CVE-2025-67490 is an authorization vulnerability classified under CWE-863 affecting the Auth0 Next.js SDK, a widely used library for implementing user authentication in Next.js applications. The flaw arises from improper handling of simultaneous requests on the same client, which causes incorrect lookups in the TokenRequestCache. This cache is responsible for storing token request results, and the concurrency issue can lead to one request accessing or receiving tokens intended for another request. Such a mix-up can result in unauthorized disclosure of sensitive authentication tokens, compromising user confidentiality and potentially allowing limited unauthorized actions due to token misuse. The vulnerability affects versions 4.11.0 through 4.11.2 and 4.12.0, with fixes released in 4.11.2 and 4.12.1. Exploitation requires network access, low privileges, and user interaction, making it moderately difficult but feasible in multi-request scenarios typical in web applications. The CVSS v3.1 score of 5.4 reflects medium severity, with high impact on confidentiality, low impact on integrity, and no impact on availability. No known exploits are currently reported in the wild, but the risk remains for organizations using vulnerable versions. The issue underscores the importance of proper concurrency control in authentication token management within web SDKs.

For European organizations, this vulnerability poses a risk of unauthorized access to authentication tokens, potentially leading to data breaches or unauthorized actions within applications using the affected Auth0 Next.js SDK versions. Confidentiality of user sessions can be compromised, which is critical for sectors handling sensitive personal data such as finance, healthcare, and e-commerce. The integrity impact is limited but could allow attackers to perform actions under another user's session if tokens are misappropriated. Although availability is unaffected, the reputational damage and regulatory consequences under GDPR for data exposure could be significant. Organizations relying on Next.js with Auth0 for authentication in customer-facing or internal applications must consider this vulnerability a priority. The medium CVSS score indicates moderate urgency, but the widespread use of Next.js and Auth0 in Europe amplifies potential exposure. The lack of known exploits suggests a window for proactive mitigation before active attacks emerge.

The primary mitigation is to upgrade the Auth0 Next.js SDK to versions 4.11.2 or 4.12.1 or later, where the concurrency issue in TokenRequestCache is resolved. Organizations should audit their applications to identify usage of affected versions and enforce upgrade policies. Additionally, developers should review token caching and session management logic to ensure thread-safe and request-isolated handling of authentication tokens. Implementing strict session validation and monitoring for anomalous token usage can help detect exploitation attempts. Where immediate upgrades are not feasible, applying application-level concurrency controls to serialize token requests or isolate user sessions can reduce risk. Security teams should also review logs for unusual authentication patterns and prepare incident response plans for potential token leakage. Finally, educating developers on secure token management practices in asynchronous environments is recommended to prevent similar issues.