CVE-2026-42857

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.

CVSS v3.1 Score

4.6

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N

EPSS — Exploit Prediction

0.0003

Probability of exploitation

0.09%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-79 Cross-site Scripting (XSS)

Affected Products

Vendor	Product	Versions
openedx	openedx	< 2026-04-24

References

Advisories & Patches

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

Exploits

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

Frequently Asked Questions

What is CVE-2026-42857? +

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. It has a CVSS v3.1 base score of 4.6 (MEDIUM).

How severe is CVE-2026-42857? +

CVE-2026-42857 has a CVSS v3.1 score of 4.6 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0003, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-42857? +

CVE-2026-42857 affects products from openedx, specifically: openedx. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-42857? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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