---
id: "CVE-2026-115349"
kind: "vulnerability"
aliases: ["CVE-2026-115349"]
title: "Mistune: nested square brackets cause quadratic parsing, exhausting server CPU"
severity: "high"
cvss: 8.7
epss: 0.00343
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["markdown-rendering", "mistune", "package", "python"]
not_affected_if: ["mistune-3.3.0-or-later", "mistune-below-3.0.0"]
published_at: "2026-10-22T19:17:40+00:00"
first_seen_at: "2026-10-23T21:57:31+00:00"
updated_at: "2026-10-24T07:15:51+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-115349", "first_seen_at": "2026-10-23T21:57:31+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-115349", "first_seen_at": "2026-10-24T07:15:51+00:00"}]
---

# Mistune: nested square brackets cause quadratic parsing, exhausting server CPU

Mistune's parse_link_text function in helpers.py takes quadratic time on markdown with deeply nested square brackets. A small crafted request can tie up server CPU and cause denial of service.

_Summary, check and fix written by a model from the sources; labelled as such._

## Fix

Upgrade mistune to 3.3.0 or later.

## Timeline

- 2026-10-23T21:57:31+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-115349
- 2026-10-24T07:15:51+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-115349

## From the sources

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity denial of service vulnerability in the parse_link_text function in src/mistune/helpers.py. Attackers can submit markdown with deeply nested square brackets triggering quadratic parsing time, exhausting server CPU with small crafted requests.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_