CVE-2026-53513: Outdated @better-auth/sso Plugin Allows Unauthorized Server Access

### Am I affected?

Users are affected if all of the following are true:

- Their application uses `@better-auth/sso` at a version `>= 0.1.0, < 1.6.11` on the stable line, or any `1.7.0-beta.x` on the pre-release line.
- The `sso()` plugin is added to their application's `betterAuth({ plugins: [...] })` array.
- Any user with a valid Better Auth session can reach `POST /sso/register` (the plugin's default gate accepts any session).

For the non-blind SSRF impact (full IAM credential or internal HTTP body exfiltration), no further configuration is required.

For the account takeover escalation, additionally:

- Developers set `sso({ trustEmailVerified: true, ... })`.
- The developer's application deployment has accounts whose `email` overlaps with attacker-chosen domains.

If developers do not enable the SSO plugin, their application is not affected.

Fix:

1. Upgrade to `@better-auth/[email protected]` or later.
2. If developers cannot upgrade, see workarounds below.

### Summary

The `@better-auth/sso` plugin's `POST /sso/register` endpoint accepts attacker-controlled `oidcConfig.userInfoEndpoint`, `tokenEndpoint`, and `jwksEndpoint` URLs when `skipDiscovery: true` is set, persists them on the `ssoProvider` row without origin validation, then issues server-side fetches to those URLs during the OIDC callback. The fetched response body is reflected through the user profile, producing a non-blind SSRF reachable by any authenticated session. The same primitive exists on `POST /sso/update-provider`.

### Details

The schema field types accept bare strings: no `.url()` validator, no origin gate. The discovery branch (`skipDiscovery: false`) routes URLs through `validateDiscoveryUrl`; the skip-discovery branch persists them as-is. At callback time three fetch sites read the stored URLs: `validateAuthorizationCode` for the token endpoint, `betterFetch` for the userInfo endpoint, and `validateToken` for the JWKS endpoint.

When `trustEmailVerified: true` is configured, the attacker can escalate to account linking. A malicious userInfo response with `emailVerified: true` and a chosen `email` triggers OAuth auto-link against any pre-existing user row with that email, compounding the SSRF into account takeover.

### Patches

Fixed in `@better-auth/[email protected]`. Provider registration (`POST /sso/register` with `skipDiscovery: true`) and every `POST /sso/update-provider` request now validate each supplied OIDC endpoint URL (`authorizationEndpoint`, `tokenEndpoint`, `userInfoEndpoint`, `jwksEndpoint`, `discoveryEndpoint`) at registration time. A URL is rejected unless it satisfies one of two conditions:

1. Its host is publicly routable on the internet, evaluated through the `@better-auth/core/utils/host.isPublicRoutableHost` gate. RFC 1918 private ranges, RFC 4193 unique-local addresses, link-local addresses (including the cloud-metadata IP `169.254.169.254`), loopback, multicast, broadcast, and reserved ranges are rejected, along with cloud-metadata FQDNs.
2. Its origin is already listed in the application's `trustedOrigins` configuration. This preserves the documented escape hatch for customers running internal IdPs intentionally on private networks.

The schema also tightens from `z.string()` to `z.url()` on those fields, so malformed URLs fail at parse time rather than at fetch time. Deployments running internal IdPs that previously worked must add the IdP's origin to `trustedOrigins` to keep working after upgrade.

### Workarounds

If developers cannot upgrade immediately:

- **Disable provider self-registration**: set `sso({ providersLimit: 0 })`. The limit is enforced before the schema branch, blocking every `/sso/register` regardless of `skipDiscovery`.
- **Reverse-proxy gate**: block `POST /sso/register` and `POST /sso/update-provider` at the edge, or restrict to a denylist of source IPs and a small admin user list.
- **Network-level egress controls**: block egress from the auth server to RFC 1918, RFC 4193, link-local ranges (`169.254.0.0/16`, `fe80::/10`), and the cloud-metadata FQDN list at the firewall or VPC level. AWS users should additionally enforce IMDSv2 (`HttpTokens: required`).
- **Set `trustEmailVerified: false`** until upgrade. This caps the impact at non-blind SSRF and removes the account-takeover escalation, but does not stop the SSRF.

### Impact

- **Server-Side Request Forgery (non-blind)**: the attacker reads response bodies from any HTTP endpoint reachable from the auth server, including cloud metadata services (AWS IMDS, GCP metadata FQDN), internal-only APIs, and infrastructure services such as Redis or admin panels bound to localhost.
- **Account takeover** (when `trustEmailVerified: true`): the attacker mints a malicious userInfo response asserting `emailVerified: true` for an arbitrary email, triggering OAuth auto-link against pre-existing user rows.

### Credit

Reported by Vaadata.

### Resources

- [CWE-918: Server-Side Request Forgery (SSRF)](https://cwe.mitre.org/data/definitions/918.html)
- [CWE-20: Improper Input Validation](https://cwe.mitre.org/data/definitions/20.html)
- [CWE-441: Unintended Proxy or Intermediary](https://cwe.mitre.org/data/definitions/441.html)
- [CWE-345: Insufficient Verification of Data Authenticity](https://cwe.mitre.org/data/definitions/345.html)

CWE-20 Improper Input Validation

CWE-345

CWE-441

CWE-918 Server-Side Request Forgery (SSRF)