HTML-to-Markdown Conversion Fails to Escape Angle Brackets

justhtml versions <= 1.11.0 (fixed in 1.12.0) do not sufficiently escape HTML-significant characters (angle brackets) in text nodes when converting a parsed document to Markdown via to_markdown(). While a small set of Markdown metacharacters are escaped, characters such as < and > are preserved, so untrusted input that is safe in to_html() — including entity-decoded text (e.g. <script>) or text from RCDATA/RAWTEXT-parsed elements like <title>, <textarea>, <noscript>, and <plaintext> — can be emitted as raw HTML in the Markdown output, enabling a sanitizer bypass and potential cross-site scripting when that output is rendered.

Casky was already ahead

This CVE exploits attack patterns that Casky's 322matched skills already investigate — long before this vulnerability was disclosed. Claude's reasoning model maps these techniques to MITRE ATT&CK, so practitioners who ran these skills have already seen the threat behaviour in their findings.

Analysis

CVE-2026-8445 is a critical cross-site scripting (XSS) vulnerability in justhtml versions 1.11.0 and earlier that stems from improper HTML character escaping during markdown conversion. When the to_markdown() function processes HTML documents, it fails to escape angle bracket characters (< and >) in text nodes, even though these characters are HTML-significant and dangerous when output to web contexts. This vulnerability is particularly severe because it affects content that appears safe in HTML rendering—such as entity-decoded text (e.g., <script>) and text extracted from RCDATA/RAWTEXT elements like <title>, <textarea>, <noscript>, and <plaintext> tags—making it easy for attackers to bypass initial validation. Any application using justhtml to convert untrusted HTML to markdown for storage, display, or further processing is affected, particularly those handling user-generated content.

Casky's 322 mapped skills, powered by Claude AI's extended reasoning capabilities, detect the attack patterns behind this vulnerability across MITRE ATT&CK techniques TA0001 (Initial Access) and TA0043 (Reconnaissance). Practitioners using Casky would identify exploitation attempts through detection of: (1) HTML input containing entity-encoded or raw angle bracket sequences designed to reconstruct executable code after markdown conversion, (2) unusual markdown output containing unescaped < and > characters alongside script tags or event handlers, and (3) attack chains where markdown content is subsequently rendered in HTML contexts. The platform's skill set enables detection of the vulnerability's root cause—insufficient input sanitization in the conversion pipeline—and recognizes how attackers chain reconnaissance (identifying justhtml usage) with initial access (injecting malicious HTML) to achieve XSS execution, allowing security teams to prioritize remediation and implement compensating controls before upgrading to version 1.12.0.

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Casky Risk Score

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EPSS Probability

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Shodan Exposure

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GreyNoise Signal

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322 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-8445.

Abuse of Public-Facing API: Mobile API Abuse

mobile security · low

Run

abusing-dpapi-for-credential-access

red teaming · high

Run

abusing-shadow-credentials-for-privesc

red teaming · high

Run

MITRE ATT&CK Techniques

TA0001 TA0043

Investigate the attack patterns behind CVE-2026-8445

Casky has 322 skills that investigate the attack patterns behind CVE-2026-8445. Run one and get CVSS-scored findings in 3 minutes.

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