CVE-2026-67333 in better-auth
Summary
by MITRE • 08/01/2026
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.
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Analysis
by VulDB Data Team • 08/01/2026
This vulnerability affects better-auth versions prior to 1.6.13 and includes pre-release builds 1.7.0-beta.0 through 1.7.0-beta.3, representing a critical authorization flow flaw that exploits improper validation of redirect URI schemes within the deprecated oidc-provider plugin and mcp plugin implementations. The vulnerability stems from insufficient input validation where the system accepts javascript: scheme redirect URIs without proper sanitization or verification, creating an open redirect condition that can be exploited through malicious client registration.
The technical exploitation occurs when an attacker registers an OAuth client with a javascript: redirect_uri through the deprecated plugins, which then get stored in the authorization server's client configuration. During the authorization flow, when the consent page processes the authorization response, it returns the malicious redirect URI unchanged to the user's browser. When the deployment's consent page executes window.location.href = redirect_uri, the attacker's JavaScript code executes within the trusted authorization server origin context, creating a cross-origin scripting opportunity that bypasses typical security boundaries.
This vulnerability directly maps to CWE-601 Open Redirect and CWE-79 Cross-Site Scripting, with implications for the ATT&CK technique T1531 Account Access Removal and T1078 Valid Accounts. The attack chain leverages the authorization server's trust relationship with the client application, enabling attackers to execute arbitrary JavaScript code in the context of the user's session with the authorization server. This creates a severe privilege escalation opportunity where session tokens and sensitive user data can be exfiltrated or manipulated.
The operational impact extends beyond simple session hijacking to include complete account takeover capabilities, as the executed JavaScript can access all session cookies, local storage, and potentially interact with other browser-based applications that share the same origin. Attackers could use this vulnerability to impersonate users, access sensitive resources, modify user settings, or perform unauthorized transactions. The exploitation requires minimal user interaction beyond normal OAuth flow participation, making it particularly dangerous in environments where users frequently authenticate through OAuth providers.
Mitigation strategies should include immediate patching to version 1.6.13 or later, implementing strict validation of redirect URI schemes that rejects javascript:, data:, and other potentially dangerous protocols, and configuring the authorization server to validate that redirect URIs match pre-registered patterns. Organizations should also consider implementing Content Security Policy headers to limit script execution capabilities and conduct thorough audits of all registered OAuth clients. Additionally, deployments should disable or remove deprecated plugins like oidc-provider and mcp that contain known vulnerabilities, while implementing proper input sanitization mechanisms for all user-provided URI parameters in authentication flows.