mitigations.md

Mitigation Strategies Analysis for identityserver/identityserver4

Mitigation Strategy: Strict Client Configuration with Principle of Least Privilege (IS4 Configuration)

• 1. Mitigation Strategy: Strict Client Configuration with Principle of Least Privilege (IS4 Configuration)

  • Description:

    1. Access IS4 Configuration: Navigate to your IdentityServer4 configuration files (typically appsettings.json or a database, depending on your setup).
    2. Review Client Definitions: Examine the configuration for each registered client (Clients section).
    3. AllowedGrantTypes: For each client, set AllowedGrantTypes to the minimum required. Choose from:
       • GrantTypes.AuthorizationCode: For web apps (with PKCE).
       • GrantTypes.ClientCredentials: For machine-to-machine.
       • GrantTypes.ResourceOwnerPassword: Avoid if possible; only if absolutely necessary and with extreme caution.
       • GrantTypes.Hybrid: Avoid unless you have a specific, well-understood need.
       • GrantTypes.Implicit: Strongly discouraged; avoid unless absolutely necessary and you understand the risks.
    4. AllowedScopes: For each client, define AllowedScopes to include only the scopes the client needs. Use fine-grained scopes (e.g., read:profile, not api_access).
    5. RedirectUris: For each client, set RedirectUris to the exact, HTTPS URLs where the client can receive authorization responses. No wildcards.
    6. RequirePkce: Set RequirePkce = true for all clients using GrantTypes.AuthorizationCode, especially public clients.
    7. ClientSecrets: For confidential clients (RequireClientSecret = true), generate strong, random ClientSecrets. Store these outside of the IS4 configuration (e.g., in a key vault). Reference the secret in the IS4 config.
    8. AllowOfflineAccess: Only set AllowOfflineAccess = true if the client needs refresh tokens.
    9. AccessTokenLifetime: Set a short AccessTokenLifetime (e.g., in seconds, like 300 for 5 minutes).
    10. RefreshTokenUsage: Set to ReUse or OneTimeOnly. OneTimeOnly is recommended for better security (refresh token rotation).
    11. RefreshTokenExpiration: Set to Absolute or Sliding. If Sliding is used, also set AbsoluteRefreshTokenLifetime.
    12. AbsoluteRefreshTokenLifetime: Set a maximum lifetime for refresh tokens (e.g., in seconds).
    13. UpdateAccessTokenClaimsOnRefresh: Consider setting to true if claims might change during the refresh token lifetime.
    14. Save Changes: Save the updated configuration. Restart IdentityServer4 if necessary.

  • Threats Mitigated:

    • Unauthorized Access (High Severity): Incorrect grant types or scopes allow unauthorized access.
    • Token Leakage (High Severity): Implicit flow or weak secrets expose tokens.
    • Privilege Escalation (High Severity): Overly permissive clients gain excessive access.
    • Authorization Code Interception (High Severity): Lack of PKCE allows code interception.
    • Open Redirect (Medium Severity): Incorrect RedirectUris allow redirection to malicious sites.

  • Impact:

    • All listed threats are significantly reduced by correctly configuring clients.

  • Currently Implemented:

    • Authorization Code Flow with PKCE is enforced for the SPA client.
    • Client Credentials Flow is used for machine-to-machine.
    • Client secrets are referenced from Azure Key Vault.
    • Basic Redirect URI validation is in place.
    • AccessTokenLifetime is set to a short duration.
    • RefreshTokenExpiration and AbsoluteRefreshTokenLifetime are configured.

  • Missing Implementation:

    • Review and refine AllowedScopes to be more granular.
    • Implement stricter RedirectUris validation (exact matches).
    • Set RefreshTokenUsage to OneTimeOnly to enable refresh token rotation.

Mitigation Strategy: Fine-Grained Scope and API Resource Definition (IS4 Configuration)

• 2. Mitigation Strategy: Fine-Grained Scope and API Resource Definition (IS4 Configuration)

  • Description:

    1. Access IS4 Configuration: Open your IdentityServer4 configuration.
    2. Define ApiResources: Create ApiResource definitions for each protected API. Give each resource a unique name.
    3. Define ApiScopes: Within each ApiResource, define ApiScope objects. Each scope should represent a specific permission or action (e.g., read:users, create:orders).
       • Name: The unique name of the scope (e.g., read:users).
       • DisplayName: A user-friendly name for the scope (e.g., "Read User Data").
       • Description: A more detailed description of the scope.
       • UserClaims: (Optional) List of user claims that should be included in the access token when this scope is granted.
    4. Associate Scopes with Resources: Ensure each ApiScope is associated with the correct ApiResource.
    5. Client Configuration (Refer to Strategy #1): Ensure clients only request the ApiScopes they need via their AllowedScopes property.
    6. Identity Resources (Optional): If you need to include standard OpenID Connect claims (e.g., profile, email), define IdentityResource objects.
    7. Save Changes: Save the updated configuration and restart IdentityServer4.

  • Threats Mitigated:

    • Unauthorized Data Access (High Severity): Overly broad scopes grant excessive access.
    • Privilege Escalation (High Severity): Clients can access resources they shouldn't.

  • Impact:

    • Both threats are significantly reduced by defining granular scopes and associating them with specific API resources.

  • Currently Implemented:

    • Basic ApiResources are defined.
    • Some ApiScopes are defined, but not consistently granular.

  • Missing Implementation:

    • Refine ApiScopes to be more atomic and specific to individual operations within each ApiResource.

