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

High 8.0 Published 5 Mar 2026 Updated 6 Mar 2026

Attackers can inject malicious scripts into student submissions, allowing them to execute actions in the instructor context.

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: None listed

What is CVE-2026-28405?

CVE-2026-28405 is a high-severity cross-site scripting vulnerability in MarkUsProject Markus, published on 5 Mar 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

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.

Weakness: CWE-79

Which versions are affected by CVE-2026-28405?

It affects 1 product.

Products include Markusproject Markus.

How do I fix CVE-2026-28405?

No fix has been published for CVE-2026-28405 yet. Limit exposure of the affected software until one is available.

Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.

Is CVE-2026-28405 being exploited?

There is no known exploitation of CVE-2026-28405 in the wild and no public exploit code.

Sources

Also known as GHSA-p5pc-pxrj-3893.