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

Severity: medium (CVSS 4.9 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-03-06 · updated 2026-06-17
Weakness: CWE-776

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.

Affected products

MarkUsProject Markus: before 2.9.4 — fixed in 2.9.4

How to fix

Upgrade to a fixed version: MarkUsProject Markus 2.9.4. Check the vendor advisory for mitigations.

References

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

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