Mistune Markdown Parser Denial-of-Service Vulnerability

A denial-of-service vulnerability has been identified in Mistune version 3.2.0, a Python Markdown parser. This vulnerability stems from the parse_link_title() function within helpers.py, which is susceptible to excessive backtracking and parsing loops when processing malformed reference links. An attacker can exploit this by providing specially crafted Markdown input that causes the application to consume excessive CPU resources, leading to application hangs and service unavailability. Publicly available PoC exploit code demonstrates the vulnerability. This poses a significant threat to applications that rely on Mistune to parse untrusted Markdown content, such as web applications and APIs.

Attack Chain

1. An attacker crafts a malicious Markdown document containing specially crafted reference links with excessive escape character sequences.
2. The attacker submits the malicious Markdown document to a web application or API that uses Mistune for Markdown parsing.
3. The application calls the mistune.html() function to render the Markdown content into HTML.
4. Within mistune.html(), the parse method in mistune/markdown.py is invoked.
5. The parse_ref_link function in mistune/block_parser.py is called to process the reference links.
6. The parse_link_title function in mistune/helpers.py is then called to parse the link title.
7. Due to the malformed reference link structure, parse_link_title enters an excessive parsing loop with significant backtracking.
8. The excessive parsing consumes CPU resources, eventually leading to a denial-of-service condition as the application hangs and becomes unresponsive.

Impact

Successful exploitation of this vulnerability results in a denial-of-service (DoS) condition. Specifically, the targeted application experiences high CPU usage and ultimately hangs, rendering it unavailable to legitimate users. This can disrupt services, cause financial losses, and damage the reputation of organizations that rely on the affected application. The vulnerability impacts any application using Mistune 3.2.0 to parse untrusted markdown, including web applications and APIs.

Recommendation

• Apply mitigations suggested by the vendor, including parsing depth and iteration limits within parse_link_title().
• Implement input validation to limit reference-link title length, mitigating the impact of excessively long titles.
• Deploy the Sigma rule Mistune_DOS_Process_CPU_Spike to detect processes exhibiting high CPU usage during Markdown parsing.
• Deploy the Sigma rule Mistune_DOS_Request_Pattern to detect suspicious request patterns indicative of the exploit being attempted.
• Monitor web server logs for suspicious requests containing excessive escape character sequences indicative of the provided PoC exploit.