Affected: rust-ammonia ammonia
ammonia is a Rust library for repairing and securing untrusted HTML. Prior to 3.3.2, 4.0.2, and 4.1.3, applications that explicitly allow the math tag, annotation-xml, and at least one raw-text HTML element while disallowing the encoding attribute can permit mutation cross-site scripting. ammonia does not correctly implement annotation-xml's encoding-dependent MathML integration-point behavior after attribute filtering removes encoding, so the sanitizer validates a descendant under one namespace interpretation while the browser reparses the output under another. An attacker can use that namespace transition and an allowed raw-text HTML element to execute JavaScript when a user renders the sanitized output. The required tags are not enabled by default, so applications that do not explicitly allow them are not affected. This issue is fixed in versions 3.3.2, 4.0.2, and 4.1.3.
Affected: crates.io: ammonia
If a certain set of MathML tags are enabled, an attacker can inject arbitrary JavaScript code into the user's browser.
The `annotation-xml` tag has slightly different behavior than the other "integration point" tags in MathML and SVG, but ammonia didn't handle it, so it didn't correctly strip the namespace-incompatible tags.
This vulnerability only has an effect when the `math` and `annotation-xml` tags are both enabled, but the `encoding` attribute is disabled, because it relies on the following sequence of steps:
1. User writes code like `<math><annotation-xml encoding="text/html"><gadget></annotation-xml></math>`. 2. Namespace filtering checks the DOM, and it passes. `<gadget>` is parsed as HTML. 3. Attribute filter strips it down to `<math><annotation-xml><gadget></annotation-xml></math>`. Because the encoding attribute is gone, `<gadget>` is now parsed as MathML. 4. The gadget is written in such a way that it exploits the parsing differences between HTML and MathML.
Additionally, the gadget can only be written using a tag that is parsed as raw text in HTML. These [elements] are:
* title * textarea * xmp * iframe * noembed * noframes * plaintext * noscript * style * script
Applications that do not explicitly allow any of these tags should not be affected, since none are allowed by default.
[elements]: https://github.com/servo/html5ever/blob/045a0378f2b0f8d4a350793899cf722a2a9b3d11/html5ever/src/tree_builder/rules.rs
---
**Discovered by:** [Ivan Ivančić](https://…