Better Auth: OAuth callback accepts mismatched `state` when cookie-backed state storage is used without PKCE

Summary

### Am I affected? Users are affected if all of the following are true: - The application uses `better-auth` at a version below `1.6.2` (or `@better-auth/sso` paired with such a version). - `betterAuth({ account: { storeStateStrategy } })` is set to `"cookie"`. The default `"database"` is not affected. - The application wires at least one OAuth provider through `genericOAuth({ config })` with `pkce: false`, or it supplies a custom `getToken` or `tokenUrl` that does not require the stored `codeVerifier`. Stock social providers with PKCE on are not affected. - The provider returns arbitrary `code` values to the configured callback URL. If users are on `[email protected]` or later, they are not affected. Fix: 1. Upgrade to `[email protected]` or later (current stable is `1.6.10`). 2. If users cannot upgrade, see workarounds below. ### Summary In `parseGenericState`, the cookie branch decrypted the `oauth_state` cookie and validated expiry, but did not compare the incoming OAuth `state` query parameter to the nonce that `generateGenericState` issued at sign-in. Any callback to `/api/auth/oauth2/callback/<providerId>` that arrived with a forged `state` and any `code` was therefore accepted as long as the browser still held a live `oauth_state` cookie. With `pkce: false` (or any `getToken` path that does not enforce a code-verifier round-trip), an attacker who forced the victim to deliver an attacker-controlled authorization code to the callback would mint a session bound to the attacker's external identity in the victim's browser. Account-linking flows behaved the same way, binding the attacker's external account to an authenticated victim row. ### Details The cookie branch of `parseGenericState` did not compare the cookie's stored nonce to the incoming `state` parameter. The database branch (the default) was not affected because the verification row is keyed by `state` and the lookup itself enforces equality. The fix re-binds the cookie to the nonce: `generateGenericState` writes `oauthState: state` into the encrypted payload before storage, and `parseGenericState` rejects when `parsedData.oauthState !== state`. The same primitive covers every caller (`generic-oauth`, social, account-link, oauth-proxy passthrough, OIDC SSO, SAML relay state). ### Patches Fixed in `[email protected]` via [PR #8949](https://github.com/better-auth/better-auth/pull/8949) (commit `9deb7936a`, merged 2026-04-09). The cookie branch of `parseGenericState` now rejects when the encrypted payload's nonce does not match the incoming `state` parameter; the database branch gained a defense-in-depth equality check. ### Workarounds If users cannot upgrade immediately: - **Switch `storeStateStrategy` back to `"database"`** (the default). This closes the cookie-only bypass without a code change. - **Enable `pkce: true`** on every affected `genericOAuth` provider. The `codeVerifier` is the missing primitive that the attacker cannot supply. ### Impact - **Forced-login (CSRF on OAuth callback)**: the attacker forces the victim's browser into an authenticated session bound to the attacker's external identity, allowing the attacker to observe the victim's actions inside the application. - **Persistent account linking**: account-link flows bind the attacker's external account to the victim's authenticated row, granting persistent access until the link is removed. ### Credit Reported by @Jvr2022 via private advisory disclosure, and by @alavesa (PatchPilots audit) via the public duplicate [issue #8897](https://github.com/better-auth/better-auth/issues/8897). ### Resources - [CWE-352: Cross-Site Request Forgery (CSRF)](https://cwe.mitre.org/data/definitions/352.html) - [CWE-345: Insufficient Verification of Data Authenticity](https://cwe.mitre.org/data/definitions/345.html) - [CWE-287: Improper Authentication](https://cwe.mitre.org/data/definitions/287.html) - [RFC 6749 §10.12: Cross-Site Request Forgery](https://datatracker.ietf.org/doc/html/rfc6749#section-10.12) - [RFC 7636: Proof Key for Code Exchange](https://datatracker.ietf.org/doc/html/rfc7636)