CVE-1999-0994 is a vulnerability affecting Microsoft Windows NT 4.0 systems that utilize the SYSKEY feature for securing the Security Account Manager (SAM) database password hashes. SYSKEY is a mechanism designed to add an additional layer of encryption to the SAM database, which stores user account credentials. However, in this specific implementation, the keystream used for encrypting the SAM password hashes is reused, which is a cryptographic weakness. Reusing a keystream in encryption schemes, especially stream ciphers, can allow attackers to perform cryptanalysis and recover plaintext data—in this case, the password hashes. Once an attacker obtains these hashes, they can attempt offline password cracking to reveal user passwords, potentially leading to unauthorized access. The vulnerability does not require authentication or user interaction and can be exploited remotely over the network (AV:N), with low attack complexity (AC:L). The impact is primarily on confidentiality (C:P), as password hashes can be exposed, but integrity and availability are not directly affected. Microsoft has released patches addressing this issue, as documented in security bulletin MS99-056. No known exploits have been reported in the wild, but the vulnerability remains significant due to the sensitive nature of password hashes and the potential for privilege escalation if compromised.

For European organizations, the exploitation of CVE-1999-0994 could lead to the compromise of user credentials stored on Windows NT 4.0 systems. Although Windows NT 4.0 is an outdated operating system, some legacy systems may still be in use within certain industries or government sectors. If attackers recover password hashes and successfully crack them, they could gain unauthorized access to critical systems, leading to potential data breaches, insider threats, or lateral movement within networks. This could impact confidentiality of sensitive information and potentially disrupt business operations if privileged accounts are compromised. Given the age of the vulnerability, the risk is mitigated if organizations have migrated to supported operating systems; however, legacy systems without patches remain vulnerable. The medium CVSS score reflects moderate risk, but the actual impact depends on the presence of vulnerable systems and the value of the accounts stored therein.

European organizations should first identify any remaining Windows NT 4.0 systems using SYSKEY encryption for SAM databases. The primary mitigation is to apply the official Microsoft patch as detailed in security bulletin MS99-056. If patching is not feasible due to legacy constraints, organizations should consider isolating these systems from the network to prevent remote exploitation. Additionally, enforcing strong password policies and regularly auditing account credentials can reduce the risk of successful password cracking. Transitioning away from Windows NT 4.0 to supported operating systems is strongly recommended to eliminate this and other legacy vulnerabilities. Network segmentation and monitoring for unusual access patterns to legacy systems can also help detect potential exploitation attempts. Finally, organizations should ensure that backups and incident response plans account for potential credential compromise scenarios.