---
id: "CVE-2026-115348"
kind: "vulnerability"
aliases: ["CVE-2026-115348"]
title: "Mistune: crafted Markdown with unbalanced brackets exhausts server CPU"
severity: "high"
cvss: 8.7
epss: 0.00359
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["markdown-rendering", "mistune", "package", "python", "self-hosted"]
not_affected_if: ["mistune-not-installed", "no-user-markdown-input", "version-3.3.0-or-later"]
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-2026-115348", "first_seen_at": "2026-10-23T21:57:31+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-115348", "first_seen_at": "2026-10-24T07:15:51+00:00"}]
---

# Mistune: crafted Markdown with unbalanced brackets exhausts server CPU

Mistune, a Python Markdown parser, has an algorithmic complexity flaw in its inline link parser. An unauthenticated attacker can send Markdown with many unbalanced '[' or '![' characters to tie up worker processes for minutes.

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

## How to check

pip show mistune (or check the installed version in your lockfile) and see whether it is between 3.0.0 and 3.2.x.

## 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-2026-115348
- 2026-10-24T07:15:51+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-115348

## From the sources

Mistune from 3.0.0 before 3.3.0 contains an algorithmic complexity vulnerability in the inline link parser's parse_link_text function that allows unauthenticated attackers to exhaust CPU with unbalanced brackets. Attackers can submit Markdown containing many unbalanced '[' or '![' characters, forcing repeated unbounded forward scans and tying up worker processes for minutes.

---

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