Vulnerability · Published Aug 23, 2026 · Assigned by VulnCheck

CVE-2026-5389justhtml before 1.13.0 XSS via code fence breakout

Medium 6.1

In plain English

A medium-severity security flaw in justhtml from EmilStenstrom. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 14% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)6.1Medium

Exploit odds (30 days)0.2%top 86.1% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

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.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
PyPI	justhtml	1.13.0	GHSA-5vp3-3cg6-2rq3

Affected products (as reported)

Vendor	Product	Versions
EmilStenstrom	justhtml	0 – before 1.13.0

Weakness: CWE-80 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

References

GitHub Security Advisory (GHSA-5vp3-3cg6-2rq3) vendor-advisory
VulnCheck Advisory: justhtml before 1.13.0 XSS via code fence breakout third-party-advisory