What’s Vulnerable › Vulnerabilities

CVE-2026-5388: justhtml before 1.15.0 Multiple Security Issues

• Severity: critical (CVSS 9.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-23 · updated 2026-10-12
• Weakness: CWE-20

Description

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.

Affected products

• EmilStenstrom justhtml: before 1.15.0 — fixed in 1.15.0

How to fix

Upgrade to a fixed version: EmilStenstrom justhtml 1.15.0. Check the vendor advisory for mitigations.

References

• https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-c9vm-hv86-f23r
• https://www.vulncheck.com/advisories/justhtml-before-multiple-security-issues

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