GHSA-pw9m-5jxm-xr6h Better Auth: OAuth refresh-token replay via missing client authentication on oidc-provider and mcp plugins

• npm better-auth: < 1.6.11 → fixed in 1.6.11

Am I affected? Users are affected if all of the following are true: • Their application uses `better-auth` and has enabled at least one of: `oidcProvider()` (imported from `better-auth/plugins/oidc-provider`), or `mcp()` (imported from `better-auth/plugins/mcp`). • Their application has at least one confidential OAuth client registered (any client with `type: "web" | "native" | "user-agent-based"` in the `oauthApplication` table, or any `trustedClients` entry without `type: "public"`). Public clients with PKCE are not affected. • Their application uses `better-auth` at a version below the patched release. If an application only uses `@better-auth/oauth-provider` (the canonical replacement for `oidc-provider`) and the `mcp` plugin is not enabled, it is not affected. Fix: 1. Upgrade to `[email protected]` or later. 2. Migrate from the deprecated `oidcProvider()` to `@better-auth/oauth-provider` when feasible. The new package enforces client authentication on both grants by default. 3. If developers cannot upgrade their applications, see workarounds below. Summary The legacy `oidcProvider` and `mcp` plugins each expose an OAuth 2.0 token endpoint whose `refresh_token` grant authenticates the request entirely on possession of the bound `refreshToken` row and a matching `client_id`. Neither plugin verifies the registered confidential client's `client_secret` on the refresh path. An attacker who obtains any valid `refresh_token` (via database read, log capture, browser-side XSS, or CORS-amplified script in the mcp case) and the public `client_id` can mint fresh access tokens and rotated refresh tokens until the chain is revoked. Details RFC 6749 §6 and OAuth 2.1 §4.3 require confidential clients to authenticate to the token endpoint on every grant, including refresh. The same plugins' `authorization_code` grant correctly enforces `client_secret` (the oidc-provider via `verifyStoredClientSecret`, the mcp plugin via raw equality), which proves the omission on the refresh path is a regression rather than a design choice. Token rotation issues a new `refresh_token` with each call, so a single leaked refresh-token grants indefinite access until the row is revoked or its `refreshTokenExpiresAt` (default 7 days) passes; rotation refreshes that window each call. Two adjacent issues on the mcp surface ship in the same patch. The mcp `authorization_code` grant uses raw `===` for client-secret comparison and ignores the `storeClientSecret: "encrypted" | "hashed"` configuration; the fix routes both grants through `verifyStoredClientSecret`. The mcp `/mcp/token` endpoint sets `Access-Control-Allow-Origin: *` unconditionally, which amplifies the refresh bypass in browser contexts; the fix narrows the CORS allowlist. The newer `@better-auth/oauth-provider` package routes both grants through `validateClientCredentials` and is not affected. Patches Fixed in `[email protected]`. The legacy `oidcProvider` and `mcp` token endpoints now require `client_secret` on the `refresh_token` grant for confidential clients, using the same constant-time comparison the `authorization_code` grant already used. Public clients are unaffected (they have no secret to enforce, and PKCE substitutes on the auth-code grant). The `Authorization: Basic` parser is fixed to follow RFC 6749 §2.3.1: the credential is split on the first colon and each half is percent-decoded. Client IDs and secrets that contain reserved characters now authenticate correctly. The `/mcp/token` endpoint's CORS configuration is narrowed in the same change (the wildcard `Access-Control-Allow-Origin: *` header is removed), matching the standalone `@better-auth/oauth-provider` package. The deprecated `oidc-provider` plugin remains deprecated. The recommended migration path is `@better-auth/oauth-provider`. Workarounds None of these close the bug fully without a code patch. • Migrate to `@better-auth/oauth-provider` if your deployment can adopt the new plugin. It enforces `client_secret` on both grants. • Force all clients to public + PKCE: set every client's `type: "public"` and require PKCE. The bug is unreachable when there is no `client_secret` to verify. • Network-layer ingress restriction: limit `/api/auth/oauth2/token` and `/api/auth/mcp/token` to known client IPs at the load balancer. Practical for server-to-server flows, not for end-user-device clients. • Out-of-band refresh-token rotation: on any suspicion of leak, run `db.deleteMany({ model: "oauthAccessToken", where: [{ field: "clientId", value: }] })` to invalidate all refresh tokens for the affected client. • For the mcp endpoint specifically: drop the wildcard CORS at an upstream proxy and replace with a tight allowlist. Impact • Indefinite confidential-client impersonation: an attacker holding any valid `refresh_token` and the public `client_id` can mint access tokens and rotated refresh tokens indefinitely, until the row is revoked. Rotation refreshes the expiration window each call. • Resource access at the user's authorized scope: every minted access token carries the original user's authorization scope, so the attacker reads or writes whatever the resource server grants for that scope. Credit Reported by @subhanUmer. Resources • CWE-306: Missing Authentication for Critical Function • CWE-287: Improper Authentication • CWE-345: Insufficient Verification of Data Authenticity • CWE-863: Incorrect Authorization • RFC 6749 §6: Refreshing an Access Token • OAuth 2.1 §4.3: Refresh Token