Possible Cause: Inadequate Logging and Lack of WAF Visibility

B.

Based on the information provided, the SOC analyst is reviewing malicious activity on an external, exposed web server and has found that specific traffic is not being logged, and there is no visibility from the WAF for the web application. This means that some traffic is not being inspected or logged by the WAF, which can indicate a security issue.

Looking at the answer choices:

A. The user agent client is not compatible with the WA.

This is unlikely to be the cause of the issue, as if the user agent client was not compatible with the WAF, it would not be able to communicate with the web server at all, rather than having some traffic go unlogged.

B. A certificate on the WAF is expired.

An expired certificate on the WAF would cause SSL/TLS errors for clients connecting to the web server, but it would not cause specific traffic to be unlogged or unmonitored by the WAF.

C. HTTP traffic is not forwarding to HTTPS to decrypt.

This is a possibility, as the WAF may not be able to inspect encrypted traffic if it is not configured to decrypt HTTPS traffic. However, this would not explain why specific traffic is not being logged, as all HTTPS traffic would be unlogged.

D. Old, vulnerable cipher suites are still being used.

This is the most likely cause of the issue. If old and vulnerable cipher suites are still being used, it is possible that the WAF is unable to inspect or log traffic using these cipher suites. Attackers can exploit vulnerabilities in these cipher suites to bypass security controls and evade detection by the WAF.

Therefore, the answer is likely to be D - old, vulnerable cipher suites are still being used.