CVE-2026-44897 Mistune Heading ID Attribute Injection XSS
An attacker can inject arbitrary HTML attributes into heading elements via unsanitized ID values, leading to cross-site scripting in applications using Mistune.
- 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-2026-44897?
CVE-2026-44897 is a medium-severity cross-site scripting vulnerability in lepture mistune, published on 26 May 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 3.2.1, HTMLRenderer.heading() builds the opening <hN> tag by string-concatenating the id attribute value directly into the HTML — with no call to escape(), safe_entity(), or any other sanitisation function. A double-quote character " in the id value terminates the attribute, allowing an attacker to inject arbitrary additional attributes (event handlers, src=, href=, etc.) into the heading element. This vulnerability is fixed in 3.2.1.
Weakness: CWE-79
Which versions are affected by CVE-2026-44897?
3 open source packages ship an affected version, including mistune. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
mistune PyPI | All versions | 3.2.1 |
mistune PyPI | All versions | 3.2.1 |
mistune PyPI | All versions | 3.2.1 |
Products include Mistune Project Mistune.
How do I fix CVE-2026-44897?
Upgrade mistune to 3.2.1 or later.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-44897 being exploited?
There is no known exploitation of CVE-2026-44897 in the wild and no public exploit code.
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-88400, CLEANSTART-2026-DO33300, CLEANSTART-2026-QT72309, DEBIAN-CVE-2026-44897, ECHO-b76f-887d-88ac, GHSA-v87v-83h2-53w7 and 2 more.