TI-2026-59142 Aliases CVE-2026-86434 GHSA-jcmh-chhq-rh2x

commonmark 2.0.0 through 2.8.3 Denial of Service via Slug Collision

MEDIUM 44CWE-407Published 2026-09-07 · Updated 2026-10-08

Key facts

Attack vector: Network
Privileges required: None
User interaction: None
Attack complexity: Low
Attack requirements: None
Impact type: Denial of service
Confidentiality · integrity · availability: None · None · High
Fix status: Fixed

Attack profile from CVSS 4.0 · CNA. Fix status from CNA. Impact type from CWE-407.

Summary

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV

EPSS 0.00509 43rd percentileScored 2026-11-07

Fix Fixed commonmark (thephpleague): 2.8.4

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation: None
Automatable: Yes
Technical impact: Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

Vendor	Product	Affected versions	Fixed in	Status	Source
thephpleague	commonmark	2.0.0 to before 2.8.4	2.8.4	Patch available	CNA

commonmark Vendorthephpleague Affected 2.0.0 to before 2.8.4 Fixed in 2.8.4 Patch available CNA

CPE names Threatist (inferred) · 1

Inferred from how NVD named this product before; not given by a source.

CPE	Versions	Role	Vendor
cpe:2.3:a:thephpleague:commonmark::::::::	—	—	thephpleague

cpe:2.3:a:thephpleague:commonmark:*:*:*:*:*:*:*:* Versions — — ·thephpleague

Remediation

Fix · CNA commonmark (thephpleague) Fixed in 2.8.4

Fixed in the same release

Records fixed by the same vendor advisory or release, as the CNA lists them.

Release · 2026-09-07
thephpleague commonmark 2.8.4
4 records Highest priority Priority MEDIUM 3 MEDIUM, 1 LOW
Other records, highest priority first
- TI-2026-59143 commonmark 1.5.0 before 2.8.4 Denial of Service via FootnotePriority MEDIUM 44
- TI-2026-59141 commonmark 1.5.0 before 2.8.4 Denial of Service via AttributesPriority MEDIUM 44
- TI-2026-59140 commonmark 2.0.0 before 2.8.4 Denial of Service via XMLPriority LOW 37
Show all 4 records →

Timeline

2026-10-02 Threatist record created from CVE-2026-86434
2026-10-03 Alias added: GHSA-jcmh-chhq-rh2x

References

Advisory 1

GitHub Security Advisory (GHSA-mh25-x5hq-wrqp)

TI-2026-69535 league/commonmark Quadratic DoS via TableStartParser CVSS 4.0HIGH 8.7
TI-2026-50093 league/commonmark: AttributesExtension href/src unsafe-link filter bypass via embedded control bytes CVSS 3.1MEDIUM 6.1
TI-2026-69534 Cross-site scripting in League CommonMark bypasses DisallowedRawHtml CVSS 4.0MEDIUM 5.3
TI-2026-65445 Flysystem: WhitespacePathNormalizer's control-character (CorruptedPathDetected) check is… CVSS 3.1LOW 3.5
TI-2021-12376 Time-of-check Time-of-use (TOCTOU) Race Condition in league/flysystem CVSS 3.1CRITICAL 9.8
TI-2026-59143 commonmark 1.5.0 before 2.8.4 Denial of Service via Footnote CVSS 4.0HIGH 8.7

See all 20 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences