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Vulnerability · Published Jun 24, 2026 · Assigned by GitHub_M

CVE-2026-49851Mistune: Potential DoS via quadratic-time parsing in parse_link_text

High 7.5

In plain English

A high-severity uncontrolled resource consumption in mistune from lepture. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 44% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.5High
Exploit odds (30 days)0.5%top 55.6% of all CVEs
Exploited in the wildNo reportsCISA SSVC: poc
Attack toolingNoneno public tooling known

Official description

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, Mistune is vulnerable to a CPU exhaustion DoS due to superlinear (approximately O(n²)) behavior in parse_link_text. When parsing Markdown containing many consecutive [ characters, parse_link_text repeatedly scans the input using a regex search inside a loop. Each iteration re-scans a large portion of the remaining string, resulting in quadratic-time behavior. An attacker-controlled Markdown input can therefore trigger excessive CPU usage with a very small payload. This vulnerability is fixed in 3.3.0.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Debian:13mistuneno fix listedDEBIAN-CVE-2026-49851
Debian:14mistuneno fix listedDEBIAN-CVE-2026-49851
PyPImistune3.3.0GHSA-qcq2-496w-v96p
PyPImistune3.3.0PYSEC-2026-2652
Ubuntu:22.04:LTSmistuneno fix listedUBUNTU-CVE-2026-49851
Ubuntu:24.04:LTSmistuneno fix listedUBUNTU-CVE-2026-49851
Ubuntu:25.10mistuneno fix listedUBUNTU-CVE-2026-49851
Ubuntu:26.04:LTSmistuneno fix listedUBUNTU-CVE-2026-49851
Ubuntu:26.10mistuneno fix listedUBUNTU-CVE-2026-49851

Affected products (as reported)

VendorProductVersions
lepture mistune < 3.3.0

Weakness: CWE-400, CWE-770, CWE-407, CWE-1333 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References