Missing case-insensitive matching for CSS url function

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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. ```rust let mut builder = Builder::new(); // allow inline styles builder.filter_style_properties(hashset!["background-image"]); builder.add_generic_attributes(&["style"]); // notice that "http" URLs aren't allowed builder.url_schemes(["cid"].into()); // attacker-controlled, presumably let cleaned = builder.clean(r#"<div style='background:URL("https://evil.com/ping");'>"#); // this is the expected result assert_eq!(String::from(&cleaned), r#"<div style="">ok</p>"#); // this is the result that the vulnerable version actually gets assert_eq!(String::from(&cleaned), r#"<div style="background-image:URL(&quot;https://evil.com/ping&quot;);">ok</p>"#); ``` 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](https://github.com/zhixuli0406).

Source published

16 Oct 2026 12:00:00 UTC

Source modified

17 Oct 2026 15:30:02 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

OSV · upstream source license applies

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