The legacy oidcProvider and mcp plugins in better-auth versions before 1.6.11 exhibit two main cryptographic weaknesses: (1) their OIDC discovery documents advertise the 'none' algorithm in id_token_signing_alg_values_supported (and resource_signing_alg_values_supported for mcp), which can cause relying parties that negotiate algorithms from discovery metadata without pinning to accept unsigned tokens; (2) they enable the PKCE 'plain' code challenge method by default, including silently downgrading missing code_challenge_method parameters to 'plain', contrary to OAuth 2.1 RFC 9700 §2.1.1 which forbids 'plain'. The patched version 1.6.11 removes 'none' from discovery metadata, disables plain PKCE by default, and rejects requests missing code_challenge_method. The recommended migration is to @better-auth/oauth-provider, which excludes these insecure defaults.

Relying parties that do not pin JWT signing algorithms and rely on discovery metadata may accept unsigned tokens, risking token forgery or impersonation. The acceptance of PKCE 'plain' weakens the protection of authorization codes, making interception attacks more feasible if the authorization URL leaks. These issues can lead to compromised authentication flows and elevated risk of unauthorized access.

Upgrade to better-auth version 1.6.11 or later, which removes the insecure 'none' algorithm from discovery metadata and disables acceptance of plain PKCE by default. Migrate from the deprecated oidcProvider and mcp plugins to the @better-auth/oauth-provider package, which is not affected by these issues. If immediate upgrade is not possible, explicitly disable plain PKCE by setting allowPlainCodeChallengeMethod to false in oidcProvider or mcp configurations, and override the discovery metadata to exclude 'none' from id_token_signing_alg_values_supported. Verify changes by inspecting the .well-known endpoint. These mitigations align runtime and discovery behavior to secure defaults.