CVE-1999-0366 in Windows
Summary
by MITRE
In some cases, Service Pack 4 for Windows NT 4.0 can allow access to network shares using a blank password, through a problem with a null NT hash value.
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Analysis
by VulDB Data Team • 04/18/2026
The vulnerability described in CVE-1999-0366 represents a critical authentication flaw in Windows NT 4.0 Service Pack 4 that fundamentally undermines network security controls. This issue emerged from the improper handling of null NT hash values during the authentication process, creating an unintended pathway for unauthorized network access. The vulnerability specifically affects systems where Service Pack 4 has been installed, making it particularly concerning given the widespread deployment of Windows NT 4.0 in enterprise environments during the late 1990s. The flaw stems from the Windows authentication subsystem's treatment of accounts with empty or null password hashes, which should typically be rejected as invalid credentials but were instead being accepted under certain conditions.
The technical mechanism behind this vulnerability involves the NTLM authentication protocol's handling of null hash values in the authentication challenge-response process. When a user attempts to access network resources, the system should validate that the provided credentials correspond to a legitimate account with a valid password hash. However, the Service Pack 4 implementation contained a logic error where accounts with null NT hash values were not properly rejected during authentication. This occurs because the system's credential validation routine fails to adequately check for null hash values before proceeding with the authentication process, allowing attackers to bypass password requirements entirely. The flaw is particularly insidious because it operates at the protocol level rather than being a simple configuration error, making it difficult to detect and remediate through standard security audits.
The operational impact of this vulnerability extends far beyond simple unauthorized access, creating significant risks for enterprise network security. Systems affected by this vulnerability become susceptible to lateral movement attacks where malicious actors can traverse network segments without proper authentication, potentially accessing sensitive data repositories, administrative shares, and critical business applications. The vulnerability aligns with attack patterns documented in the MITRE ATT&CK framework under the credential access and lateral movement techniques, specifically targeting the use of valid accounts and authentication protocols to gain unauthorized access. Organizations with multiple Windows NT 4.0 systems in their network infrastructure faced particularly high risk, as this single vulnerability could compromise entire network segments. The implications were severe enough that Microsoft classified this as a critical security issue requiring immediate attention from system administrators.
Mitigation strategies for this vulnerability required immediate action from affected organizations, including the installation of updated Service Packs and security patches from Microsoft. The recommended approach involved applying the official security updates that corrected the null hash validation logic in the authentication subsystem. Additionally, system administrators were advised to implement network segmentation and access control measures to limit the potential damage from unauthorized access attempts. The vulnerability highlighted the importance of proper credential validation and the need for comprehensive security testing of authentication protocols. Organizations should have also considered implementing network monitoring solutions to detect unusual authentication patterns and unauthorized access attempts. This vulnerability serves as a classic example of how seemingly minor implementation flaws in authentication systems can create significant security risks, emphasizing the critical importance of robust credential validation mechanisms in network security architectures. The issue demonstrates the necessity of following security best practices such as those outlined in the CWE database, specifically addressing weaknesses related to authentication and credential handling in system implementations.