# CVE-2026-66370: html_sanitize_ex HTML5 scrubber keeps attacker-supplied form-association attributes, allowing form hijacking

> An attacker can hijack existing forms in sanitized HTML to steal submitted data, including credentials, by injecting form-association attributes.

- **Severity:** Medium 4.8 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 6 Aug 2026
- **Updated:** 19 Aug 2026
- **Also known as:** EEF-CVE-2026-66370, GHSA-r8xc-fmjg-37c5, GHSA-w3f9-jjhw-wwvq

## What is CVE-2026-66370?

CVE-2026-66370 is a medium-severity vulnerability in rrrene html_sanitize_ex, published on 6 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official 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.

Weakness: CWE-601 (URL Redirection to Untrusted Site)

## Which versions are affected by CVE-2026-66370?

1 open source package ship an affected version, including html_sanitize_ex. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `html_sanitize_ex` | Hex | From 0.3.1 | 1.4.5, 1.5.3 |

Products: Rrrene Htmlsanitizeex.

## How do I fix CVE-2026-66370?

Upgrade html_sanitize_ex to a fixed version for your release line: 1.4.5 or 1.5.3.

## Is CVE-2026-66370 being exploited?

There is no known exploitation of CVE-2026-66370 in the wild and no public exploit code.

## Other vulnerabilities in rrrene html_sanitize_ex

- [CVE-2026-68749](https://insights.sbomify.com/CVE-2026-68749/): Quadratic regex backtracking in the html_sanitize_ex CSS scrubber allows CPU-exhaustion denial of service (high 8.2)
- [CVE-2026-68750](https://insights.sbomify.com/CVE-2026-68750/): Quadratic sibling re-flattening in the html_sanitize_ex traversal engine allows CPU-exhaustion denial of service (high 8.2)
- [CVE-2026-66829](https://insights.sbomify.com/CVE-2026-66829/): html_sanitize_ex HTML5 scrubber keeps attacker-supplied meta refresh, allowing forced cross-origin redirection (low 2.3)
- [CVE-2026-66843](https://insights.sbomify.com/CVE-2026-66843/): html_sanitize_ex HTML5 scrubber keeps attacker-supplied `<object>` elements, allowing untrusted content embedding (low 2.3)
- [CVE-2026-68747](https://insights.sbomify.com/CVE-2026-68747/): CSS sanitizer allowlist bypass in html_sanitize_ex via non-declaration input (low 2.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-66370 and fix it if it is affected.

What it is: CVE-2026-66370 is a medium-severity vulnerability in rrrene html_sanitize_ex, published on 6 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-66370 in the wild and no public exploit code.

Affected packages and fixed versions:
- html_sanitize_ex (Hex): affected from 0.3.1; fixed in 1.4.5, 1.5.3

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-66370/ (Markdown: https://insights.sbomify.com/CVE-2026-66370/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-66370
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-66370
- OSV: https://osv.dev/vulnerability/CVE-2026-66370

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-66370/
Summaries are generated and may contain mistakes; the official description above is the record.