Have you ever tried chrome inspector with Facebook? If so, I am sure you have seen this. This warning message is to help prevent Self-XSS scams.
Self-XSS is a social engineering attack that is designed to gain control of your social media account. In a self-XSS attack, an attacker convinces a user to runs malicious code on the address bar of his/her web browser.
Following video covers both share-baiting (a pure social engineering attack) and self-XSS (a combination of social engineering and a browser vulnerability).
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