This tweet describes a bypass payload for the Cloudflare Web Application Firewall (WAF) specifically targeting Cross-Site Scripting (XSS) vulnerabilities. The payload is designed to be sneaky by using mixed casing in attribute names (e.g., 'sRc', 'oNlY', 'oNeRrOr') which can evade pattern matching rules that are case-sensitive or less strict. The payload uses an <img> tag with an invalid source 'x', and includes an 'onerror' event handler that triggers a JavaScript alert showing the domain name of the current document (document.domain). The use of 'alert(document.domain)' is a common demonstration to prove XSS execution. This bypass works because some WAFs may improperly detect or filter event handlers or code inside mixed-case attributes, allowing the malicious script to run despite the WAF's protection. In simple terms, the attacker cleverly obfuscates the event handler code by writing attribute names with unusual casing which bypasses Cloudflare's XSS filters, allowing the script to execute. Knowing such bypasses helps both security researchers to improve WAF rules and helps defenders understand how attackers exploit weaknesses in WAF filtering logic.
For more details, check out the original tweet here: https://twitter.com/hackme_xyz/status/2043006326121345217