CVE-2026-86435: commonmark 1.5.0 before 2.8.4 Denial of Service via Footnote
commonmark versions from 1.5.0 before 2.8.4 contain a denial of service vulnerability in the Footnote extension that fails to deduplicate footnote definitions. Attackers can craft documents with duplicate footnote definitions and references to create quadratic output expansion, consuming excessive memory and CPU to exhaust server resources.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
commonmark/Footnote extensionto a version that resolves this vulnerability.Fixed in 2.8.4 - Compensating control
Apply an update for the CommonMark Footnote extension vulnerability where versions from 1.5.0 before 2.8.4 fail to deduplicate footnote definitions; ensure the Footnote extension is not used with CommonMark versions in the affected range (1.5.0 through 2.8.3).
Event History
Frequently Asked Questions
Which deployments are exposed?
Deployments using commonmark versions from 1.5.0 through versions before 2.8.4 are affected when the Footnote extension processes attacker-controlled documents.
What does an attacker need to do to trigger the issue?
An attacker needs to submit or otherwise cause processing of a document containing duplicate footnote definitions and references. No authentication or user interaction is required according to the supplied CVSS vector.
How can I determine whether my service is at risk?
Check the installed commonmark version and whether the Footnote extension is used to render untrusted or externally supplied documents. Versions before 2.8.4 within the stated affected range can produce quadratic output expansion from duplicate footnotes.
What is the remediation?
Upgrade commonmark to version 2.8.4 or later. The vulnerability can exhaust CPU and memory, causing a denial of service.