CVE-2026-86435 in commonmark
Summary
by MITRE • 09/07/2026
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.
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Analysis
by VulDB Data Team • 09/07/2026
The CommonMark library, specifically versions ranging from 1.5.0 up to but not including 2.8.4, contains a critical implementation flaw within its Footnote extension that leads to resource exhaustion through quadratic output expansion. This vulnerability arises because the parser fails to properly deduplicate footnote definitions when processing documents containing multiple references to the same footnote identifier or redundant definition blocks. In standard Markdown parsing logic, footnotes are typically collected and rendered once at the end of a document regardless of how many times they are referenced inline. However, in this affected version range, the internal data structures do not enforce uniqueness constraints on footnote definitions during the rendering phase. Consequently, if an attacker crafts a malicious input file that includes numerous duplicate footnote definitions or excessive references to existing footnotes without proper deduplication logic, the parser will generate redundant output entries for each instance rather than consolidating them into a single representation.
This technical flaw directly results in a Denial of Service condition characterized by exponential growth in memory consumption and CPU utilization relative to the size of the input document. The quadratic expansion occurs because the processing time and memory allocation scale with the square of the number of duplicate entries or references, rather than linearly as expected in well-optimized parsers. For example, a document containing thousands of repeated footnote definitions can cause the server hosting the CommonMark parser to allocate excessive amounts of heap memory and consume significant processor cycles attempting to render these redundant structures. This behavior effectively allows an unauthenticated attacker to trigger a resource exhaustion attack by simply uploading or submitting a specially crafted Markdown file through any application that utilizes this library for rendering user-generated content, such as forums, documentation sites, or collaborative editing platforms.
From a security classification perspective, this vulnerability aligns with CWE-400, which describes Uncontrolled Resource Consumption, and more specifically relates to the mechanism of Denial of Service via resource exhaustion due to inefficient algorithmic complexity in input processing. In terms of adversary tactics, this flaw can be leveraged within the MITRE ATT&CK framework under the technique T1496, Resource Hijacking, where an attacker uses compromised or untrusted resources to perform computationally intensive tasks that degrade service availability for legitimate users. The impact is primarily operational rather than confidentiality-related, as it does not involve data leakage but severely impairs system stability and responsiveness.
To mitigate this vulnerability, organizations must immediately upgrade the CommonMark library to version 2.8.4 or later, where the footnote extension has been patched to enforce strict deduplication of definitions before rendering. Additionally, defensive measures should include implementing input validation limits on document size and complexity, particularly restricting the number of allowed footnotes per document. Deploying rate limiting and resource quotas at the application gateway level can also help absorb such attacks by terminating connections that exhibit abnormal processing times or memory usage patterns associated with quadratic expansion exploits. Regular security audits of third-party dependencies are essential to ensure that similar algorithmic inefficiencies in other parsing libraries do not present comparable risks.