CVE-2026-116695: Crass: Incorrect tokenization of bad URLs enables CSS sanitizer bypass

Q: Which applications are exposed to this issue?

Applications are exposed if they accept untrusted CSS, parse it with Crass before version 1.0.8, and reserialize or otherwise accept the parsed output without rejecting malformed bad URL tokens. Using Crass alone is not sufficient to make an application vulnerable.

Q: What does an attacker need to exploit the bypass?

An attacker needs a way to supply CSS that is processed by the affected sanitizer. The crafted CSS uses a bad URL containing a closing parenthesis after an escaped backslash, causing Crass and the browser to interpret the remaining content differently.

Q: Is script execution possible through this issue?

No. The issue does not enable script execution in modern browsers, but it can permit CSS-based UI redress, external resource loading, or CSS-based data exfiltration.

Q: What should teams do if they cannot immediately upgrade?

Reject untrusted CSS containing malformed bad URL constructs rather than parsing and reserializing them as acceptable output. The documented fix is to upgrade Crass to version 1.0.8.

Published Oct 27, 2026

Updated

Crass is a Ruby CSS parser that is compliant with the CSS Syntax Level 3 specification. Prior to 1.0.8, Crass incorrectly treats a closing parenthesis after an escaped backslash as escaped while consuming the remnants of a bad URL. Crass therefore keeps declarations, style rules, or at-rules inside a bad URL token even though a browser treats them as live CSS. A sanitizer that parses untrusted CSS with Crass and reserializes it without rejecting the bad URL can allow CSS used for UI redress, external resource loading, or CSS-based data exfiltration, but this issue does not enable script execution in modern browsers. This issue is fixed in version 1.0.8.

Affected Software

1 affected component

rubygems/crass<1.0.8

Remediation

Recommended actions to resolve this vulnerability, in priority order.

Upgrade
Upgrade Crass to a version that resolves this vulnerability.
Fixed in 1.0.8

Event History

Oct 27, 2026

CVE Published

via MITRE·08:10 PM

Data Sourced

via MITRE·08:10 PM

DescriptionWeakness

Frequently Asked Questions

Which applications are exposed to this issue?

Applications are exposed if they accept untrusted CSS, parse it with Crass before version 1.0.8, and reserialize or otherwise accept the parsed output without rejecting malformed bad URL tokens. Using Crass alone is not sufficient to make an application vulnerable.

What does an attacker need to exploit the bypass?

An attacker needs a way to supply CSS that is processed by the affected sanitizer. The crafted CSS uses a bad URL containing a closing parenthesis after an escaped backslash, causing Crass and the browser to interpret the remaining content differently.

Is script execution possible through this issue?

No. The issue does not enable script execution in modern browsers, but it can permit CSS-based UI redress, external resource loading, or CSS-based data exfiltration.

What should teams do if they cannot immediately upgrade?

Reject untrusted CSS containing malformed bad URL constructs rather than parsing and reserializing them as acceptable output. The documented fix is to upgrade Crass to version 1.0.8.

CVE-2026-116695: Crass: Incorrect tokenization of bad URLs enables CSS sanitizer bypass

Affected Software

Remediation

Event History

Frequently Asked Questions

Which applications are exposed to this issue?

What does an attacker need to exploit the bypass?

Is script execution possible through this issue?

What should teams do if they cannot immediately upgrade?

Company

Resources

Contact

Frequently Asked Questions

Which applications are exposed to this issue?

What does an attacker need to exploit the bypass?

Is script execution possible through this issue?

What should teams do if they cannot immediately upgrade?