Horizontal Privilege Escalation occurs when a user is able to access another user’s resources at the same privilege level. This typically happens when applications rely on user-supplied identifiers (e.g., `user_id`, email address) without proper server-side authorisation checks. An adversary can exploit this by tampering with identifiers in requests to view or modify another user’s data, such as changing `account_id=123` to `account_id=124`.

Remediation

• Avoid using easily guessable or public identifiers (e.g., sequential database primary keys, email addresses) as access control tokens.
• Always enforce server-side authorisation by verifying that the authenticated session is permitted to access the requested resource.
• Use indirect object references (e.g., opaque or random identifiers) instead of exposing raw user IDs to clients.
• Apply logging and monitoring to detect unusual access patterns that could indicate lateral movement.