Better Auth has an account takeover issue via OAuth auto-link to unverified pre-registered email

Am I affected?

Users are affected if all of the following are true:

• Their application uses better-auth at a version < 1.6.11 on the stable line, or any current next pre-release.
• emailAndPassword.enabled: true is set in their application's betterAuth({ ... }) configuration.
• At least one OAuth or SSO provider is configured (any built-in social provider, or genericOAuth(...), or any provider via @better-auth/sso).
• account.accountLinking.disableImplicitLinking is not set to true.
• account.accountLinking.enabled is not set to false.

Setting either disableImplicitLinking: true or enabled: false closes the hole at the cost of breaking the standard "add another login method" UX. emailAndPassword.requireEmailVerification: true does not mitigate, because the link-time emailVerified flip promotes the attacker's row to verified, after which the password login becomes usable.

Fix:

1. Upgrade to better-auth@1.6.11 or later.
2. If developers cannot upgrade, see workarounds below.

Summary

The OAuth callback's auto-link gate in handleOAuthUserInfo admits an implicit account link whenever the provider asserts email_verified: true, without requiring the local user row's emailVerified to also be true. An attacker who pre-registers a victim's email through /sign-up/email (which writes a row with emailVerified: false) can have the victim's later OAuth identity bound to the attacker's user row, granting both a password login and the victim's OAuth identity on the same account. This is the pre-account-hijacking class — the same shape as Microsoft "nOAuth" (2023) and the Sign in with Apple JWT flaw (2020).

Details

The auto-link gate validates only the OAuth provider's userInfo.emailVerified claim. The local row's emailVerified field is never read. When no (accountId, providerId) match exists, the user lookup falls back to email, which surfaces any pre-registered row at that email.

A separate post-link step promotes the local emailVerified to true when the provider's claim is true and the local email matches the provider's email. This step is correct for legitimate first-time linking, but combined with the missing local-side check it becomes load-bearing for the takeover: after the link, the attacker's password row is treated as verified, defeating requireEmailVerification: true as a mitigation.

The fix adds the local-side ownership check to the gate: implicit linking now also rejects when dbUser.user.emailVerified is false. The same primitive lives in one-tap and inherits the same fix shape; the SSO domainVerified short-circuit follows separately as a hardening change.

Patches

Fixed in better-auth@1.6.11. Implicit linking now refuses to attach an OAuth identity to a local account whose emailVerified flag is false. The same gate change applies in the one-tap sign-in plugin, which previously had its own simpler linking path. The Google ID-token email_verified claim is also normalized through toBoolean so a string "false" is treated as falsy (some Google responses send the string, which the prior code treated as truthy).

The public surface for the new gate is account.accountLinking.requireLocalEmailVerified, defaulted to true. Applications whose users sign up through OAuth without ever verifying their email locally can opt out with account: { accountLinking: { requireLocalEmailVerified: false } } to retain the legacy permissive behavior. The option is marked @deprecated; the gate at each call site carries a FIXME pointing at the next-minor follow-up that drops the option and makes the check unconditional.

Test fixtures across the admin, oidc-provider, mcp, generic-oauth, last-login-method, and oauth-provider suites now pre-verify created users via a databaseHooks.user.create.before hook (or the disableTestUser opt-in on the oauth-provider RP fixture) so those suites continue to exercise their role and flow logic rather than tripping the new gate.

Workarounds

If developers cannot upgrade their applications immediately:

• Disable implicit linking: set account.accountLinking.disableImplicitLinking: true. Forces all linking through the authenticated /link-social endpoint where the user must already be signed in.
• Disable linking entirely: set account.accountLinking.enabled: false. Closes the hole but breaks the multi-login-method UX entirely.

emailAndPassword.requireEmailVerification: true alone does not mitigate, because the link-time emailVerified flip promotes the attacker's row to verified.

Impact

• Account takeover via pre-account hijacking: the attacker holds a working password login plus the victim's OAuth identity on the same account, granting persistent access.
• requireEmailVerification: true bypass: the attacker's password login becomes usable post-link.
• Cross-flow reach: every OAuth and SSO sign-in path that calls handleOAuthUserInfo is affected (built-in social providers, generic-oauth, oauth-proxy, SSO OIDC, SSO SAML, one-tap).

Credit

Reported by @avrmeduard.

Resources

• CWE-287: Improper Authentication
• CWE-345: Insufficient Verification of Data Authenticity
• Sudhodanan & Paverd, Pre-hijacked accounts: an empirical study of security failures in user account creation on the web (USENIX Security 2022)