Better Auth SSO EUVD-2026-44733

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/sso@1.6.11 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/sso@1.6.11. 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)
• CWE-20: Improper Input Validation
• CWE-441: Unintended Proxy or Intermediary
• CWE-345: Insufficient Verification of Data Authenticity