threat-modeling.md

Threat Model Analysis for identityserver/identityserver4

Threat: Malicious Client Registration and Impersonation

• Description: An attacker attempts to register a new client application with a ClientId that mimics a legitimate, existing client. If successful, and if client secrets are weak or not properly validated, the attacker can then request authorization codes or tokens as if they were the legitimate client. The attacker might also try to register a client with overly permissive redirect URIs (e.g., using wildcards) to intercept authorization codes.
• Impact: The attacker can gain unauthorized access to user data and resources protected by the legitimate client. This can lead to data breaches, account takeovers, and other significant security incidents.
• Affected IdentityServer4 Component:
  • IClientStore implementation (typically InMemoryClientStore, ConfigurationClientStore, or a custom database-backed store). The logic that validates client registration and retrieves client details.
  • Client registration endpoint (if exposed).
  • Authorization Endpoint (/connect/authorize).
• Risk Severity: Critical
• Mitigation Strategies:
  • Enforce Strict Client Secret Management: Use strong, randomly generated client secrets. Store secrets securely (e.g., using a secrets management service, not in source code). Rotate secrets regularly.
  • Mandatory PKCE: Enforce Proof Key for Code Exchange (PKCE) for all client types, including confidential clients. This prevents authorization code interception, a key step in client impersonation.
  • Restrict Redirect URIs: Avoid using wildcard characters in redirect URIs. Use exact matching whenever possible. If wildcards are necessary, implement strict validation logic.
  • Client Authentication: For confidential clients, consider using client certificates (mTLS) or JWT client assertions for stronger authentication.
  • Auditing: Log all client registration and modification attempts. Monitor for suspicious activity.
  • Manual Approval: For high-security environments, consider requiring manual approval of new client registrations.

Threat: Authorization Code Interception and Replay (Without PKCE)

• Description: An attacker intercepts the authorization code returned by IS4 after a user authenticates. This typically happens on the redirect back to the client application. Without PKCE, the attacker can exchange this intercepted code for an access token and ID token.
• Impact: The attacker gains unauthorized access to user data and resources, impersonating the user.
• Affected IdentityServer4 Component:
  • Authorization Endpoint (/connect/authorize).
  • Token Endpoint (/connect/token) - specifically, the code exchange logic.
• Risk Severity: Critical (if PKCE is not used),
• Mitigation Strategies:
  • Mandatory PKCE: Implement and enforce PKCE for all clients. This is the primary defense against authorization code interception.
  • HTTPS: Use HTTPS for all communication between the client, IS4, and the resource server. Ensure proper certificate validation.

Threat: Token Tampering (JWT Modification)

• Description: An attacker intercepts a JWT (ID token or access token) issued by IdentityServer4 and modifies its contents (e.g., changing the sub claim to impersonate another user, adding roles, extending the expiration). This assumes the attacker can intercept the token in transit (e.g., via a man-in-the-middle attack if HTTPS is not properly enforced).
• Impact: The attacker can gain unauthorized access to resources, elevate their privileges, or extend their access beyond the intended duration. This bypasses intended authorization checks.
• Affected IdentityServer4 Component:
  • Token Endpoint (/connect/token) - specifically, the token signing logic. While validation happens on the resource server, the issuance of the tampered-with token originates from IS4 (even if the tampering happens in transit).
• Risk Severity: Critical
• Mitigation Strategies:
  • HTTPS: Use HTTPS for all token exchanges to prevent interception. This is crucial to prevent the attacker from obtaining the token to tamper with.
  • Signature Verification: (While primarily a resource server responsibility, it's relevant to IS4's overall security posture) Ensure resource servers verify the JWT signature. IS4 must sign tokens with a strong key.
  • Key Management: Securely manage the signing key used by IS4 (see "Compromised Signing Key" threat below).

