A vulnerability in the JSON processing of the backend system allows an attacker to bypass the WAF using unicode obfuscation. By injecting a malicious payload into the PUT request at /api/user/profile, the attacker can trigger a stored XSS attack on /dashboard/profile. This bypass technique exploits the trust placed on JSON fields by the backend system, even though client-side sanitization is in place. The WAF evasion technique showcases the importance of thorough input validation and security measures to prevent such attacks.