Horizontal Privilege Escalation involves a user gaining access to resources or functionalities that are normally available to other users with the same privilege level, rather than increasing their own level of access. Unlike vertical privilege escalation, where a user moves from a lower to a higher privilege level, horizontal privilege escalation occurs when a user accesses or manipulates data or functions assigned to other users at the same level.

• Exploiting Application Flaws: Attackers may exploit weaknesses in an application, such as improper access controls or lack of user authentication, to gain unauthorized access to data or features belonging to other users.
• Session Hijacking: By hijacking or impersonating another user's session, attackers can gain access to resources and actions that are restricted to that particular user.
• Insecure Direct Object References (IDOR): An attacker might manipulate input parameters or URLs to access objects or data that should be restricted to other users with the same privilege level.

• Access Control Implementation: Implement robust access control mechanisms to ensure that users can only access resources and functionalities that are explicitly permitted for their role. This includes proper validation of user input and session management.
• Regular Security Audits: Conduct regular security audits and penetration testing to identify and address potential vulnerabilities that could be exploited for horizontal privilege escalation.
• Least Privilege Principle: While horizontal privilege escalation focuses on accessing other users' data at the same level, applying the principle of least privilege helps minimize the impact by ensuring that users have access only to what is necessary for their tasks.

• Incident Detection and Response: Establish procedures for detecting and responding to unauthorized access or activities indicative of horizontal privilege escalation. This includes monitoring user activities and investigating unusual behavior.
• Access Reviews: Regularly review user access permissions and logs to ensure that access controls are functioning as intended and to detect any unauthorized access or privilege misuse.

• https://en.wikipedia.org/wiki/Privilege_escalation
• https://owasp.org/www-community/attacks/Insecure_Direct_Object_References
• https://www.csoonline.com/article/3258977/what-is-horizontal-privilege-escalation-how-to-prevent-it.html