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JSON.parse reviver to sanitise data (stopping XSS)

I'm trying to reduce vulenrabilities in an old code base, specifically XSS attacks. The general pattern is:

The site is a collection of HTML pages with tags
It uses jQuery
Each page runs its own scripts
Each page's scripts make an $.ajax request to a PHP file which returns text/plain JSON (via json_encode($data))
This data is then used to create dynamic HTML markup to inject into the DOM via either $(selector).html, $(selector).append, or occasionnally document.getElementById(id).innerHTML =

Here's a simplified example of the data flow: page.html

<head>
    <script src="script.js"></script>
</head>
<body>
    <table id="myTable"></table>
</body>

script.js

$.ajax({url: "phpFile.php"}, function(data){
    data = JSON.parse(data);
    $("#myTable tbody").html(data.redcuce(row=>"<tr><td>"+row.text"+</td></tr>", ""))
}

phpFile.php

//database query resulting in $data = [text->'<img src="x" onerror="alert(1)">', text->'Innocent value']
echo json_encode($data);

Some of the data retrieved is user input, so if a field contains <img src="x" onerror="alert(1)">, this executes when rendered in the table.

Solution?

I've passed the below sanitising function to JSON.parse to sanitise the data

const sanitiseJson = (key, value) => typeof value === "string" ? DOMPurify.sanitize(value, { USE_PROFILES: { html: true } }) : value;

This uses the DOMPurify library, which would be very easy for me to implement with search and replace.

As I understand it, this will remove potentially malicious HTML from the JSON object. This will prevent XSS attacks in this scenario.

Is this sufficient for sanitising fetched data that is then inserted into the DOM?
What are the other XSS vulenrabilities that are not addressed by this?

Edit: note I'm well aware that .html() etc is not the proper way to insert data, but this is an old, large code base (45k+ lines) and it's heavily entrenched, so my question is more about if this is an 'acceptable' solution in a pinch or if it doesn't come close

From my limited testing it seems to work, and as I (poorly) understand the way the data is processed I can't see how this would be circumvented easily

about 4 years ago · Juan Pablo Isaza

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JSON.parse reviver to sanitise data (stopping XSS)

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Andres GPT