Mitigation Strategy: Secure Key Management and Rotation (IS4 Configuration)

• 3. Mitigation Strategy: Secure Key Management and Rotation (IS4 Configuration)

  • Description:

    1. Key Generation: Generate strong signing keys:
       • RSA: At least 2048 bits.
       • ECDSA: Use a strong curve (e.g., NIST P-256, P-384).
    2. Secure Storage: Store keys outside of the IS4 configuration files. Use a key management service (Azure Key Vault, AWS KMS, HashiCorp Vault).
    3. IS4 Configuration: Configure IdentityServer4 to use the keys from the key vault:
       • AddSigningCredential: Use the appropriate method to load the key from your key vault. This often involves providing connection details or a key identifier. The specific method depends on your chosen key vault. Do not store the key directly in the configuration file.
    4. Key Rotation (Configuration): Configure IS4 for key rotation:
       • AddValidationKey: Add the previous signing key as a validation key. This allows IS4 to validate tokens signed with the old key while transitioning to the new key.
       • Automated Rotation (External): Implement an external process (e.g., a scheduled task, a script) to:
         • Generate a new key.
         • Store the new key in the key vault.
         • Update the IS4 configuration (e.g., by updating the key identifier in the key vault and restarting IS4, or by using a configuration provider that automatically reloads).
         • Add the old key as a validation key.
         • Remove old validation keys after a sufficient grace period (long enough to cover the longest possible access token lifetime).
    5. Restart IS4: After making changes to the signing key configuration, restart IdentityServer4.

  • Threats Mitigated:

    • Token Forgery (Critical Severity): Weak or compromised keys allow token forgery.
    • Key Compromise (Critical Severity): Rotation limits the impact of a compromised key.

  • Impact:

    • Both threats are significantly reduced by using strong keys, secure storage, and regular rotation.

  • Currently Implemented:

    • Strong keys are used (RSA 2048-bit).
    • Keys are stored in Azure Key Vault.
    • AddSigningCredential is used to load the key.

  • Missing Implementation:

    • Implement a regular key rotation schedule and automate the process (including AddValidationKey usage).

Mitigation Strategy: Rigorous returnUrl Validation (IS4 Configuration and Code)

• 4. Mitigation Strategy: Rigorous returnUrl Validation (IS4 Configuration and Code)

  • Description:

    1. IS4 Configuration (Limited): IdentityServer4 provides some built-in returnUrl validation, but it's not sufficient on its own.
    2. Custom Validation (Code): Implement custom returnUrl validation within your IdentityServer4 implementation, before any redirect occurs. This is typically done in a custom IIdentityServerInteractionService implementation or within a custom grant validator.
    3. Whitelist: Create a whitelist of allowed returnUrl values. This should be stored securely (e.g., in configuration, a database).
    4. Exact Matching: Validate the returnUrl against the whitelist using exact string matching.
    5. Rejection: If the returnUrl is invalid, reject the request or redirect to a safe, default URL.
    6. Logging: Log any invalid returnUrl attempts.
    7. Consider using ValidatedReturnUrl: If you are using a custom interaction service, make sure to use the ValidatedReturnUrl property of the AuthorizationRequest object, which contains the URL-decoded and validated return URL.

  • Threats Mitigated:

    • Open Redirect (Medium Severity): Prevents attackers from redirecting users to malicious sites.

  • Impact:

    • The risk of open redirect is significantly reduced.

  • Currently Implemented:

    • Basic domain-level checks are performed.

  • Missing Implementation:

    • Implement a strict whitelist with exact matching in a custom IIdentityServerInteractionService or grant validator.
    • Log invalid returnUrl attempts.

Mitigation Strategy: Enable and Configure Front-Channel and Back-Channel Logout (IS4 Configuration)

• 5. Mitigation Strategy: Enable and Configure Front-Channel and Back-Channel Logout (IS4 Configuration)

  • Description:

    1. Front-Channel Logout (IS4 Configuration):
       • FrontChannelLogoutUri: For each client that supports front-channel logout, configure the FrontChannelLogoutUri property. This is the URL that IS4 will redirect to when the user logs out of IS4. This URL should be on the client application.
       • FrontChannelLogoutSessionRequired: Set to true to ensure that a session ID is included in the logout request.
    2. Back-Channel Logout (IS4 Configuration):
       • BackChannelLogoutUri: For each client that supports back-channel logout, configure the BackChannelLogoutUri property. This is the URL that IS4 will make an HTTP request to when the user logs out. This URL should be on the client application and should not be accessible directly by the user.
       • BackChannelLogoutSessionRequired: Set to true to ensure that a session ID is included in the logout request.
    3. Client-Side Implementation: The client applications must implement the corresponding endpoints (FrontChannelLogoutUri and BackChannelLogoutUri) to handle the logout requests from IS4 and terminate their own sessions.
    4. Enable EnableSignOutPrompt: If you want to show a confirmation prompt to the user before logging them out, set EnableSignOutPrompt = true in the IdentityServerOptions.

  • Threats Mitigated:

    • Session Hijacking (High Severity): Ensures that sessions are properly terminated across all applications when a user logs out.
    • Incomplete Logout (Medium Severity): Prevents users from remaining logged in to some applications after logging out of IS4.

  • Impact:

    • Significantly reduces the risk of session-related vulnerabilities.

  • Currently Implemented:

    • None.

  • Missing Implementation:

    • Configure FrontChannelLogoutUri and BackChannelLogoutUri for all relevant clients.
    • Implement the corresponding logout endpoints in the client applications.
    • Consider enabling EnableSignOutPrompt.