CVE-2026-67336 in better-auth
Summary
by MITRE • 08/01/2026
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.
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Analysis
by VulDB Data Team • 08/01/2026
The vulnerability present in better-auth versions prior to 1.6.11 represents a critical security flaw within the OpenID Connect and Multi-Client Provider authentication plugins. This issue stems from insecure cryptographic defaults that fundamentally compromise the authentication integrity of systems relying on these components. The vulnerability specifically affects the oidcProvider and mcp plugins where the software advertises the none algorithm during token exchange processes, creating an avenue for attackers to exploit the authentication flow.
The technical flaw manifests through two primary vectors that together create a significant attack surface. First, the system's algorithm negotiation process accepts unsigned tokens when the none algorithm is presented, bypassing essential cryptographic verification mechanisms. This occurs because the software fails to properly validate or reject algorithm choices that should never be permitted in production environments. Second, the implementation accepts plain PKCE (Proof Key for Code Exchange) by default rather than enforcing the required S256 method, which creates a vulnerability for authorization code interception attacks.
From an operational impact perspective, this vulnerability enables sophisticated attackers to perform token manipulation and session hijacking attacks with relatively low complexity. The none algorithm acceptance allows unauthorized parties to forge authentication tokens without proper cryptographic signatures, potentially leading to complete system compromise. When combined with the plain PKCE vulnerability, attackers can intercept and reuse authorization codes, effectively bypassing the entire OAuth 2.0 authorization flow protection mechanisms. This vulnerability directly impacts the confidentiality, integrity, and availability of authentication systems relying on these plugins.
The security implications extend beyond simple credential theft to encompass full system compromise potential, particularly in environments where better-auth serves as a core authentication component. Attackers can leverage this vulnerability to impersonate legitimate users, gain unauthorized access to protected resources, and potentially escalate privileges within the affected systems. The impact is amplified when considering that these plugins are likely used in enterprise environments where proper cryptographic practices are essential for maintaining security boundaries. Organizations utilizing these vulnerable versions face significant risk of data breaches and unauthorized system access.
Mitigation strategies should focus on immediate version upgrades to 1.6.11 or later releases that address the cryptographic defaults. System administrators must ensure that all instances of better-auth are updated and that algorithm validation is properly enforced within the authentication flow. The configuration should explicitly disable the none algorithm and mandate the use of S256 for PKCE implementation. Additionally, organizations should implement monitoring to detect any attempts to negotiate insecure cryptographic algorithms or use of deprecated authentication flows. Security teams should conduct thorough audits of all systems using these plugins to verify proper cryptographic implementations and ensure that no legacy configurations remain in place that could expose the system to these vulnerabilities.
This vulnerability aligns with CWE-327 (Use of a Broken or Risky Cryptographic Algorithm) and CWE-347 (Improper Verification of Cryptographic Signature) categories, while also mapping to ATT&CK techniques involving credential access and privilege escalation through authentication bypass methods. The vulnerability demonstrates the critical importance of proper cryptographic implementation and the dangers of insecure defaults in authentication systems that can undermine entire security frameworks.