CVE-2026-59928CWE-1333CWE-407

Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions

High · published July 8, 2026

CVSS v3.1
7.5
EPSS
1%
Percentile
50.6
In the wild
Unconfirmed
What it is

Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, a Markdown document containing many repeated or distinct reference-link definitions causes quadratic work in src/mistune/block_parser.py and the ref_links environment dictionary handling, allowing denial of service through CPU exhaustion. This issue is fixed in version 3.3.0.

The record
Technical detail
CVSS v3.1
7.5 · HIGH
Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CVSS v4.0
Not supplied
EPSS
0.00651 · 50.6th percentile
Weaknesses
CWE-1333 · Inefficient Regular Expression Complexity; CWE-407 · Inefficient Algorithmic Complexity
Published
2026-07-08T16:23Z
EPSS history
Timeline
  • 08 JUL 16:23Z
    Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions
    cvelistv5