CVE-2026-5389: Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS) in EmilStenstrom justhtml
Description
justhtml versions before 1.13.0 have a cross-site scripting (XSS) vulnerability in the to_markdown() function. This flaw allows attackers to inject backticks inside sanitized pre elements, breaking out of fixed-length code fences and enabling execution of raw HTML when rendered by CommonMark or GitHub Flavored Markdown (GFM) renderers.
CVSS v4.0
Score 5.1medium
Affected software
EmilStenstrom
justhtml
pkg:github/emilstenstrom/justhtmlRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
CVE-2026-5389 describes a basic cross-site scripting vulnerability in EmilStenstrom's justhtml library versions prior to 1.13.0. The vulnerability arises in the to_markdown() function during serialization of attacker-controlled pre content. By injecting backticks within sanitized pre elements, an attacker can break out of the intended fixed-length code fences, causing raw HTML to be executed when the Markdown output is rendered by CommonMark or GFM-style renderers.
Potential Impact
Successful exploitation can lead to execution of arbitrary HTML code in the context of the victim's browser when viewing rendered Markdown, potentially enabling XSS attacks. The CVSS 4.0 score is 5.1 (medium severity), indicating that the vulnerability is remotely exploitable without privileges or user interaction, but requires user interface interaction and has limited scope and impact.
Mitigation Recommendations
Upgrade justhtml to version 1.13.0 or later, where this vulnerability is fixed. No other mitigation details are provided. Patch status is confirmed by the version boundary indicating fixed in 1.13.0.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- VulnCheck
- Date Reserved
- 2026-04-01T21:14:36.435Z
- Cvss Version
- 4.0
- State
- PUBLISHED
Threat ID: 6a8afb27acd9273b49f5f723
Added to database: 08/23/2026, 13:52:39 UTC
Last enriched: 09/10/2026, 23:32:47 UTC
Last updated: 10/07/2026, 06:48:17 UTC
Views: 51
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