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attack-surface.md

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Attack Surface Analysis for identityserver/identityserver4

Attack Surface: Client Misconfiguration (Overly Permissive Settings)

  • Description: Clients are applications that request tokens from IdentityServer4. Misconfigured clients, especially those with excessive permissions, are a major attack vector directly managed within IS4.
  • How IdentityServer4 Contributes: IS4 provides the entire framework for defining client configurations (grant types, scopes, redirect URIs, secrets). This inherent flexibility is where the risk originates.
  • Example: A client configured with AllowedGrantTypes = { GrantType.ResourceOwnerPassword, GrantType.AuthorizationCode } when it only needs GrantType.AuthorizationCode with PKCE. An attacker could brute-force credentials via the ResourceOwnerPassword grant – a grant type enabled by IS4.
  • Impact: Unauthorized access to protected resources, data breaches, account takeover.
  • Risk Severity: High to Critical (depends on exposed resources).
  • Mitigation Strategies:
    • Principle of Least Privilege: Grant clients only the absolute minimum necessary permissions within IS4's configuration.
    • Mandatory PKCE: For public clients, enforce PKCE through IS4's settings. Never allow the Implicit flow (disable it in IS4).
    • Strict RedirectUri Validation: Avoid wildcards in IS4's RedirectUris configuration. Use exact matching. IS4 must validate the redirect_uri on every request.
    • Strong Client Secrets: Use strong, random secrets for confidential clients, managed securely outside of IS4 but configured within IS4.
    • Regular Configuration Audits: Regularly audit client configurations within IS4 to ensure they remain appropriate.
    • Secure Client Credentials Flow: If using this IS4-provided flow, ensure proper client authentication and authorization within IS4's settings.

Attack Surface: Weak or Default Cryptographic Keys

  • Description: IdentityServer4 uses cryptographic keys for signing tokens (JWTs). Weak or default keys allow attackers to forge tokens, a vulnerability directly tied to IS4's key management.
  • How IdentityServer4 Contributes: IS4 provides the mechanisms for managing signing keys. The vulnerability stems from how these mechanisms are used (or misused).
  • Example: Using the default development signing key ("idsrv3test") in production. An attacker can use this publicly known key to create valid JWTs, granting access – a direct consequence of IS4's configuration.
  • Impact: Complete system compromise; attackers can impersonate any user/client.
  • Risk Severity: Critical.
  • Mitigation Strategies:
    • Generate Strong Keys: Use strong, random keys with appropriate lengths, configured within IS4.
    • Secure Key Storage: Store keys securely, outside of the application, but the configuration to use those keys resides within IS4.
    • Regular Key Rotation: Implement and enforce key rotation through IS4's configuration.
    • Asymmetric Keys for Signing: Use asymmetric keys (RSA, ECDSA) for signing, configured within IS4.
    • X.509 Certificates: Use X.509 certificates for signing keys, managed and configured within IS4.

Attack Surface: Open Redirect Vulnerabilities (via redirect_uri and post_logout_redirect_uri)

  • Description: Attackers manipulate redirect_uri (post-authorization) or post_logout_redirect_uri (post-logout) to redirect users to malicious sites – a vulnerability directly within IS4's handling of these parameters.
  • How IdentityServer4 Contributes: IS4 uses these parameters as part of the OAuth/OIDC flows. The vulnerability is in IS4's validation (or lack thereof) of these parameters.
  • Example: An attacker crafts a login URL with a malicious redirect_uri: https://your-is4.com/connect/authorize?client_id=...&redirect_uri=https://evil.com. IS4, if misconfigured, redirects the user after authentication.
  • Impact: Phishing, credential theft, session hijacking.
  • Risk Severity: High.
  • Mitigation Strategies:
    • Strict RedirectUri Validation: Avoid wildcards; use exact matching in IS4's configuration. IS4 must validate against a whitelist.
    • Whitelist post_logout_redirect_uri: Maintain and enforce a whitelist within IS4 for allowed post_logout_redirect_uri values.
    • User Confirmation (Optional): While not a direct IS4 feature, consider a confirmation page before IS4 performs the redirect.

Attack Surface: Custom Grant Type Vulnerabilities

  • Description: Custom grant types, if insecurely implemented, introduce vulnerabilities directly within the IdentityServer4 framework.
  • How IdentityServer4 Contributes: IS4 allows developers to implement custom grant types, extending its core functionality. This extensibility point is the source of the risk.
  • Example: A custom grant type that bypasses proper authentication, allowing attackers to obtain tokens without credentials – a flaw within the IS4 extension.
  • Impact: Unauthorized access, data breaches, account takeover.
  • Risk Severity: High to Critical (depends on the specific flaw).
  • Mitigation Strategies:
    • Rigorous Security Review: Thoroughly review and test custom grant type code within the IS4 context.
    • Secure Coding Practices: Adhere to secure coding and OAuth/OIDC best practices within the IS4 extension.
    • Input Validation: Validate all input within the custom grant type handler.
    • Proper Authentication/Authorization: Ensure the custom grant type correctly authenticates and authorizes within IS4's framework.
    • Prefer Standard Grant Types: Avoid custom grant types if standard IS4 grant types suffice.