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

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 better-auth@1.6.2 or later, they are not affected.

Fix:

1. Upgrade to better-auth@1.6.2 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 better-auth@1.6.2 via PR #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.

Resources

• CWE-352: Cross-Site Request Forgery (CSRF)
• CWE-345: Insufficient Verification of Data Authenticity
• CWE-287: Improper Authentication
• RFC 6749 §10.12: Cross-Site Request Forgery
• RFC 7636: Proof Key for Code Exchange