The tester identified an HTTPS downgrade attack (e.g., SSL stripping). The best mitigation is to enforce HSTS (HTTP Strict Transport Security).
HSTS (Option A):
HSTS (Strict-Transport-Security) ensures that the browser always uses HTTPS, preventing downgrade attacks.
Example header:
Strict-Transport-Security: max-age=31536000; includeSubDomains
Reference: CompTIA PenTest+ PT0-003 Official Study Guide - "Web Security Headers and HTTPS Enforcements" Incorrect options:
Option B (httponly flag): Protects cookies from JavaScript access but does not enforce HTTPS.
Option C (Firewall rule on port 80): Helps, but does not force browsers to use HTTPS.
Option D (Removing x-content-type-options): Unrelated; nosniff prevents MIME-type sniffing.