CVE-2026-53518

Better Auth is an authentication and authorization library for TypeScript. From 1.6.0 until 1.6.11, the @better-auth/oauth-provider POST /oauth2/token endpoint for the authorization_code grant redeems a single-use authorization code through a non-atomic find-then-delete sequence, allowing two concurrent requests to pass the read step and mint independent access tokens, refresh tokens, and ID tokens; legacy /oauth2/token and /mcp/token paths in oidc-provider and mcp plugins share the same primitive. This issue is fixed in version 1.6.11.

Better Auth is an authentication and authorization library for TypeScript. From 1.6.0 until 1.6.11, the @better-auth/oauth-provider POST /oauth2/token endpoint for the authorization_code grant redeems a single-use authorization code through a non-atomic find-then-delete sequence, allowing two concurrent requests to pass the read step and mint independent access tokens, refresh tokens, and ID tokens; legacy /oauth2/token and /mcp/token paths in oidc-provider and mcp plugins share the same primitive. This issue is fixed in version 1.6.11.

Am I affected?

Users are affected if all of the following are true:

• Their project depends on @better-auth/oauth-provider at a version >= 1.6.0, < 1.6.11, or uses the embedded plugin in better-auth >= 1.4.8-beta.7, < 1.6.0, or enables the legacy oidc-provider or mcp plugins from better-auth/plugins.
• Their application exposes /api/auth/oauth2/token (or the legacy plugins' /oauth2/token and /mcp/token) as a token endpoint to OAuth/OIDC clients, including internal MCP clients (Claude Desktop, custom MCP tool callers, AI agents).
• Their application has not implemented an external mitigation: a load-balancer-level idempotency cache keyed by code, a database trigger that rejects duplicate token issuance for the same authorization code, or a custom adapter override that performs an atomic compare-and-delete.

Fix:

1. Upgrade to @better-auth/[email protected] or later. If developers use the legacy plugin paths from better-auth/plugins, upgrade better-auth to 1.6.11 or later.
2. If developers cannot upgrade, see workarounds below.

Summary

The OAuth provider's POST /oauth2/token endpoint, on the authorization_code grant, redeems a single-use authorization code through a non-atomic find-then-delete sequence. Two concurrent requests with the same code value both pass the read step before either delete completes, then both proceed to PKCE verification and createUserTokens. Each surviving request mints a fresh access token, refresh token, and id token. RFC 6749 §4.1.2 requires authorization codes to be single-use; this primitive does not enforce that under concurrency.

Details

The same architectural primitive (find a single-use verification row, then delete it, then trust the row to authorize) is used in 20 other call sites across the codebase. The deletion primitive returns Promise<void>, discarding the row count surfaced by adapter.deleteMany, so no call site can detect "another caller already claimed this row". The fix lands at the primitive layer rather than at any individual call site.

The fix introduces a claimVerificationByIdentifier primitive at the internal-adapter layer that performs an atomic claim-and-return, replaces the find-then-delete pair at this call site, and migrates the highest-impact variant sites in the same release.

Patches

Fixed in @better-auth/[email protected] and [email protected] for the legacy oidc-provider and mcp plugin paths. All three token-exchange call sites now consume the verification row through internalAdapter.consumeVerificationValue, an atomic claim primitive that deletes the row and returns its prior value in one operation. The first request to arrive takes the row and mints tokens; concurrent racers observe an empty result and return invalid_grant.

Error-code consistency is also tightened on the @better-auth/oauth-provider token endpoint: the malformed-verification-value branches previously returned a project-specific invalid_verification code, which is not part of RFC 6749 §5.2's response error set. Both branches now return invalid_grant so spec-compliant clients can branch on the standard code without a special case.

Workarounds

None of these close the bug fully without a code patch. Upgrading is the only good path.

• Network-layer: deploy an authorization-server-aware reverse proxy (Envoy, NGINX with Lua, custom Cloudflare Worker) that holds an in-flight registry keyed by the code parameter and serializes concurrent requests for the same code. Fragile under multi-instance deployments unless the registry is shared (Redis-backed).
• Database-layer: add a SQL or Mongo uniqueness constraint that prevents two oauthAccessToken rows from being created with the same upstream code reference. Adapter-specific and not always feasible since the schema does not currently store the source code.
• Application-layer: wrap deleteVerificationByIdentifier with a custom hook that uses…

Better Auth is an authentication and authorization library for TypeScript. From 1.6.0 until 1.6.11, the @better-auth/oauth-provider POST /oauth2/token endpoint for the authorization_code grant redeems a single-use authorization code through a non-atomic find-then-delete sequence, allowing two concurrent requests to pass the read step and mint independent access tokens, refresh tokens, and ID tokens; legacy /oauth2/token and /mcp/token paths in oidc-provider and mcp plugins share the same primitive. This issue is fixed in version 1.6.11.

