Executive Summary

[{'type': 'paragraph', 'content': 'CVE-2026-27807 is a moderate severity vulnerability in the MarkUs web application that affects versions prior to 2.9.4. It arises from the way MarkUs handles YAML configuration file uploads by course instructors. The YAML parser processes aliases without restrictions, allowing an attacker to craft a specially designed "billion laughs" style YAML payload. This payload triggers exponential alias expansion, which causes excessive CPU and RAM consumption, leading to a denial of service (DoS). The root cause is improper restriction of recursive entity references in Document Type Definitions (DTDs), classified under CWE-776.'}] [2]

Useful 0 Not Useful 0

Impact Analysis

This vulnerability can impact you by causing a denial of service (DoS) condition on the MarkUs application. An attacker with network access and high privileges can exploit the unrestricted YAML alias expansion to consume excessive CPU and memory resources, potentially making the application unavailable to legitimate users. There is no impact on confidentiality or integrity, but availability is affected.

Useful 0 Not Useful 0

Compliance Impact

I don't know

Useful 0 Not Useful 0

Detection Guidance

This vulnerability involves the processing of malicious YAML files with recursive aliases that cause excessive CPU and RAM consumption, leading to denial of service. Detection can focus on monitoring unusual resource usage patterns on systems running vulnerable MarkUs versions (prior to 2.9.4).

Specifically, you can monitor for high CPU or memory usage spikes related to the MarkUs application process, especially during YAML file uploads or processing.

While no specific detection commands are provided in the resources, general system commands to monitor resource usage include:

• Using top or htop to observe CPU and memory usage: `top` or `htop`
• Checking process-specific resource consumption: `ps aux --sort=-%mem | grep markus` or `ps aux --sort=-%cpu | grep markus`
• Monitoring logs for unusual or repeated YAML upload attempts or errors related to YAML parsing in MarkUs logs (location depends on deployment)

Network detection could involve monitoring for suspicious upload activity targeting the YAML upload feature by course instructors, but no specific network commands or signatures are provided.

Useful 0 Not Useful 0

Mitigation Strategies

The primary mitigation step is to upgrade MarkUs to version 2.9.4 or later, which includes patches that limit YAML file sizes and the number of nodes, preventing the exploitation of recursive YAML aliases.

Additional mitigations introduced in version 2.9.4 include configuration settings to control zip extraction limits, which help protect against zip bomb attacks.

Until the upgrade can be applied, consider restricting or disabling the ability for course instructors to upload YAML files, or implement monitoring to detect and block suspicious YAML uploads.

Useful 0 Not Useful 0