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

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

- **Severity:** High 8.0 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H`)
- **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 available:** No
- **Published:** 5 Mar 2026
- **Updated:** 6 Mar 2026
- **Also known as:** GHSA-p5pc-pxrj-3893

## 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 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-28405?

It affects 1 product.


Products: 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.

## Is CVE-2026-28405 being exploited?

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

## Other vulnerabilities in MarkUsProject Markus

- [CVE-2026-25057](https://insights.sbomify.com/CVE-2026-25057/): Zip Slip in MarkUs config upload allowing RCE (critical 9.1)
- [CVE-2024-51499](https://insights.sbomify.com/CVE-2024-51499/): MarkUs Arbitrary File Write leading up to remote code execution (student accounts) (high 7.1)
- [CVE-2024-51743](https://insights.sbomify.com/CVE-2024-51743/): Arbitrary File Write leading up to remote code execution (instructor accounts) (high 7.1)
- [CVE-2026-25962](https://insights.sbomify.com/CVE-2026-25962/): MarkUs: Zip bomb in config upload enables DoS (medium 6.5)
- [CVE-2026-24900](https://insights.sbomify.com/CVE-2026-24900/): MarkUs has a submission-view IDOR exposes all student submissions (medium 6.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-28405 and fix it if it is affected.

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

Affected product: MarkUsProject Markus. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-28405/ (Markdown: https://insights.sbomify.com/CVE-2026-28405/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-28405
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-28405
- OSV: https://osv.dev/vulnerability/CVE-2026-28405

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-28405/
Summaries are generated and may contain mistakes; the official description above is the record.