CVE-2026-67336

Am I affected?

Users are affected if all of the following are true:

• Their application uses better-auth at a version below the patched release.
• Their application enables oidcProvider() from better-auth/plugins/oidc-provider or mcp() from better-auth/plugins/mcp (the mcp plugin delegates to oidcProvider and inherits both defaults).
• For the algorithm-negotiation impact: relying parties of the application's OIDC server use a JWT verification library that performs algorithm negotiation from the discovery document without pinning to a specific signing algorithm.
• For the PKCE impact: the authorization URL is exposed to any party other than the user agent and the application's OP.

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:

1. Upgrade to better-auth@1.6.11 or later.
2. Migrate from the deprecated oidcProvider and mcp plugins to @better-auth/oauth-provider when feasible.
3. If developers cannot upgrade their applications, see workarounds below.

Summary

The legacy oidcProvider and mcp plugins exhibit two related defects in their OIDC discovery and authorize surfaces.

The discovery document advertises "none" in idtokensigningalgvalues_supported (and, for mcp, in resourcesigningalgvaluessupported 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 codechallengemethod=plain under this default, and a missing codechallengemethod parameter is silently downgraded to "plain" before the allowlist check. Discovery advertises codechallengemethods_supported: ["S256"], contradicting the runtime acceptance of plain. RFC 9700 §2.1.1 (OAuth 2.1) explicitly forbids plain.

Details

The metadata builders unconditionally inject "none" into the alg list. The runtime authorize gate is structured so a buggy client that strips the codechallengemethod 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 codechallengemethod: z.literal("S256").optional(), which rejects plain at parse time.

Patches

Fixed in better-auth@1.6.11. The legacy oidcProvider and mcp plugins now:

• Drop "none" from idtokensigningalgvalues_supported (both plugins) and from resourcesigningalgvaluessupported (mcp). Discovery no longer advertises the unsigned-token option.
• Default allowPlainCodeChallengeMethod to false. A request that explicitly passes codechallengemethod=plain is rejected with invalid_request unless the integrator opts in.
• Reject a code_challenge without an accompanying codechallengemethod instead of silently rewriting the missing value to plain. Clients that send code_challenge must also send codechallengemethod=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 codechallengemethod 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.

Workarounds

If developers cannot upgrade their applications immediately:

• Disable plain PKCE explicitly: set oidcProvider({ allowPlainCodeChallengeMethod: false }) (and the equivalent on mcp). Closes the runtime acceptance of plain even though the silent downgrade still rewrites missing methods.
• Override the metadata to drop "none" from idtokensigningalgvalues_supported. For oidcProvider, pass metadata: { idtokensigningalgvalues_supported: ["RS256"] }. For mcp, set the same on options.oidcConfig.metadata. Verify by curling the .well-known endpoint.
• Migrate to @better-auth/oauth-provider: the package is the deprecation target and is unaffected by both defects.

Impact

• Algorithm-negotiation downgrade: relying parties that read the discovery document without pinning may accept unsigned (alg: "none") tokens.
• Authorization-code interception: PKCE 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.
• OAuth 2.1 / RFC 9700 non-conformance: deployments shipping the defaults are non-compliant with current standards.

Credit

Reported by @subhanUmer.

Resources

• CWE-327: Use of a Broken or Risky Cryptographic Algorithm
• CWE-757: Selection of Less-Secure Algorithm During Negotiation
• CWE-1188: Insecure Default Initialization of Resource
• RFC 9700 §2.1.1: PKCE
• RFC 9700 §2.1.2: Token Replay Prevention
• RFC 8414 §2: Authorization Server Metadata