Better Auth is an authentication and authorization library for TypeScript. From 1.6.0 until 1.6.11, the @better-auth/oauth-provider POST /oauth2/token endpoint for the authorization_code grant redeems a single-use authorization code through a non-atomic find-then-delete sequence, allowing two concurrent requests to pass the read step and mint independent access tokens, refresh tokens, and ID tokens; legacy /oauth2/token and /mcp/token paths in oidc-provider and mcp plugins share the same primitive. This issue is fixed in version 1.6.11.

Am I affected?

Users are affected if all of the following are true:

• Their project depends on @better-auth/oauth-provider at a version >= 1.6.0, < 1.6.11, or uses the embedded plugin in better-auth >= 1.4.8-beta.7, < 1.6.0, or enables the legacy oidc-provider or mcp plugins from better-auth/plugins.
• Their application exposes /api/auth/oauth2/token (or the legacy plugins' /oauth2/token and /mcp/token) as a token endpoint to OAuth/OIDC clients, including internal MCP clients (Claude Desktop, custom MCP tool callers, AI agents).
• Their application has not implemented an external mitigation: a load-balancer-level idempotency cache keyed by code, a database trigger that rejects duplicate token issuance for the same authorization code, or a custom adapter override that performs an atomic compare-and-delete.

Fix:

1. Upgrade to @better-auth/[email protected] or later. If developers use the legacy plugin paths from better-auth/plugins, upgrade better-auth to 1.6.11 or later.
2. If developers cannot upgrade, see workarounds below.

Summary

The OAuth provider's POST /oauth2/token endpoint, on the authorization_code grant, redeems a single-use authorization code through a non-atomic find-then-delete sequence. Two concurrent requests with the same code value both pass the read step before either delete completes, then both proceed to PKCE verification and createUserTokens. Each surviving request mints a fresh access token, refresh token, and id token. RFC 6749 §4.1.2 requires authorization codes to be single-use; this primitive does not enforce that under concurrency.

Details

The same architectural primitive (find a single-use verification row, then delete it, then trust the row to authorize) is used in 20 other call sites across the codebase. The deletion primitive returns Promise<void>, discarding the row count surfaced by adapter.deleteMany, so no call site can detect "another caller already claimed this row". The fix lands at the primitive layer rather than at any individual call site.

The fix introduces a claimVerificationByIdentifier primitive at the internal-adapter layer that performs an atomic claim-and-return, replaces the find-then-delete pair at this call site, and migrates the highest-impact variant sites in the same release.

Patches

Fixed in @better-auth/[email protected] and [email protected] for the legacy oidc-provider and mcp plugin paths. All three token-exchange call sites now consume the verification row through internalAdapter.consumeVerificationValue, an atomic claim primitive that deletes the row and returns its prior value in one operation. The first request to arrive takes the row and mints tokens; concurrent racers observe an empty result and return invalid_grant.

Error-code consistency is also tightened on the @better-auth/oauth-provider token endpoint: the malformed-verification-value branches previously returned a project-specific invalid_verification code, which is not part of RFC 6749 §5.2's response error set. Both branches now return invalid_grant so spec-compliant clients can branch on the standard code without a special case.

Workarounds

None of these close the bug fully without a code patch. Upgrading is the only good path.

• Network-layer: deploy an authorization-server-aware reverse proxy (Envoy, NGINX with Lua, custom Cloudflare Worker) that holds an in-flight registry keyed by the code parameter and serializes concurrent requests for the same code. Fragile under multi-instance deployments unless the registry is shared (Redis-backed).
• Database-layer: add a SQL or Mongo uniqueness constraint that prevents two oauthAccessToken rows from being created with the same upstream code reference. Adapter-specific and not always feasible since the schema does not currently store the source code.
• Application-layer: wrap deleteVerificationByIdentifier with a custom hook that uses…