CVE-2025-71403 in better-auth
Summary
by MITRE • 08/01/2026
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.
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Analysis
by VulDB Data Team • 08/01/2026
The vulnerability in better-auth versions prior to 1.1.20 represents a critical authorization bypass flaw that undermines the security of web applications relying on trusted origins validation. This issue specifically affects the implementation of origin checking mechanisms within the authentication flow, where the system fails to properly validate absolute URLs and wildcard domain patterns. The flaw exists in the callback URL processing logic that is designed to prevent open redirect attacks by verifying that incoming requests originate from approved domains. When attackers exploit this vulnerability, they can craft malicious callbackURL parameters that appear legitimate to the origin validation checks, thereby circumventing the security controls intended to protect against unauthorized redirections.
The technical implementation of this vulnerability stems from insufficient input sanitization and validation within the trustedOrigins configuration handling. The system's validation logic does not adequately handle edge cases involving absolute URLs that contain wildcards or partial domain matching patterns, allowing attackers to construct URLs that satisfy the validation criteria while still redirecting users to malicious destinations. This weakness enables what is classified as an open redirect vulnerability under CWE-601, where the application redirects users to untrusted domains without proper verification. The flaw operates at the application layer and can be exploited through manipulation of HTTP parameters, specifically targeting the callbackURL parameter that is commonly used in OAuth flows and other authentication protocols.
The operational impact of this vulnerability is severe and directly enables account takeover attacks through token theft. When successful, attackers can redirect users to malicious domains where they capture authentication tokens, session identifiers, or other sensitive credentials that are automatically included in the redirect process. This creates a pathway for unauthorized access to user accounts and potential compromise of entire user bases, particularly when combined with other attack vectors such as credential stuffing or session hijacking. The vulnerability affects the core authentication mechanism, making it particularly dangerous as it can be exploited regardless of the specific authentication method being used within better-auth.
Effective mitigation strategies must address both the immediate validation logic flaws and implement comprehensive input sanitization measures. Organizations should upgrade to better-auth version 1.1.20 or later where the trustedOrigins validation has been properly patched to handle absolute URLs and wildcard domains correctly. Additionally, implementing strict callback URL verification that explicitly validates domain patterns without allowing wildcard expansion, enforcing HTTPS-only redirects, and maintaining a whitelist of approved origins are essential defensive measures. The fix aligns with ATT&CK technique T1566.002 which covers social engineering through phishing, as this vulnerability enables attackers to craft convincing redirect attacks that can harvest credentials. Organizations should also implement monitoring for unusual redirect patterns and consider implementing additional authentication layers such as multi-factor authentication to provide defense in depth against potential exploitation attempts.