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 from 4.9.0 up to but not including 4.13.0 suffer from an input-validation flaw in the handling of 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 inject additional OAuth query parameters into the authorization request by manipulating the returnTo value. This injection can cause the Auth0 authorization server to issue tokens containing unintended or malicious parameters, potentially allowing attackers to escalate privileges, bypass intended authorization constraints, or redirect users to malicious endpoints. The vulnerability requires the attacker to have high privileges (PR:H) and involves user interaction (UI:R), making exploitation more complex but still feasible in targeted scenarios. The CVSS v3.1 base score is 5.7, reflecting medium severity with high impact on confidentiality and integrity but no impact on availability. The flaw was addressed and fixed in version 4.13.0 of the SDK. No known exploits are currently reported in the wild, but the risk remains significant for applications using vulnerable versions.

For European organizations, this vulnerability poses a risk to the confidentiality and integrity of authentication tokens issued by Auth0 when used with Next.js applications. Successful exploitation could allow attackers to manipulate OAuth tokens, potentially leading to unauthorized access to sensitive systems or data. This is particularly concerning for industries with strict data protection requirements such as finance, healthcare, and government sectors. The flaw could also facilitate phishing or session hijacking attacks by redirecting users to malicious URLs post-authentication. Since many European companies rely on cloud-based identity providers like Auth0 for secure authentication, the vulnerability could have widespread implications if not promptly addressed. The requirement for high privileges and user interaction somewhat limits the attack surface but does not eliminate the risk, especially in environments with complex user roles or where insider threats exist.

The primary mitigation is to upgrade the Auth0 Next.js SDK to version 4.13.0 or later, where the input validation flaw is corrected. Organizations should audit their applications to identify usage of the returnTo parameter and ensure that any user-supplied inputs are strictly validated and sanitized against a comprehensive whitelist of allowed URLs or parameters. Implementing additional server-side checks to verify the legitimacy of OAuth parameters before processing can reduce risk. Monitoring authentication logs for unusual parameter patterns or unexpected redirects can help detect exploitation attempts. Security teams should also educate developers about secure OAuth implementation practices and conduct regular dependency reviews to promptly apply security patches. Finally, consider implementing multi-factor authentication and anomaly detection to mitigate potential impacts from compromised tokens.