CVE-2025-71400

Better Auth library before version 1.6.11 has a vulnerability in the @better-auth/sso plugin. The POST /sso/register and POST /sso/update-provider endpoints accept attacker-controlled URLs (oidcConfig.userInfoEndpoint, tokenEndpoint, jwksEndpoint) when skipDiscovery: true is set, store them without origin validation, and fetch them during OIDC callback, allowing non-blind server-side request forgery (SSRF) and potential account linking when trustEmailVerified: true is configured.

Better Auth before version 1.6.11 has a vulnerability in the legacy oidcProvider and mcp plugins where OAuth token endpoints authenticate only possession of a valid refresh_token and client_id, without verifying the client_secret. This allows an attacker with a valid refresh_token to mint access tokens and rotated refresh tokens via /api/auth/oauth2/token or /api/auth/mcp/token.

better-auth (npm) before version 1.4.2 allows an external request to configure baseURL when it is not otherwise defined (e.g., BETTER_AUTH_URL is unset). An attacker able to make the very first request to the server after startup can poison the router's base path, causing all routes to return 404 for all users (denial of service). The issue is not reachable when baseURL is explicitly configured or on typical managed hosting platforms.

Better Auth relies on the rou3 router library, which normalizes URL paths by removing empty segments. In Better Auth versions prior to 1.4.5, an attacker can bypass disabledPaths configuration and path-based rate limits by submitting requests with extra slashes in the URL path. The issue does not apply when the proxy or platform normalizes URLs by collapsing multiple slashes.

Better-auth before version 1.4.9 contains a two-factor authentication bypass vulnerability when session.cookieCache is enabled. Attackers with valid primary credentials can access authenticated routes without completing second-factor verification by exploiting premature session caching.

better-auth before version 1.6.11 contains insecure cryptographic defaults in the oidcProvider and mcp plugins that advertise the none algorithm and accept plain PKCE by default. Attackers can exploit algorithm negotiation to accept unsigned tokens or intercept authorization codes when PKCE plain is used instead of the required S256 method.

Better-auth before version 1.6.2 fails to validate the OAuth state parameter against the stored nonce when using cookie-backed state storage without PKCE. Attackers can forge the state parameter and supply an attacker-controlled authorization code to create authenticated sessions bound to the attacker's external identity or persistently link attacker accounts to victim profiles.

better-auth versions before 1.6.11 fail to delete cached sessions when removing users via admin, anonymous, or SCIM endpoints when secondaryStorage is configured and storeSessionInDatabase is false. Attackers can reuse deleted user session tokens to maintain authentication for up to seven days after account deletion.

better-auth before version 1.6.13 (and pre-release builds 1.7.0-beta.0 through 1.7.0-beta.3) fail to validate the scheme of redirect_uris registered via the deprecated oidc-provider plugin and the mcp plugin. An attacker can register an OAuth client with a javascript: redirect_uri, which the authorization server later returns unchanged in the consent response. If the deployment's consent page navigates the browser to the returned redirectURI, the attacker's JavaScript executes in the authorization-server origin, exposing the victim's session and enabling account takeover.

better-auth SCIM versions from 1.5.0 before 1.7.0-beta.4 fail to bind non-organization SCIM providers to their creator by default, allowing authenticated users to manage other users' providers. Attackers can regenerate SCIM bearer tokens, invalidate legitimate tokens, and authenticate to SCIM API routes with the attacker-controlled token.