CVE-2026-86434 thephpleague CVE debrief

Who should care

Defenders of systems using league/commonmark versions >= 2.0.0 and < 2.8.4, especially those using HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension, should assess exposure and prioritize verification and updates.

Why it matters

Defenders should prioritize verifying and updating to version 2.9.0 or later, especially for systems using HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension, as the vulnerability can cause denial of service via excessive CPU consumption.

• Denial of service via excessive CPU consumption
• Potential for unauthenticated attackers to disrupt service

Technical summary

The vulnerability exists in the UniqueSlugNormalizer::normalize() method, 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. This can cause denial of service via excessive CPU consumption when many headings are forced onto a single base slug, especially when HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension is registered. Defenders should prioritize verifying and updating to version 2.9.0 or later.

Defensive priority

Defenders should prioritize verifying and updating to version 2.9.0 or later, especially for systems using HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension.

Recommended defensive actions

• Verify and update to version 2.9.0 or later
• Review and limit usage of HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension
• Monitor for excessive CPU consumption
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The vulnerability is caused by the UniqueSlugNormalizer::normalize() method restarting 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.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.