In the realm of cybersecurity and identity & access management (IAM), the term Subject denotes the active entity initiating a request or seeking to interact with a system's resources. Understanding subjects is fundamental to establishing secure access controls and maintaining the integrity and confidentiality of information systems.

What is a Subject in Cybersecurity?

A subject is the "who" or "what" that initiates an operation within a security context. This entity can take many forms:

• A human user attempting to log into an application
• A service account requesting data from another service
• An IoT device sending sensor readings to a central server
• An application seeking to access an API endpoint

The core function of a subject is to act as the initiator of an operation. The subject's identity and associated attributes form the basis for all subsequent access control decisions.

Why is Subject Identification Critical in Security?

Proper subject identification is essential for several reasons:

• Accountability: Tracking which subject performed what action enables audit trails and forensic analysis
• Access Control: Security policies can only be enforced when the subject is clearly identified
• Risk Assessment: Subject attributes help determine the risk level of each access request
• Compliance: Regulatory frameworks like NIST SP 800-63 require proper identity verification

Without accurate subject identification, organizations cannot implement the principle of least privilege or detect unauthorized access attempts.

How is a Subject Authenticated?

Before a subject can perform any action, its identity must be verified through authentication. Common methods include:

• Knowledge factors: Passwords, PINs, or security questions
• Possession factors: Hardware tokens, smart cards, or mobile devices
• Inherence factors: Biometrics such as fingerprints or facial recognition
• Token-based authentication: OAuth 2.0 tokens, API keys, or JWT tokens as defined in OpenID Connect Core 1.0

Once authenticated, the subject's authorization (permissions) is checked against predefined policies before granting access to resources.

When is a Subject Established in a Session?

A subject is established at the beginning of a session, typically during the authentication process. This establishment includes:

1. The subject presents credentials or proof of identity
2. The identity provider validates these credentials
3. A session or token is created containing subject attributes
4. These attributes persist throughout the session for access decisions

In frameworks like OAuth 2.0, the subject's identity is embedded in access tokens that accompany each request.

Which Entities Can Be Considered a Subject?

Modern IAM systems recognize various types of subjects:

Practical Examples

Example 1: Human User Authentication

When an employee logs into an enterprise application using their username and password, they become the subject. The system authenticates their credentials, retrieves their role assignments and group memberships, and uses these attributes to determine which resources they can access.

Example 2: Service-to-Service Communication

In a microservices architecture, when one microservice requests data from another using an API key or bearer token, the calling service acts as the subject. The receiving service validates the token and checks whether the calling service has permission to access the requested data, implementing proper authorization as recommended by CISA security guidelines.

Subject Attributes in Access Control

The attributes associated with a subject are crucial for making granular access decisions, especially in frameworks like Attribute-Based Access Control (ABAC). Common subject attributes include:

• Roles and group memberships
• Department or organizational unit
• Security clearance level
• Device context and location
• Time of access request

These attributes enable dynamic, context-aware access control that adapts to changing conditions and security requirements.