code.
Sources
Also known as GHSA-f4hw-w632-m6wh.