Diverse data sources pose a significant challenge in ETL data validation. These sources may have varying formats, structures, or data quality, making it complex to validate and ensure the consistency and integrity of data across different sources during the ETL process. Testers need to address this challenge effectively to ensure accurate data validation.

An INNER JOIN retrieves records from both tables where there is a match between the columns specified in the JOIN condition. Since you need to find customers who have placed orders, you want to retrieve records from both the "Customers" and "Orders" tables where there is a match on the customer ID in both tables, making INNER JOIN the appropriate choice.

The WHERE clause is used to filter rows based on a specified condition. In this scenario, you would use the WHERE clause to specify the date range for which you want to calculate the total revenue from the "Sales" table. This clause ensures that only the relevant rows within the specified date range are included in the calculation.

Social Engineering is a form of cyberattack where the attacker manipulates people into revealing confidential information or performing actions. It often involves disguising malicious activities as legitimate requests to trick victims. Phishing, Malware, and DDoS attacks are different types of cyber threats.

SSL/TLS uses asymmetric keys, including a public key for encryption and a private key for decryption, to secure data transmission.

The principle of "least privilege" in web application security means providing users or processes with the minimum level of access necessary to perform their tasks. This helps limit potential damage and unauthorized actions.

Authentication mechanisms, such as passwords, biometrics, or smart cards, are used to verify the identity of users. Once verified, the system grants access to specific resources, ensuring that only authorized users can access them.

An Application Layer Gateway (ALG) is a firewall filtering technique that makes decisions based on the application layer protocol. ALGs are responsible for understanding and processing application-specific protocols, making them suitable for filtering at the application layer.

The primary function of an Intrusion Detection System (IDS) is to monitor and detect suspicious activities on a network or system for enhanced security.

Burp Suite is a popular web security assessment tool that focuses on spidering and analyzing web pages to find potential vulnerabilities. It's widely used for web application security testing, including scanning for common web vulnerabilities like SQL injection and Cross-Site Scripting (XSS).