# CVE-2026-50571: WebSub: Reflected script execution through intent-verification challenges


- **Severity:** Medium 5.7 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Fix available:** No
- **Published:** 27 Oct 2026
- **Updated:** 28 Oct 2026
- **Also known as:** GHSA-f4hw-w632-m6wh

## 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 (Improper Neutralization of Input During Web Page Generation)

## 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-50571 and fix it if it is affected.

What it is: CVE-2026-50571 is a medium-severity vulnerability in w3c websub, published on 27 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-50571 in the wild and no public exploit code.

Affected product: w3c websub. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-50571/ (Markdown: https://insights.sbomify.com/CVE-2026-50571/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-50571
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-50571
- OSV: https://osv.dev/vulnerability/CVE-2026-50571

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-50571/
Summaries are generated and may contain mistakes; the official description above is the record.