npm · better-auth
Better Auth has insecure cryptographic defaults in oidcProvider: alg=none advertised and plain PKCE accepted by default
Users are affected if all of the following are true:
better-auth at a version below the patched release.oidcProvider() from better-auth/plugins/oidc-provider or mcp() from better-auth/plugins/mcp (the mcp plugin delegates to oidcProvider and inherits both defaults).If the application only uses @better-auth/oauth-provider (the canonical replacement) and have not enabled the legacy plugins, it is not affected. The new package's discovery document excludes none and its authorize schema rejects plain at parse time.
Fix:
better-auth@1.6.11 or later.oidcProvider and mcp plugins to @better-auth/oauth-provider when feasible.The legacy oidcProvider and mcp plugins exhibit two related defects in their OIDC discovery and authorize surfaces.
The discovery document advertises "none" in id_token_signing_alg_values_supported (and, for mcp, in resource_signing_alg_values_supported on the OAuth protected-resource metadata). Any relying party that performs algorithm negotiation from this metadata without pinning to a real signing algorithm may accept unsigned tokens.
PKCE plain is enabled by default. The runtime gate in the authorize handler accepts code_challenge_method=plain under this default, and a missing code_challenge_method parameter is silently downgraded to "plain" before the allowlist check. Discovery advertises code_challenge_methods_supported: ["S256"], contradicting the runtime acceptance of plain. RFC 9700 §2.1.1 (OAuth 2.1) explicitly forbids plain.
The metadata builders unconditionally inject "none" into the alg list. The runtime authorize gate is structured so a buggy client that strips the code_challenge_method parameter still enters the plain code path because the handler rewrites the missing value to "plain" before the allowlist check fires.
@better-auth/oauth-provider (the deprecation target for oidcProvider) is not affected by either defect. The metadata builder uses a JWSAlgorithms type union that structurally excludes "none". The authorize schema is code_challenge_method: z.literal("S256").optional(), which rejects plain at parse time.
Fixed in better-auth@1.6.11. The legacy oidcProvider and mcp plugins now:
"none" from id_token_signing_alg_values_supported (both plugins) and from resource_signing_alg_values_supported (mcp). Discovery no longer advertises the unsigned-token option.allowPlainCodeChallengeMethod to false. A request that explicitly passes code_challenge_method=plain is rejected with invalid_request unless the integrator opts in.code_challenge without an accompanying code_challenge_method instead of silently rewriting the missing value to plain. Clients that send code_challenge must also send code_challenge_method=S256.Discovery and runtime behavior align on S256 only by default.
Integrators who must keep plain PKCE for legacy clients can restore the previous shape with oidcProvider({ allowPlainCodeChallengeMethod: true }) (and likewise for mcp). With the opt-in set, a request that omits code_challenge_method is treated as plain again, preserving backwards compatibility while keeping the secure default for everyone else. Both legacy plugins are deprecated long-term; the recommended migration is @better-auth/oauth-provider, which never advertised none or accepted plain PKCE.
If developers cannot upgrade their applications immediately:
oidcProvider({ allowPlainCodeChallengeMethod: false }) (and the equivalent on mcp). Closes the runtime acceptance of plain even though the silent downgrade still rewrites missing methods."none" from id_token_signing_alg_values_supported. For oidcProvider, pass metadata: { id_token_signing_alg_values_supported: ["RS256"] }. For mcp, set the same on options.oidcConfig.metadata. Verify by curling the .well-known endpoint.@better-auth/oauth-provider: the package is the deprecation target and is unaffected by both defects.alg: "none") tokens.plain does not protect the authorization code if the URL leaks (Referer headers, browser history, screen capture, proxy logs). PKCE S256 is what protects against that exposure; with plain the protection is absent.Reported by @subhanUmer.
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