CVE-2026-59928 Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions
An attacker can cause CPU exhaustion and denial of service by submitting Markdown documents with many repeated reference-link definitions to applications using Mistune.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-59928?
CVE-2026-59928 is a high-severity denial of service vulnerability in lepture mistune, published on 8 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Which versions are affected by CVE-2026-59928?
3 open source packages ship an affected version, including mistune. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
mistune PyPI | All versions | 3.3.0 |
mistune PyPI | All versions | 3.3.0 |
mistune PyPI | All versions | 3.3.0 |
Products include Mistune Project Mistune.
How do I fix CVE-2026-59928?
Upgrade mistune to 3.3.0 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-59928 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-59928 exists.
Other vulnerabilities in lepture mistune
- CVE-2026-59922 Mistune plugins/formatting: quadratic-time parsing on long runs of `~~x~~`, `==x==`, and `^^x^^` markers (strikethrough / mark / insert)
- CVE-2026-59925 inline_parser: quadratic-time parsing on long runs of `**x**` and `***x***` emphasis pairs
- CVE-2026-59923 Mistune: XSS via percent-encoded javascript URI bypass in safe_url()
- CVE-2026-59929 Mistune renderers/html.safe_url: HARMFUL_PROTOCOLS list misses legacy and chained schemes that historically chain to `javascript:` execution
- CVE-2026-44708 Mistune Math Plugin XSS Escape Bypass
Sources
Also known as AZL-92211, DEBIAN-CVE-2026-59928, ECHO-059a-fd72-cb55, GHSA-ffq3-xpv3-j92q, PYSEC-2026-2216, UBUNTU-CVE-2026-59928.