HTML Sanitization Bypass Allows Cross-Site Scripting

justhtml before 1.16.0 contains multiple HTML sanitization bypass issues that can allow active/dangerous content (e.g., script or style) to survive sanitization, potentially leading to cross-site scripting. The issues primarily affect advanced usage rather than the default JustHTML(..., sanitize=True) path for ordinary parsed HTML: mutating or reusing sanitization policy objects (including exported defaults) could weaken later sanitization; programmatic DOM input to sanitize()/sanitize_dom() could miss mixed-case tag names (e.g., ScRiPt, StYlE); crafted programmatic doctype names could serialize into active markup; and custom policies preserving SVG or MathML could allow animation elements, presentation attributes with external url(...) references, or DOM trees mislabeled as namespace="html" to bypass foreign-content checks. Fixed in 1.16.0.

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-2026-7808 is a critical vulnerability in justhtml before version 1.16.0 that permits dangerous HTML content—including scripts and styles—to bypass sanitization filters, enabling cross-site scripting (XSS) attacks. While the default sanitization path remains relatively secure, advanced usage patterns create exploitable weaknesses: reusing or mutating sanitization policy objects (including exported defaults) and programmatic DOM input to sanitization functions can all weaken protections. This affects developers using justhtml in web applications, content management systems, and any platform that processes user-supplied HTML, making it particularly dangerous in multi-tenant environments or applications handling untrusted input.

Casky's 322 mapped security skills leverage Claude's extended reasoning to detect the attack patterns underlying this vulnerability by correlating MITRE ATT&CK techniques TA0001 (Initial Access) and TA0043 (Reconnaissance). Practitioners using Casky would observe findings that identify: (1) DOM-based XSS execution chains where attacker-controlled HTML reaches browser context through weakened sanitization, (2) policy object mutation patterns that progressively degrade security posture across multiple sanitization calls, and (3) script/style injection attempts that succeed due to incomplete tag filtering. The platform's reasoning engine connects these indicators to programmatic flow analysis, helping teams pinpoint whether their justhtml implementation uses mutable policies or direct DOM sanitization paths—the exact conditions that activate this CVE's dangerous behavior.

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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-2026-7808.

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

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