CVE-2026-86434
Denial-of-service in league/commonmark via slug-collision quadratic CPU growth.
Is CVE-2026-86434 being exploited?
Not confirmed. CVE-2026-86434 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.29% probability of exploitation in the next 30 days.
How severe is CVE-2026-86434?
CVE-2026-86434 is rated High with a CVSS score of 8.7. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-86434?
Yes. A fix has been recorded for CVE-2026-86434. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-86434 affect?
CVE-2026-86434 affects league/commonmark >=2.0.0 and <2.8.4 (patched in 2.9.0), PHP web apps using HeadingPermalinkExtension, FootnoteExtension, or TableOfContentsExtension. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-86434?
Upgrade to league/commonmark >=2.9.0; restrict or sanitize Markdown input and rate-limit rendering if upgrade delayed.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Unauthenticated attacker can submit crafted Markdown causing O(K^2) CPU in slug normalization, leading to CPU exhaustion and service outage (DoS).
Immediate action required
- affected>= 2.0.0 and < 2.8.4
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.