# CVE-2026-42857: Open edX Platform: Stored CSS Injection in Email Notifications via Incomplete HTML Sanitization

> Attackers can inject CSS into email notifications to track users and spoof content, requiring a patch to the HTML sanitizer.

- **Severity:** Medium 4.6 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 11 May 2026
- **Updated:** 13 May 2026
- **Also known as:** GHSA-4xv3-5j4x-q8g4

## What is CVE-2026-42857?

CVE-2026-42857 is a medium-severity cross-site scripting vulnerability in openedx-platform, published on 11 May 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official description

Open edX Platform enables the authoring and delivery of online learning at any scale. The HTML sanitizer clean_thread_html_body() used for discussion notification emails fails to remove <style> tags from user-generated discussion post content. This content is rendered with Django's |safe template filter in email notification templates, allowing any enrolled student to inject arbitrary CSS into email notifications sent to other users. This enables email tracking (IP address disclosure), content spoofing, and phishing attacks. This vulnerability is fixed with commit cddc25cd791bb78f76833896e4778f668861df12.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-42857?

It affects 1 product.


Products: Openedx Openedx.

## How do I fix CVE-2026-42857?

No fix has been published for CVE-2026-42857 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-42857 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-42857 exists.

## Other vulnerabilities in openedx-platform

- [CVE-2026-42858](https://insights.sbomify.com/CVE-2026-42858/): Open edX Platform: Server-Side Request Forgery (SSRF) in SAML Provider Data Sync Endpoint (high 8.5)
- [CVE-2024-43782](https://insights.sbomify.com/CVE-2024-43782/): openedx-translations's Atlas translations for Open edX missing validation (high 7.7)
- [CVE-2026-35404](https://insights.sbomify.com/CVE-2026-35404/): Open edX Platform has an Open Redirect in Survey Views via Unvalidated redirect_url Parameter (medium 4.7)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-42857 and fix it if it is affected.

What it is: CVE-2026-42857 is a medium-severity cross-site scripting vulnerability in openedx-platform, published on 11 May 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-42857 exists.

Affected product: openedx-platform. 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-2026-42857/ (Markdown: https://insights.sbomify.com/CVE-2026-42857/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-42857
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-42857
- OSV: https://osv.dev/vulnerability/CVE-2026-42857

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-42857/
Summaries are generated and may contain mistakes; the official description above is the record.