XSS in Openedx

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.

Vulnerability class: XSS (Cross-Site Scripting)

Published · last modified .

CVSS v3 metric

CVSS v3 base score 4.6 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.002 (8.0th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 8th percentile — 8.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-42857: held from 0.003 to 0.002.

EPSS over last 30 days · oldest: 0.003 · newest: 0.002 · change: -0.001

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-42857?
CVE-2026-42857 is a medium-severity vulnerability in Openedx, classified under Cross-site Scripting. CVSS score: 4.6/10. Published 2026-05-11.
How severe is CVE-2026-42857?
Medium severity. CVSS v3 base score is 4.6 out of 10.