AIKIDO-2026-204536

better-auth is vulnerable to Insecure Cryptographic Defaults

CVE-2026-67336Published Aug 5, 2026

CVE Score

High

Insecure Cryptographic Defaults

TL;DR

The legacy oidcProvider and mcp plugins advertise "none" in id_token_signing_alg_values_supported (and resource_signing_alg_values_supported for mcp) and accept PKCE code_challenge_method=plain by default, including silently rewriting a missing method to plain before the allowlist check. Relying parties that negotiate algorithms from discovery without pinning may accept unsigned tokens, and authorization-code interception becomes easier when the authorize URL leaks because plain PKCE provides no real challenge binding. The fix drops "none" from discovery metadata, defaults allowPlainCodeChallengeMethod to false, and rejects a code_challenge without an explicit code_challenge_method.

How do I fix this?

Upgrade the better-auth library to the patch version.

Who does this affect?

You are affected if you are using a version that falls within the vulnerable range and your application enables the legacy oidcProvider or mcp plugins from better-auth.