better-auth versions from 1.3.8-beta.3 up to but not including 1.7.0-rc.3 contain a UI misrepresentation vulnerability in the deviceAuthorization plugin. This vulnerability occurs because the GET /device endpoint omits the requesting client_id and scope, allowing attackers to trick signed-in users into approving a device code. The attacker can then redeem the code for a live session token, potentially gaining unauthorized access.
Better Auth versions prior to 1.4.5 rely on the rou3 router library, which normalizes URL paths by removing empty segments. This behavior causes different URL paths with extra slashes (e.g., /path, //path, ///path) to resolve to the same route. As a result, attackers can bypass disabledPaths configuration and path-based rate limits by submitting requests with extra slashes in the URL. This vulnerability does not affect deployments where the proxy or platform normalizes URLs by collapsing multiple slashes. The issue is fixed in Better Auth version 1.4.5, which bundles the fixed rou3 library.
better-auth versions before 1.6.2 fail 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 1.4.9 contain 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 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 versions before 1.6.11 contain 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 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 (which wraps the same provider). 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 (e.g. assigning it to window.location.href), the attacker's JavaScript executes in the authorization-server origin, exposing the victim's session and enabling account takeover.
better-auth versions >= 1.1.3 and < 1.6.22 (and pre-release versions >= 1.7.0-beta.0 and < 1.7.0-beta.10) are vulnerable to account takeover via pre-account hijacking on magic-link and email-OTP sign-in when open email/password registration is enabled. An attacker registers an account with the victim's email address and an attacker-chosen password; the account remains unverified. When the legitimate owner later signs in via the magic-link or email-OTP passwordless flow, the account is marked verified without removing the pre-existing password or revoking existing sessions, so the attacker's password remains valid, granting persistent access to the victim's account. Fixed in 1.6.22 and 1.7.0-beta.10.
better-auth versions after v0.0.2 and before 1.1.16 contain a reflected cross-site scripting (XSS) vulnerability on the /api/auth/error page, where the value of the 'error' URL parameter is reflected as HTML without proper neutralization. An attacker who coerces a user into visiting a specially-crafted URL can execute arbitrary JavaScript in the context of the user's browser. The issue is fixed in version 1.1.16.
better-auth versions before 1.1.20 contain a bypass vulnerability in trustedOrigins validation logic affecting absolute URLs and wildcard domains. Attackers can construct malicious callbackURL parameters that pass origin checks and trigger open redirects to steal sensitive tokens for account takeover.
better-auth versions greater than 1.3.34 and before 1.4.0 contain a vulnerability in the multi-session plugin's /sign-out after-hook, which trusts raw multi-session cookies and forwards extracted values to internalAdapter.deleteSessions without verifying the cookie signature (e.g., via getSignedCookie). An attacker can supply a forged _multi-* cookie to trigger deletion of arbitrary session tokens.