thephpleague commonmark up to 2.8.3 UniqueSlugNormalizer normalize denial of service

CVSS Meta Temp Score
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CTI Interest Score
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6.3$0-$5k0.77+

Summaryinfo

A vulnerability classified as problematic was found in thephpleague commonmark up to 2.8.3. This affects the function UniqueSlugNormalizer::normalize of the component UniqueSlugNormalizer. The manipulation results in denial of service. This vulnerability was named CVE-2026-86434. The attack may be performed from remote. There is no available exploit.

Detailsinfo

A vulnerability was found in thephpleague commonmark up to 2.8.3. It has been classified as problematic. Affected is the function UniqueSlugNormalizer::normalize of the component UniqueSlugNormalizer. The manipulation with an unknown input leads to a denial of service vulnerability. CWE is classifying the issue as CWE-404. The product does not release or incorrectly releases a resource before it is made available for re-use. This is going to have an impact on availability. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability is traded as CVE-2026-86434 since 09/07/2026. The exploitability is told to be easy. It is possible to launch the attack remotely. The exploitation doesn't require any form of authentication. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1499.

Upgrading to version 2.9.0 eliminates this vulnerability.

Once again VulDB remains the best source for vulnerability data.

Productinfo

Vendor

Name

Version

Website

CPE 2.3info

CPE 2.2info

CVSSv4info

VulDB Vector: 🔒
VulDB Reliability: 🔍

CNA CVSS-B Score: 🔒
CNA CVSS-BT Score: 🔒
CNA Vector: 🔒

CVSSv3info

VulDB Meta Base Score: 6.4
VulDB Meta Temp Score: 6.3

VulDB Base Score: 5.3
VulDB Temp Score: 5.1
VulDB Vector: 🔒
VulDB Reliability: 🔍

CNA Base Score: 7.5
CNA Vector (VulnCheck): 🔒

CVSSv2info

AVACAuCIA
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VectorComplexityAuthenticationConfidentialityIntegrityAvailability
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VulDB Base Score: 🔒
VulDB Temp Score: 🔒
VulDB Reliability: 🔍

Exploitinginfo

Class: Denial of service
CWE: CWE-404
CAPEC: 🔒
ATT&CK: 🔒

Physical: No
Local: No
Remote: Yes

Availability: 🔒
Status: Not defined
Price Prediction: 🔍
Current Price Estimation: 🔒

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Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: Upgrade
Status: 🔍

0-Day Time: 🔒

Upgrade: commonmark 2.9.0

Timelineinfo

09/07/2026 Advisory disclosed
09/07/2026 +0 days CVE reserved
09/07/2026 +0 days VulDB entry created
09/07/2026 +0 days VulDB entry last update

Sourcesinfo

Product: github.com

Advisory: github.com
Status: Confirmed

CVE: CVE-2026-86434 (🔒)
GCVE (CVE): GCVE-0-2026-86434
GCVE (VulDB): GCVE-100-399588

Entryinfo

Created: 09/07/2026 15:28
Changes: 09/07/2026 15:28 (77)
Complete: 🔍
Cache ID: 216::103

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