CVE-2025-13953 is a critical authentication bypass vulnerability identified in the GTT Sistema de Información Tributario, a tax information system application widely used for managing tax data and authentication via Active Directory (LDAP). The vulnerability stems from the application's use of a local WebSocket to perform authentication, where the application fails to properly validate the authenticity or origin of data received through this channel. This flaw allows an attacker who has access to the local machine or internal network to spoof the WebSocket connection by injecting manipulated data. Consequently, the attacker can impersonate any user within the domain and gain unauthorized access without needing valid credentials. This bypass compromises the confidentiality of sensitive tax data, the integrity of the system by allowing unauthorized changes, and the availability by potentially disrupting legitimate access. The vulnerability affects all versions of the product and has been assigned a CVSS 4.0 score of 9.3, reflecting a network-based attack vector with no required privileges or user interaction, and high impact on confidentiality, integrity, and availability. Although no known exploits are currently in the wild, the ease of exploitation and critical nature of the affected system make this a significant threat. The root cause is inadequate validation of WebSocket data origin and authenticity, which should be addressed by implementing strict origin checks, authentication tokens, or mutual TLS for WebSocket connections. Additionally, internal network access controls and monitoring are essential to detect and prevent exploitation attempts.

For European organizations, particularly tax authorities and financial institutions using the GTT Sistema de Información Tributario, this vulnerability poses a severe risk. Exploitation could lead to unauthorized access to sensitive tax data, enabling data theft, fraud, or manipulation of tax records. The ability to authenticate as any domain user without credentials threatens the entire domain's security, potentially allowing lateral movement and further compromise of internal systems. This could result in significant financial losses, regulatory penalties under GDPR due to data breaches, and erosion of public trust in government services. The disruption of tax information systems could also impact critical government operations and citizen services. Given the critical nature of tax data and the central role of such systems, the impact extends beyond individual organizations to national economic and security concerns. The vulnerability's network-based attack vector and lack of required privileges mean that even insider threats or compromised internal devices could exploit it, increasing the threat surface within European networks.

To mitigate this vulnerability, European organizations should implement multiple layers of defense: 1) Apply strict validation of WebSocket connections by enforcing origin checks and verifying authentication tokens or implementing mutual TLS to ensure only legitimate clients can establish connections. 2) Restrict internal network access to the tax information system using network segmentation and firewall rules to limit exposure to trusted devices and users. 3) Monitor authentication logs and WebSocket traffic for anomalies indicative of spoofing or injection attempts, employing intrusion detection systems tailored to WebSocket protocols. 4) Enforce strong endpoint security to prevent local machine compromise, including up-to-date antivirus, endpoint detection and response (EDR), and regular patching. 5) Engage with the vendor (GTT) for patches or updates addressing this vulnerability as they become available, and plan for rapid deployment. 6) Conduct regular security audits and penetration testing focused on internal network and WebSocket communication security. 7) Educate internal users about the risks of internal network threats and enforce strict access controls and least privilege principles. These measures, combined, reduce the likelihood of successful exploitation and limit potential damage.