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

Severity: medium (CVSS 6.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-08-06 · updated 2026-10-24
Weakness: CWE-601

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.

Affected products

rrrene html_sanitize_ex: from 0.3.1 before 1.5.3 — fixed in 1.5.3

How to fix

Upgrade to a fixed version: rrrene html_sanitize_ex 1.5.3. Check the vendor advisory for mitigations.

References

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