Broken Access Control is the correct choice because the scenario describes a user being able to access and modify resources that should be restricted to other users or roles. In CEH-aligned web testing, access control flaws occur when an application fails to enforce authorization checks consistently on the server side. Manipulating session parameters and then retrieving “sensitive medical records not associated with their own account” is a classic indicator of an authorization bypass, often seen as insecure direct object references, parameter tampering, or horizontal and vertical privilege escalation. Horizontal escalation is when one user accesses another user’s data at the same privilege level, while vertical escalation is when a user performs actions reserved for higher-privileged roles. The prompt explicitly states users can perform actions beyond assigned roles, which is the definition of broken authorization enforcement.

The other options do not align with the described root cause. Cryptographic Failures focuses on weak or missing encryption and does not explain why authenticated users can reach unauthorized records. Insecure Deserialization involves unsafe deserialization leading to remote code execution or data tampering via serialized objects, which is not indicated here. Security Misconfiguration is broader and can contribute to exposure, but the scenario emphasizes role and resource permission bypass rather than mis-set server headers, default accounts, or exposed admin interfaces.

Mitigation in CEH best practices includes enforcing server-side authorization on every request, using deny-by-default policies, validating that the authenticated user is allowed to access the specific record identifier, implementing robust role-based access control, logging access denials, and adding automated tests to prevent IDOR and privilege escalation regressions.