Summary

SVGO's removeScripts plugin (disabled by default) removes scripts from the SVG, however executable scripts were left intact in some cases. If a consumer relied on this plugin for sanitization and served them to users, these SVGs could open up doors to XSS.

Details

SVGO has a plugin for removing scripts from an SVG, which removes:

• <script> elements
• JavaScript URIs (v4 and v3 only)
• on… event handlers (v4 and v3 only)

While SVGO is not a sanitization library, we do continue to maintain the plugin for those already using it for this purpose.

However, there were two problems:

• We did not check namespaced/prefixed script elements, for example if one declared an explicit prefix for the SVG namespace ( <svg:script>) instead of using the default namespace ( <script>), the <svg:script> tag would be left intact.
• We case sensitively matched JavaScript URIs, but it should've been case-insensitive.

Proof of Concept

import { optimize } from 'svgo';

/** Presume that this string was obtained in some other way, such as network. */
const original = `
  <svg xmlns="http://www.w3.org/2000/svg" xmlns:svg="http://www.w3.org/2000/svg" xmlns:uwu="http://www.w3.org/1999/xlink" viewBox="0 0 100 100" version="1.1">
    <a uwu:href="JavaScript:(() =&gt; { alert(document.cookie) })();"><text y="30">uwu</text></a>
    <svg:script>
      alert(document.cookie);
    </svg:script>
  </svg>
`;

optimize(original, {
  plugins: ['removeScripts']
});
// Did not remove <svg:script> or uwu:href="JavaScript:…—still executed by browsers.

Impact

If you run SVGO on untrusted input (e.g., user uploads to a web application) and you depended on removeScripts, then some scripts may still be present. If that SVG was then opened directly by another user on the same domain, it could invoke scripts that could read local storage or cookies.

This may affect you if you have enabled one of the following:

It's unlikely to impact users who just use SVGO locally on their own SVGs or in build pipelines.

Patches

>= 3.0.0, <= 4.0.1

We've patched v4.0.2 and v3.3.4 just upgrade the dependency using your preferred package manager! For example:

yarn up svgo

# or if SVGO is a nested dependency
yarn up -R svgo

The proposed fix is to improve our namespace-aware handling to explicitly act on the default namespace, SVG namespace, and XHTML namespace only. This handles all scripts that are executed by browsers, but will leave intact custom prefixes that happen to have an element called <*:script> which clients shouldn't treat as executable.

>= 2.0.0, <= 2.8.2

We've patched v2.8.3, however SVGO v2 explicitly only implements and documents that it will remove <script> elements and nothing more. It has the namespace aware handling for tags like <svg:script> or <xhtml:script>, but has not been updated to remove JavaScript URIs or event handlers like >= v3. If this is something you need, please upgrade to v4 or v3, or reach for one of the documented workarounds at the end.

>= 1.0.0, <= 1.3.2

SVGO v1 has been deprecated for a while now and won't be patched. Please upgrade to a more recent version! If something is preventing you from doing so, please reach out! We're happy to expand our migration guides or support you if you're having trouble.

Workarounds

If your motivation for enabling the plugin is SVG sanitization, consider reaching for a dedicated SVG sanitization tool and invoke it before passing the SVG to SVGO.