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

What to do

1. See which version you have: cargo tree -i ammonia (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p ammonia. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example ammonia = "4.2.4") or update the dependency that pulls it in.
3. Check again with cargo audit, or paste your Cargo.lock into our Cargo.toml checker.

Details from the advisory

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!["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.

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This vulnerability was reported by Louis Li.

References

• Package: crates.io/crates/ammonia
• Advisory: rustsec.org/advisories/RUSTSEC-2026-0373.html
• Web: github.com/rust-ammonia/ammonia/pull/267

From the RustSec Advisory Database (CC0-1.0) via the OSV export, downloaded Oct 24, 2026. Severity score computed from the published CVSS vector.

Other advisories for ammonia

• Vulnerability HighRUSTSEC-2019-0001 · 2019-04-27Uncontrolled recursion leads to abort in HTML serialization
• VulnerabilityRUSTSEC-2021-0074 · 2021-07-08Incorrect handling of embedded SVG and MathML leads to mutation XSS
• VulnerabilityRUSTSEC-2021-0157 · 2021-10-10Incorrect escaping for `url("")` functions in CSS style attributes
• VulnerabilityRUSTSEC-2021-0158 · 2021-10-10XSS in ammonia via SVG `animateMotion`, `animateTransform`, and/or `animateColor` tags
• VulnerabilityRUSTSEC-2022-0003 · 2022-01-19Space bug in `clean_text`
• VulnerabilityRUSTSEC-2025-0071 · 2025-09-21Incorrect handling of embedded SVG and MathML leads to mutation XSS after removal
• VulnerabilityRUSTSEC-2026-0193 · 2026-06-30mXSS in ammonia via MathML `annotation-xml` encoding strip
• VulnerabilityRUSTSEC-2026-0213 · 2026-07-21XSS in ammonia via SVG `animate` and `set` animation tags

Full security history of ammonia