To identify authentication weaknesses in a customer-facing web application, the best method is dynamic analysis, also known as Dynamic Application Security Testing (DAST). Dynamic analysis evaluates an application while it is running, allowing testers to observe real-world interactions, session handling, login mechanisms, credential validation, access control failures, and runtime vulnerabilities such as brute-force weaknesses or authentication bypass conditions.

Security+ SY0-701 outlines DAST as the preferred approach for testing live web applications because it uncovers:

Weak session management

Broken authentication flows

Input validation failures

Misconfigurations in login portals

Runtime vulnerabilities that static code review cannot detect

Static analysis (A) only analyzes source code and may overlook logic flaws in authentication. Packet capture (B) inspects network traffic but cannot evaluate internal authentication logic. Agent-based scanning (C) is used for hosts, not web applications. Network-based scanning (E) finds port-level vulnerabilities but cannot assess application authentication mechanisms.

Therefore, dynamic analysis (D) is the most effective and accurate technique for discovering authentication weaknesses in live web applications.