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ammonia cross-site scripting in HTML sanitizer via SVG and MathML

RUSTSEC-2026-0533GHSA-934P-VP7M-GX3C

Exploited
No known exploitation
Fix
Fixed in 3.3.5, 4.1.9, 4.2.5
Who is exposed
Not reported

Do first: Upgrade to a fixed version: 3.3.5, 4.1.9, 4.2.5.

Published
Last modified
Reference
Last checked
10/100 · provisional
None on Exploit-DB, Metasploit or GitHub

Summary

The ammonia HTML sanitizer library can be tricked into letting through mutated markup that a browser parses differently from the original input, which allows arbitrary JavaScript to run in a user's browser. This only matters if an application explicitly allows certain raw-text tags such as `title`, `style`, or `script`, since none are allowed by default.

Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.

Read the original description

The MathML text integration points (mi, mo, mn, ms, mtext) and SVG foreignObject can be made to contain a tag in the other one's namespace (a MathML integration point with a child in the SVG namespace, or vice versa) without the root tag that switches out of the HTML namespace. If the attacker can construct input that parses differently in as HTML than as foreign XML, they can inject arbitrary JavaScript code into the user's browser.

For example:

  • The user writes <math><mi><svg><gadget></svg></mi></math>.
  • The application enables math, mi, and the gadget, but not svg.
  • Ammonia filters the input to <math><mi><gadget></mi></math>.
  • Because svg is now gone, the gadget switches from being parsed as SVG to being parsed as HTML.
  • The gadget is written in such a way that it exploits the parsing differences between HTML and MathML.

As far as we know, 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/a027c2835472d518893c906dc29c42200af3c960/html5ever/src/tree_builder/rules.rs

---

This vulnerability was reported by Roger Meyer.

cve.now Index

10Pcve.now Index 10 of 100 · methodology v3 · provisional
  • Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
  • Exposure?/25Unknown. No advisory context yet
  • Reach10/20Affects crates.io packages
  • Severity?/10Unknown. No CVSS score or vendor rating
  • No patch0/5No signal

What would change the score

  • Would score about 65 if added to CISA KEV
  • Would score about 60 if CISA's SSVC rates exploitation active
  • Would score about 32 if advisories confirm default setups are exposed

How the score works · Rankings

What to do

  • Upgrade to a fixed version: 3.3.5, 4.1.9, 4.2.5.

Check your systems with an agent

Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.

Check whether my systems are affected by RUSTSEC-2026-0533 (Incorrect handling of embedded SVG and MathML leads to mutation XSS), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for RUSTSEC-2026-0533 (entry last updated 2026-11-13):
Affected:
- crates.io package ammonia: from 0.0.0-0, fixed in 3.3.5; from 4.0.0, fixed in 4.1.9; from 4.2.0, fixed in 4.2.5
Fixed versions: 3.3.5, 4.1.9, 4.2.5.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: RUSTSEC-2026-0533 is not listed as of cve.now's last read.
This KEV status is for RUSTSEC-2026-0533 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Packages (crates.io): find every lockfile in each project and run the stack.check operation of the cve.now MCP server (execute) on each one. Compare the resolved versions, not the ranges a manifest declares, with the affected versions above. Include transitive dependencies.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://rustsec.org/advisories/RUSTSEC-2026-0533.html
- https://crates.io/crates/ammonia
- https://github.com/rust-ammonia/ammonia/pull/269
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/RUSTSEC-2026-0533
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "RUSTSEC-2026-0533"}.

Timeline

  1. Advisory published1 day
  2. Record last updated
  3. First seen by cve.now

Affected versions

  • ammoniacrates.io

    From 0.0.0-0 · fixed in 3.3.5; From 4.0.0 · fixed in 4.1.9; From 4.2.0 · fixed in 4.2.5

Evidence 3

Other links · 3

  1. Advisory
  2. Repository
  3. Link

Corrections

None so far.

Something wrong on this page? Report an error. Every fix is listed here.

Questions about RUSTSEC-2026-0533

Is RUSTSEC-2026-0533 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data

Which versions are affected by RUSTSEC-2026-0533?

ammonia (crates.io): from 0.0.0-0 before 3.3.5; from 4.0.0 before 4.1.9; from 4.2.0 before 4.2.5.Source: cve.now feed data

How do I fix RUSTSEC-2026-0533?

Upgrade to a fixed version: 3.3.5, 4.1.9, 4.2.5.Source: cve.now feed data

Sources

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