CVE-2026-88753 in markdown2
Summary
by MITRE • 10/15/2026
A Regular-Expression Denial of Service (ReDoS) in the Markdown2. extract_metadata() function (lib/markdown2.py) of trentm markdown2 through v2.5.5 allows attackers to cause a Denial of Service (DoS) via a crafted Markdown document.
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified as CVE-2024-39687 represents a significant security risk within the Python-based Markdown processing library known as markdown2, specifically affecting versions up through 2.5.5. This flaw is categorized as a Regular Expression Denial of Service, commonly referred to as ReDoS, which targets the extract_metadata function located in the lib/markdown.py module. The core issue stems from an inefficient implementation of regular expressions used for parsing metadata headers within Markdown documents. When processing input that contains specific patterns designed to exploit backtracking vulnerabilities in the regex engine, the application can be forced into a state of excessive computational consumption. This behavior allows remote attackers to craft malicious Markdown files that trigger catastrophic performance degradation or complete system unavailability without requiring any form of authentication or privileged access.
From a technical perspective, ReDoS attacks leverage the way certain regular expression engines handle complex patterns with nested quantifiers or ambiguous alternations. In this specific instance, the extract_metadata function likely employs regex constructs that do not properly constrain their matching scope or fail to utilize atomic grouping and possessive quantifiers where appropriate. When an attacker provides a specially crafted string containing sequences of characters that match multiple potential paths within the regular expression logic, the engine may engage in exponential backtracking. This process consumes CPU resources at a rate proportional to the length of the input data squared or exponentially, depending on the specific regex structure involved. Consequently, even moderately sized malicious documents can cause the processing thread to hang indefinitely or consume all available system memory and processor cycles, effectively rendering the service unable to handle legitimate requests.
The operational impact of this vulnerability is severe for any application that relies on markdown2 to process user-supplied content before displaying it or storing it in a database. Services such as blog platforms, documentation sites, issue trackers, or collaborative editing tools are particularly susceptible if they do not implement strict input validation or resource limits around the parsing step. An attacker can exploit this flaw by submitting a crafted Markdown document through any interface that accepts text input for processing by markdown2. The resulting denial of service disrupts availability, which is one of the three pillars of the CIA triad in information security. For high-traffic applications, even a small number of concurrent exploitation attempts could lead to widespread outages, causing significant financial loss and reputational damage due to perceived unreliability or lack of security maturity.
To mitigate this risk, organizations must prioritize upgrading the markdown2 library to version 2.5.6 or later, where the developers have addressed the inefficient regular expression patterns responsible for the backtracking issue. In addition to patching, it is advisable to implement defense-in-depth strategies such as setting strict timeouts on regex execution using libraries like re2 if available, or wrapping the parsing logic in a timeout mechanism that kills long-running processes. Input validation should also be enforced to reject excessively large metadata blocks or suspicious character sequences before they reach the parser. Furthermore, monitoring system resources for unusual spikes in CPU usage during markdown processing can help detect ongoing exploitation attempts in real-time.
This vulnerability aligns with Common Weakness Enumeration (CWE) ID 133, which describes Regular Expression Denial of Service attacks involving inefficient regex patterns that lead to resource exhaustion. It also maps directly to the MITRE ATT&CK framework under technique T1496, specifically Resource Hijacking via ReDoS, where attackers use computational complexity to degrade service availability. Understanding these classifications helps security teams contextualize the threat within broader industry standards and ensures that remediation efforts address both the immediate technical flaw and the underlying architectural weaknesses in input handling procedures. By addressing this issue promptly, developers can restore confidence in their markdown processing pipelines and prevent potential disruptions caused by malicious actors exploiting regex inefficiencies.