CVE-2025-53591 is a vulnerability classified under CWE-134, which pertains to the use of externally-controlled format strings. This flaw exists in QNAP Systems Inc.'s QTS operating system, specifically affecting versions 5.2.x. The vulnerability allows an attacker who has already obtained administrator-level access on the device to exploit the format string issue to read sensitive information or alter memory contents. Format string vulnerabilities arise when user-supplied input is unsafely used as a format string parameter in functions like printf, potentially leading to memory corruption or data leakage. In this case, the attacker must have administrative privileges, which limits the initial attack vector but increases the severity once access is gained. The vulnerability does not require user interaction and can be exploited remotely over the network. QNAP has addressed this vulnerability in updated builds of QTS and QuTS hero operating systems released in September 2025. The CVSS v4.0 score is 1.2, reflecting low severity due to the prerequisite of administrator privileges and limited impact scope. No public exploits have been reported, suggesting the threat is currently low but should not be ignored given the sensitive nature of NAS devices in enterprise environments.

For European organizations, the impact of CVE-2025-53591 is primarily tied to the confidentiality and integrity of data stored on QNAP NAS devices. If exploited, an attacker with admin access could extract sensitive information or manipulate system memory, potentially leading to data breaches or system instability. Given that NAS devices often store critical business data and backups, unauthorized access or data corruption could disrupt operations and lead to compliance violations under regulations such as GDPR. However, the requirement for administrative privileges significantly reduces the risk of remote exploitation by external attackers without prior access. The low CVSS score indicates limited direct impact, but the vulnerability could be leveraged as part of a multi-stage attack within compromised networks. European organizations relying heavily on QNAP NAS for storage, especially in sectors like finance, healthcare, and government, should consider this vulnerability a moderate risk until patched.

European organizations should immediately verify the QTS or QuTS hero versions running on their QNAP NAS devices and upgrade to the fixed versions: QTS 5.2.7.3256 build 20250913 or later, and QuTS hero h5.2.7.3256 build 20250913 or later. Network segmentation should be enforced to limit administrative access to NAS devices only to trusted internal hosts and administrators. Strong authentication mechanisms, including multi-factor authentication for administrator accounts, should be implemented to reduce the risk of credential compromise. Regular audits of administrator account activity and logs can help detect suspicious behavior indicative of exploitation attempts. Additionally, organizations should ensure that firmware and software updates are applied promptly as part of their patch management process. Disabling unnecessary services and restricting remote administrative access via VPN or secure channels can further reduce exposure. Finally, organizations should maintain offline backups to recover from potential data corruption or loss.