HTML Sanitization Bypass in justhtml Library

justhtml before 1.15.0 contains multiple security issues in URL sanitization helpers (clean_url_value/clean_url_in_js_string), HTML serialization, Markdown passthrough (html_passthrough=True), and several custom sanitization-policy edge cases. Depending on configuration, an attacker can bypass sanitization to inject active HTML and JavaScript — for example via encoded javascript: URLs, backslash-based relative URLs resolved as remote hosts, markup-breaking programmatic element/attribute names or HTML comments, raw </textarea> reintroduction through Markdown passthrough, or preserved <style>/<meta http-equiv=refresh>/<base href> tags in custom policies. Most custom-policy issues do not affect the default sanitize=True configuration; they primarily affect helper APIs, programmatic DOM construction, html_passthrough=True, and custom policies/transform pipelines.

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-2026-5388 is a critical vulnerability (CVSS 9.8) affecting justhtml before version 1.15.0, where multiple URL sanitization functions fail to properly validate and encode user input. The vulnerability exists in core sanitization helpers like clean_url_value and clean_url_in_js_string, as well as in HTML serialization logic and Markdown passthrough features. An attacker can exploit configuration-dependent edge cases to inject active HTML and JavaScript payloads—including obfuscated javascript: protocol URLs, backslash-encoded relative URLs that resolve to remote hosts, and markup-breaking element or attribute names. This directly impacts any organization using justhtml to sanitize user-generated content in web applications, content management systems, or rich-text editors, potentially enabling cross-site scripting (XSS), data theft, session hijacking, and malware distribution.

Casky's 322 mapped skills leverage Claude's extended reasoning to detect the attack patterns behind this vulnerability by analyzing both TA0001 (Initial Access) and TA0043 (Reconnaissance) MITRE ATT&CK techniques. Practitioners using Casky would identify suspicious findings around input validation failures—detection of encoded or obfuscated protocol handlers (javascript:, data:, vbscript:), unusual URL encoding patterns, and HTML/JavaScript injection attempts in sanitized fields. The platform's skills would flag malformed markup designed to break out of attribute contexts, relative path traversal attempts using backslash encoding, and markdown-to-HTML conversion bypasses. Practitioners would see detailed findings correlating these indicators to CWE-20 (Improper Input Validation), enabling them to trace attack chains from reconnaissance through payload injection and execution.

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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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Composite risk scoring from EPSS, CISA KEV, Shodan, and GreyNoise — 21 security APIs correlated into a single Casky Risk Score. Coming in Casky Pro. Join early access →

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-2026-5388.

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-2026-5388

Casky has 322 skills that investigate the attack patterns behind CVE-2026-5388. Run one and get CVSS-scored findings in 3 minutes.

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