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Often one wants to pass some data to JavaScript and to do that one renders into JavaScript using Django. for example:

<body>
Body Content
<script>
    let a = {{ var_1 }};
    let b = "{{ var_2|safe }}";
</script>
<script>
    let c = {{ var_3|safe }};
</script>
</body>

Why is it a problem?

This is very unsafe and it makes one vulnerable to cross site scripting (XSS) attacks [wiki]. Let us consider that the values of the variables rendered above are provided from the user. For demonstration purposes below is some view code with these variables set to such values that each of them will cause an alert:

from django.shortcuts import render
import json

def test_xss(request):
    context = {
        'var_1': 'alert(1)',
        'var_2': '";\nalert(2);\n"',
        'var_3': json.dumps('</script><script>alert(3);</script><script>')
    }
    return render(request, 'test.html', context)

This snippet might not cause much damage, but it can be much more dangerous. It can also be seen that even using json.dumps is not very safe here.

What can be done to resolve the problem?

Don’t render from Django into JavaScript, instead use the |json_script template filter [Django-doc] and parse it’s results using JSON.parse:

<body>
Body Content
{{ var_1|json_script:"var-1-json" }}
{{ var_2|json_script:"var-2-json" }}
{{ var_3|json_script:"var-3-json" }}
<script>
    let a = JSON.parse(document.getElementById('var-1-json').textContent);;
    let b = JSON.parse(document.getElementById('var-2-json').textContent);;
</script>
<script>
    let c = JSON.parse(document.getElementById('var-3-json').textContent);;
</script>
</body>