How to avoid XSS when trying to modify HTML content via DOM?

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REF: https://github.com/aleen42/PersonalWiki/blob/docs/qa/avoid_xss_when_trying_to_modify_content_via_dom.md

In most cases when we want to manipulate a string content, we may prefer to mount it on the DOM tree and do the manipulation rather than using a complicated regular expression replacement, like:

  • finding some nodes to remove
  • finding some classes to remove
  • modifying the content.

However, this approach always leads to an XSS problem when the external content contains some unexpected scripts like the following snippet:

const unsafe = '<img src=x onerror=alert(1)>';
$('<div>').html(unsafe).find('img').remove(); // leads to a XSS

const div = document.createElement('div');
div.innerHTML = unsafe; // leads to XSS
const img = div.querySelector('img');
img && img.parentNode.removeChild(img);

So how can we solve this problem?

dom html javascript
1 Answers

Roll back to a complicated regex? To avoid this problem, most developers may use some libraries like js-xss or DOMPurify to handle complicated regular expressions for them. Nevertheless, these libraries are sometimes too strict to remove some safe tags, except if you can set up the right configurations.

Here, I just want to clarify another simple way where we can rely on the iframe sandbox technics, which can also forbid browsers from accessing resources from the content like loading images:

const unsafe = '<img src=x onerror=alert(1)>';
const $frame = $('<iframe>').appendTo('body'), $sandboxDoc = $frame.contents();
$frame.remove();

// scripts have been blocked
// the document won't also access the image which throws us a 404
$('<div>', $sandboxDoc).html(unsafe).find('img').remove(); 

// by pure javascript
const frame = document.createElement('iframe');
document.body.appendChild(frame);
const sandboxDoc = frame.contentDocument;
frame.parentNode.removeChild(frame);
const div = sandboxDoc.createElement('div');
div.innerHTML = unsafe;
const img = div.querySelector('img');
img && img.parentNode.removeChild(img);

Such a document still has a side effect in which DOM nodes won't compute out any styles:

$('<img style="cursor: zoom-in">', $sandboxDoc).css('cursor'); // => ''

// by pure javascript
const img = sandboxDoc.createElement('img');
img.setAttribute('style', 'cursor:zoom-in');
img.style.cursor; // => ''

It means that we will have some unexpected results in some situations like we cannot remove the cursor:

$('<img style="cursor: zoom-in">', $sandboxDoc)
    .css({cursor : ''}).prop('outerHTML'); // => '<img style="cursor: zoom-in">'
    
// by pure javascript
img.style.cursor = '';
img.outerHTML; // => '<img style="cursor:zoom-in">'

To work around this, we should ensure to construct DOM nodes before removing the document:

function $sandbox(content) {
    const $frame = $('<iframe>').appendTo('body');
    const $elem = $(content, $frame.contents());
    $frame.remove();
    return $elem;
}

$sandbox('<img style="cursor: zoom-in">').css('cursor'); // => 'zoom-in'

// by pure javascript
function sandbox(fn) {
    const frame = document.createElement('iframe');
    document.body.appendChild(frame);
    const elem = fn(frame.contentDocument);
    frame.parentNode.removeChild(frame);
    return elem;
}

sandbox(sandboxDoc => {
    const img = sandboxDoc.createElement('img');
    img.setAttribute('style', 'cursor:zoom-in');
    return img;
}).style.cursor; // => 'zoom-in'
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