# CVE-2026-86434

## Summary

- **CVE ID:** CVE-2026-86434
- **Severity:** HIGH
- **CVSS Score:** 8.7 (CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N)
- **CWE:** CWE-407
- **Published:** Sep 7, 2026
- **Last Modified:** Sep 7, 2026

## Description

league/commonmark versions >= 2.0.0 and < 2.8.4 (patched in 2.9.0) contain a denial of service vulnerability in UniqueSlugNormalizer::normalize(), which restarts its numeric-suffix search from 1 on every slug collision, resulting in O(K^2) time complexity for K headings that collapse to the same base slug. The vulnerable path is reached when HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension is registered. An unauthenticated attacker can force many headings onto a single base slug (e.g., via empty ATX headings, identical heading text, or punctuation-only headings) in a small Markdown document, consuming excessive CPU and denying service.

## Affected Products

- thephpleague — commonmark (2.0.0)
- thephpleague — commonmark (2.8.4)

## References

- [CNA](https://github.com/thephpleague/commonmark/security/advisories/GHSA-mh25-x5hq-wrqp)
- [CNA](https://www.vulncheck.com/advisories/commonmark-2.0.0-through-2.8.3-denial-of-service-via-slug-collision)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.29%
- **EPSS Percentile:** 21.5

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._