CVE-2026-59930 Mistune toc / TableOfContents directive: heading IDs use predictable `toc_N` numbering with no slugification, allowing collision with attacker-controlled `id="toc_N"` content
Predictable heading IDs in Mistune allow attackers to collide anchors, potentially redirecting navigation or bypassing security checks.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-59930?
CVE-2026-59930 is a medium-severity vulnerability in lepture mistune, published on 8 Jul 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
Mistune is a Python Markdown parser with renderers and plugins. Prior to 3.3.0, the toc plugin and TableOfContents directive generate heading IDs as predictable toc_N values without slugifying the heading text, allowing attacker-controlled id="toc_N" content to collide with generated anchors and redirect same-page navigation, CSS selectors, or JavaScript handlers. This issue is fixed in version 3.3.0.
Which versions are affected by CVE-2026-59930?
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-2026-59930?
Upgrade mistune to 3.3.0 or later.
Is CVE-2026-59930 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-59930 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-2026-59928 Mistune block_parser: quadratic-time parsing on long lists of repeated reference-link definitions
- 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
Sources
Also known as AZL-92223, DEBIAN-CVE-2026-59930, ECHO-1288-ebec-bb75, GHSA-2hm2-hc3v-44h9, PYSEC-2026-2218, UBUNTU-CVE-2026-59930.