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:
- Upgrade to
better-auth@1.6.2 or later (current stable is 1.6.10).
- 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
References
Am I affected?
Users are affected if all of the following are true:
better-authat a version below1.6.2(or@better-auth/ssopaired with such a version).betterAuth({ account: { storeStateStrategy } })is set to"cookie". The default"database"is not affected.genericOAuth({ config })withpkce: false, or it supplies a customgetTokenortokenUrlthat does not require the storedcodeVerifier. Stock social providers with PKCE on are not affected.codevalues to the configured callback URL.If users are on
better-auth@1.6.2or later, they are not affected.Fix:
better-auth@1.6.2or later (current stable is1.6.10).Summary
In
parseGenericState, the cookie branch decrypted theoauth_statecookie and validated expiry, but did not compare the incoming OAuthstatequery parameter to the nonce thatgenerateGenericStateissued at sign-in. Any callback to/api/auth/oauth2/callback/<providerId>that arrived with a forgedstateand anycodewas therefore accepted as long as the browser still held a liveoauth_statecookie. Withpkce: false(or anygetTokenpath 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
parseGenericStatedid not compare the cookie's stored nonce to the incomingstateparameter. The database branch (the default) was not affected because the verification row is keyed bystateand the lookup itself enforces equality.The fix re-binds the cookie to the nonce:
generateGenericStatewritesoauthState: stateinto the encrypted payload before storage, andparseGenericStaterejects whenparsedData.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@1.6.2via PR #8949 (commit9deb7936a, merged 2026-04-09). The cookie branch ofparseGenericStatenow rejects when the encrypted payload's nonce does not match the incomingstateparameter; the database branch gained a defense-in-depth equality check.Workarounds
If users cannot upgrade immediately:
storeStateStrategyback to"database"(the default). This closes the cookie-only bypass without a code change.pkce: trueon every affectedgenericOAuthprovider. ThecodeVerifieris the missing primitive that the attacker cannot supply.Impact
Credit
Reported by @Jvr2022 via private advisory disclosure, and by @alavesa (PatchPilots audit) via the public duplicate issue #8897.
Resources
References