CVE-2026-5389 justhtml before 1.13.0 XSS via code fence breakout
Attackers can inject raw HTML into sanitized content via code fence breakout, leading to XSS when rendered by Markdown parsers.
- 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
- None listed
What is CVE-2026-5389?
CVE-2026-5389 is a medium-severity cross-site scripting vulnerability in EmilStenstrom justhtml, published on 23 Aug 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
justhtml versions before 1.13.0 contain a cross-site scripting vulnerability in the to_markdown() function when serializing attacker-controlled pre content. Attackers can place backticks inside sanitized pre elements to break out of fixed-length code fences, allowing raw HTML to execute when the generated Markdown is rendered by CommonMark or GFM-style renderers.
Weakness: CWE-80
Which versions are affected by CVE-2026-5389?
2 open source packages ship an affected version, including justhtml.
| Package | Affected | Fixed in |
|---|---|---|
justhtml PyPI | All versions | 1.13.0 |
justhtml PyPI | All versions | 1.13.0 |
How do I fix CVE-2026-5389?
Upgrade justhtml to 1.13.0 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-2026-5389 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-5389 exists.
Sources
Also known as GHSA-5vp3-3cg6-2rq3, GHSA-q3jf-9wv7-p29w.