CVE-2026-28405: MarkUs: Stored XSS in Submission HTML Preview Enables Instructor-Context Actions

Severity: medium (CVSS 5.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-05 · updated 2026-06-17
Weakness: CWE-79

Description

MarkUs is a web application for the submission and grading of student assignments. Prior to version 2.9.1, the courses/<:course_id>/assignments/<:assignment_id>/submissions/html_content route reads the contents of a student-submitted file and renders them without sanitization. This issue has been patched in version 2.9.1.

Affected products

MarkUsProject Markus: before 2.9.1 — fixed in 2.9.1

How to fix

Upgrade to a fixed version: MarkUsProject Markus 2.9.1. Check the vendor advisory for mitigations.

References

https://github.com/MarkUsProject/Markus/commit/55d74f2ddb72d2ec2f29aa2b4cb6b2da10755036
https://github.com/MarkUsProject/Markus/releases/tag/v2.9.1
https://github.com/MarkUsProject/Markus/security/advisories/GHSA-p5pc-pxrj-3893

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