---
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'
---

## Overview

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

- `websub = REC-websub-20180123`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.