RUSTSEC-2026-0373 Missing case-insensitive matching for CSS url function

Not scored Published 16 Oct 2026

Attackers can bypass URL filtering in CSS sanitization by using uppercase URL schemes, allowing injection of tracking pixels or limited data exfiltration.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is RUSTSEC-2026-0373?

RUSTSEC-2026-0373 is a vulnerability in ammonia, published on 16 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

URL filtering, such as relative URLs and URL scheme limits, is not always applied when CSS sanitization is used.

When the inner URL is quoted, CSS url("") is matched case-sensitively, when it should be matched case-insensitively. As a result, while url("") gets the intended domain name filtering, URL("") does not.

The following example shows how an application needs to be configured to trigger this bug.

let mut builder = Builder::new();
// allow inline styles
builder.filter_style_properties(hashset![&#34;background-image&#34;]);
builder.add_generic_attributes(&amp;[&#34;style&#34;]);
// notice that &#34;http&#34; URLs aren&#39;t allowed
builder.url_schemes([&#34;cid&#34;].into());

// attacker-controlled, presumably
let cleaned = builder.clean(r#&#34;&lt;div style=&#39;background:URL(&#34;https://evil.com/ping&#34;);&#39;&gt;&#34;#);

// this is the expected result
assert_eq!(String::from(&amp;cleaned), r#&#34;&lt;div style=&#34;&#34;&gt;ok&lt;/p&gt;&#34;#);

// this is the result that the vulnerable version actually gets
assert_eq!(String::from(&amp;cleaned), r#&#34;&lt;div style=&#34;background-image:URL(&amp;quot;https://evil.com/ping&amp;quot;);&#34;&gt;ok&lt;/p&gt;&#34;#);

This can be used to inject tracking pixels where the application intended to prevent them. It does not allow XSS, and its usefulness for data exfiltration is limited.

This vulnerability was reported by Louis Li.

Which versions are affected by RUSTSEC-2026-0373?

1 open source package ship an affected version, including ammonia.

Package	Affected	Fixed in
`ammonia` crates.io	From `0.0.0-0`	`4.1.8, 4.2.4`

How do I fix RUSTSEC-2026-0373?

Upgrade ammonia to a fixed version for your release line: 4.1.8 or 4.2.4.

Is RUSTSEC-2026-0373 being exploited?

There is no known exploitation of RUSTSEC-2026-0373 in the wild and no public exploit code.

Sources

Also known as GHSA-x43w-36pp-96g3.