Threat: Open Redirect Vulnerability

• Description: An attacker crafts a malicious URL that includes a legitimate IS4 authorization endpoint URL, but with a manipulated redirect_uri parameter pointing to an attacker-controlled site. If IS4 doesn't properly validate the redirect_uri, it will redirect the user to the attacker's site after authentication, potentially leaking authorization codes or tokens.
• Impact: The attacker can steal authorization codes or tokens, leading to unauthorized access to user data and resources.
• Affected IdentityServer4 Component:
  • Authorization Endpoint (/connect/authorize) - specifically, the redirect_uri validation logic.
• Risk Severity: High
• Mitigation Strategies:
  • Strict Redirect URI Validation: Implement strict validation of the redirect_uri parameter against a pre-registered list of allowed redirect URIs for each client. Avoid using wildcards unless absolutely necessary, and then with very careful validation.
  • Exact Matching: Prefer exact matching of redirect URIs over pattern matching.
  • Client Configuration: Ensure that client configurations in IS4 have accurate and restrictive redirect URIs.

Threat: Refresh Token Misuse

• Description: An attacker obtains a valid refresh token (e.g., through database compromise, token leakage). They can then use this refresh token to obtain new access tokens and ID tokens from IdentityServer4, potentially indefinitely, even if the user's password has been changed.
• Impact: Long-term unauthorized access to user data and resources, even after security incidents like password breaches.
• Affected IdentityServer4 Component:
  • Token Endpoint (/connect/token) - specifically, the refresh token handling logic.
• Risk Severity: High
• Mitigation Strategies:
  • Refresh Token Rotation: Implement refresh token rotation. When a refresh token is used to obtain new tokens, a new refresh token is also issued, and the old one is invalidated.
  • Refresh Token Expiration: Set reasonable expiration times for refresh tokens.
  • Refresh Token Binding: Consider binding refresh tokens to a specific client or device.
  • Secure Storage: Store refresh tokens securely (e.g., encrypted at rest). This is crucial for the persistence layer used by IS4.
  • Revocation: Implement a mechanism to revoke refresh tokens (e.g., based on user logout, password change, or suspicious activity). IS4 supports token revocation.
  • One-Time Use Refresh Tokens: Consider using one-time use refresh tokens.

Threat: Scope Escalation Attempt

• Description: A malicious client requests scopes from IdentityServer4 that it is not authorized to access. For example, a client registered for openid and profile might request api1.read and api1.write.
• Impact: If successful, the client gains unauthorized access to resources protected by the requested scopes.
• Affected IdentityServer4 Component:
  • Authorization Endpoint (/connect/authorize) - scope validation logic.
  • Token Endpoint (/connect/token) - scope validation logic.
  • IScopeStore implementation.
• Risk Severity: High
• Mitigation Strategies:
  • Strict Scope Definition: Carefully define and document the allowed scopes for each client.
  • Scope Validation: IS4 must validate the requested scopes against the allowed scopes for the client during both authorization and token issuance. Reject requests with unauthorized scopes.
  • Consent Screens: Implement user consent screens to explicitly inform users about the scopes being requested.
  • Auditing: Log all scope requests and grants.

Threat: Compromised Signing Key

• Description: An attacker gains access to the private key used by IS4 to sign tokens (JWTs). This could occur through server compromise, configuration file leaks, or other vulnerabilities.
• Impact: The attacker can forge valid tokens for any user, client, and scope, completely bypassing all security controls. This is a catastrophic compromise.
• Affected IdentityServer4 Component:
  • Token Endpoint (/connect/token) - token signing logic.
  • Key management infrastructure.
• Risk Severity: Critical
• Mitigation Strategies:
  • Secure Key Storage: Store the signing key securely, using a hardware security module (HSM) or a dedicated secrets management service. Never store the key in source code or unencrypted configuration files.
  • Key Rotation: Implement regular key rotation. IS4 supports key rotation.
  • Access Control: Strictly limit access to the signing key.
  • Auditing: Log all access to the signing key.
  • Key Rollover: Have a plan in place for key rollover in case of compromise.