CVE-2026-77088 justhtml 0.9.0 through 1.21.0 Cross-Site Scripting via code-span
Attackers can inject blank lines into code spans to bypass sanitization, causing unescaped HTML to be rendered as live Markdown, leading to XSS.
- 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-2026-77088?
CVE-2026-77088 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 0.9.0 through 1.21.0 contain a cross-site scripting vulnerability in to_markdown() where inline code spans fail to account for blank lines as block boundaries. Attackers can inject blank lines into code or pre element text to break the inline span, causing sanitized HTML to be emitted unescaped and re-parsed as live Markdown by compliant renderers.
Weakness: CWE-79
Which versions are affected by CVE-2026-77088?
2 open source packages ship an affected version, including justhtml.
| Package | Affected | Fixed in |
|---|---|---|
justhtml PyPI | From 0.9.0 | 1.22.0 |
justhtml PyPI | From 0.9.0 | 1.22.0 |
How do I fix CVE-2026-77088?
Upgrade justhtml to 1.22.0 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-2026-77088 being exploited?
There is no known exploitation of CVE-2026-77088 in the wild and no public exploit code.
Sources
Also known as GHSA-3r4x-v5v7-fm2r, GHSA-jf6w-2mvx-633j.