Denial of Service in CommonMark PHP Library

Executive Summary

CVE-2026-105830 is a high-severity denial of service vulnerability in the league/commonmark library affecting versions 2.0.0 through 2.10.1. It involves a quadratic-time flaw in the GitHub Flavored Markdown Table extension where attackers submit large paragraphs of pipe-free lines not starting with letters. This forces repeated full-buffer scans via strpos, consuming excessive CPU resources in PHP workers.

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Detection Guidance

Monitor CPU usage spikes during Markdown processing. Check for excessive strpos scans in PHP worker logs. Use tools like strace to trace system calls during Markdown parsing. Inspect network traffic for large untrusted Markdown inputs.

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Impact Analysis

Unauthenticated attackers can exploit this to exhaust CPU resources in PHP worker processes, causing denial of service. Systems processing untrusted Markdown with GitHub Flavored Markdown converter or Table extension enabled are at risk. The attack requires no privileges or user interaction.

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Compliance Impact

This vulnerability primarily causes denial of service by exhausting CPU resources through quadratic-time scanning operations. It does not directly impact data confidentiality or integrity required by standards like GDPR or HIPAA. However, prolonged service disruption could indirectly affect compliance by preventing access to systems handling personal or health data.

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Mitigation Strategies

Upgrade league/commonmark to version 2.10.2 or later. Disable the GitHub Flavored Markdown Table extension if not required. Implement rate limiting for Markdown processing endpoints. Monitor for unusual CPU usage patterns.

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