CVE-2026-53518: Better Auth OAuth Provider: Race Condition in Authorization Code Exchange Enables Multi-Use Code Redemption

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/oauth-provider@1.6.11 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 authorizationcode 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/oauth-provider@1.6.11 and better-auth@1.6.11 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 invalidgrant.

Error-code consistency is also tightened on the @better-auth/oauth-provider token endpoint: the malformed-verification-value branches previously returned a project-specific invalidverification code, which is not part of RFC 6749 §5.2's response error set. Both branches now return invalidgrant 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 adapter.deleteMany and surfaces the count, then injects an invalidgrant rejection when the count is zero. Requires forking the internal adapter.

Impact

- Multiple independent token sets from a single authorization: forked access tokens, refresh tokens, and id tokens issued from the same code, all valid for the original user's authorization scope. - Detection bypass: standard OAuth single-use enforcement does not fire for the second redemption when both requests interleave through the read step. - Legacy-plugin reach: oidc-provider and mcp plugins share the primitive on the same surface, so deployments using them inherit the same impact.

Credit

Reported by @chdanielmueller.

Resources

- CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization (Race Condition) - CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition - CWE-294: Authentication Bypass by Capture-replay - RFC 6749 §4.1.2: Authorization Response - OAuth 2.1 §4.1: Authorization Code Grant