The single quote mark (') is a common character used to test for SQL injection vulnerabilities. This character is often used to terminate a string in SQL queries. By injecting a single quote mark into an input field, a penetration tester can determine whether the application is susceptible to SQL injection based on the resulting error messages or behavior of the application.
The single quote mark is typically used first because it is straightforward and effective in revealing SQL injection flaws. Other characters like double quotes or semicolons might also be useful in specific contexts, but the single quote is the standard starting point for SQL injection testing.
References:
* OWASP SQL Injection Guide: OWASP SQL Injection
* Demonstrations of SQL injection techniques in various penetration testing scenarios.