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

Quick assessment

Affected
EmilStenstrom justhtml
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

justhtml 1.13.0 之前的版本中存在跨站脚本（XSS）漏洞。在调用 to_markdown() 函数序列化用户可控的 <pre> 内容时，攻击者可以在已处理的 <pre> 元素中插入反引号（backticks），从而突破固定长度的代码围栏（code fences）。当生成的 Markdown 被 CommonMark 或 GitHub Flavored Markdown (GFM) 风格的渲染器渲染时，会导致原始 HTML 被执行。

CVSS 6.1 · Medium

Affected Version Matrix 2

VendorProduct Version RangeStatus
EmilStenstrom justhtml < 1.13.0 affected
1.13.0 unaffected
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I. Basic Information for CVE-2026-5389

Vulnerability Information
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Vulnerability Title
justhtml before 1.13.0 XSS via code fence breakout
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
Web页面中脚本相关HTML标签转义处理不恰当(基本跨站脚本)
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
EmilStenstrom justhtml 0 ~ 1.13.0 -

II. Public POCs for CVE-2026-5389

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III. Intelligence Information for CVE-2026-5389

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Vendor Advisories for CVE-2026-5389 (2)

Same Patch Batch · EmilStenstrom · 2026-08-23 · 11 CVEs total

CVE-2026-7808 9.8 CRITICAL justhtml before 1.16.0 Multiple Security Issues via Sanitization
CVE-2026-8445 9.8 CRITICAL justhtml before 1.12.0 Sanitizer Bypass via Markdown
CVE-2026-5388 9.8 CRITICAL justhtml before 1.15.0 Multiple Security Issues
CVE-2026-9769 7.5 HIGH justhtml before 1.10.0 Denial of Service via deeply nested HTML
CVE-2026-4671 7.5 HIGH justhtml before 1.18.0 Denial of Service via CSS Selector
CVE-2026-77088 6.1 MEDIUM justhtml 0.9.0 through 1.21.0 Cross-Site Scripting via code-span
CVE-2026-74793 6.1 MEDIUM justhtml before 3.11.0 XSS via selectedcontent projection
CVE-2026-6827 6.1 MEDIUM justhtml before 1.17.0 Multiple Cross-Site Scripting Vulnerabilities
CVE-2026-8630 6.1 MEDIUM justhtml before 1.12.0 Mutation XSS via Raw Text Elements
CVE-2026-5751 6.1 MEDIUM justhtml before 1.14.0 Mutation XSS via custom sanitization policies

IV. Related Vulnerabilities

Same product: justhtml
Same vendor: EmilStenstrom
Same weakness: CWE-80

V. Comments for CVE-2026-5389

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