The vulnerability identified as CVE-2025-67716 affects the Auth0 Next.js SDK, a popular library used to implement user authentication in Next.js applications. Specifically, versions 4.9.0 through 4.12.1 suffer from an input-validation flaw related to the returnTo parameter. This parameter is intended to specify the URL to which users are redirected after authentication. However, due to an incomplete list of disallowed inputs (CWE-184), attackers can craft malicious returnTo values that inject unintended OAuth query parameters into the authorization request. This manipulation can cause Auth0 to issue tokens containing parameters not originally intended by the application, potentially enabling unauthorized access or privilege escalation. The vulnerability requires the attacker to have high privileges (PR:H) and user interaction (UI:R), and the attack complexity is high (AC:H), indicating that exploitation is non-trivial but feasible under certain conditions. The vulnerability impacts confidentiality and integrity but does not affect availability. No known exploits have been reported in the wild as of the publication date, and the issue is fixed in version 4.13.0 of the nextjs-auth0 SDK.

For European organizations, this vulnerability poses a risk to the security of authentication tokens used in web applications built with Next.js and Auth0. If exploited, attackers could manipulate OAuth parameters to gain unauthorized access or escalate privileges within affected applications, potentially leading to data breaches or unauthorized actions. This could compromise sensitive user data and internal systems, impacting confidentiality and integrity. Organizations relying on nextjs-auth0 for critical authentication workflows may face increased risk of account takeover or session hijacking. The medium CVSS score (5.7) reflects moderate risk, but the requirement for high privileges and user interaction limits the scope of exploitation. Nonetheless, given the widespread use of Next.js and Auth0 in Europe, especially in sectors like finance, healthcare, and e-commerce, the impact could be significant if not addressed promptly.

European organizations should immediately upgrade the Auth0 Next.js SDK to version 4.13.0 or later, where the vulnerability is patched. Additionally, developers should review and harden input validation logic for OAuth parameters, especially returnTo, to ensure only safe and intended URLs are accepted. Implement strict allowlists for redirect URIs and OAuth parameters to prevent injection of malicious inputs. Conduct thorough code audits and penetration testing focusing on authentication flows to detect similar issues. Employ runtime monitoring and anomaly detection to identify unusual OAuth parameter usage or token issuance patterns. Educate developers on secure OAuth implementation practices and maintain up-to-date dependencies to reduce exposure to known vulnerabilities.