Windows NTLM v1 Elevation of Privilege Vulnerability

Executive Summary

On January 13, 2025, a critical elevation of privilege vulnerability, CVE-2025-21311, was discovered in the NTLMv1 authentication protocol used by Windows. This vulnerability enables remote exploitation over the internet, allowing attackers with minimal knowledge of the system to exploit it. Given the severity of the risk, it is essential to adopt best security practices and migrate to more secure authentication protocols to mitigate potential threats.

Background

LAN Manager (LANMAN) authentication, which includes the LM, NTLM, and NTLMv2 variants, is a protocol used to authenticate client devices running the Windows operating system [3]. This protocol is applied when devices join a domain, authenticate between Active Directory forests, authenticate with domains running older versions of the Windows operating system, authenticate with non-Windows operating systems (starting with Windows 2000), and authenticate with computers that are not part of the domain. The LAN Manager Security Authentication Level setting determines the challenge/response protocol used for network logons. This setting influences the authentication protocol version, session security level, and the authentication requirements accepted by servers.

The critical severity score for this vulnerability is 9.8 on the Common Vulnerability Scoring System (CVSS), indicating a severe risk [2]. This high score is largely attributed to the vulnerability’s ease of exploitation, requiring minimal effort for attackers to achieve unauthorized access. With a network attack vector, it can be exploited remotely over the internet. With its low attack complexity, attackers do not need significant system knowledge and can be repeatedly exploited successfully against the vulnerable system [6].

Exploitation

The vulnerability stems from an incorrect implementation of the authentication algorithm, allowing attackers to bypass authentication [5]. By exploiting this critical weakness in the NTLM authentication protocol, attackers can escalate privileges and gain administrative control over affected systems.

Significance and Impact

Attackers primarily seek to bypass protection mechanisms to gain unauthorized access to vulnerable systems. Once successful, they can view sensitive data, modify system configurations, and potentially cause security breaches [1]. This risk is especially high in sectors that rely on older NTLM authentication protocols, as well as in industries like healthcare, finance, and government, which have strict compliance requirements. Organizations with inadequate patch management processes are also particularly vulnerable to exploitation.

Mitigation

Mitigations depend on specific security and authentication requirements. Implementing the following mitigations can help minimize risk and exposure [4]. Setting the LmCompatibilityLevel to its maximum value of 5 on all machines prevents the use of the older NTLMv1 protocol, allowing only for NTLMv2. A Registry Security Level of 5 is the most secure LAN Manager authentication setting, as it will only send NTLMv2 responses and only accept NTLMv2 authentication while refusing LM & NTLM. This setting applies NTLMv2 authentication and NTLMv2 session security if supported by the server.

A more secure protocol, such as Kerberos, should be used along with the implementation of multi-factor authentication. Extended Protection for Authentication, Windows Defender Credential Guard, and SIEM tools should be enabled to monitor unusual login attempts. Additionally, systems and networks should be segmented, staff educated, and the least privilege principle enforced.

Conclusion

The CVE-2025-21311 vulnerability allows attackers to bypass Windows NTLMv1 authentication due to insecure authentication implementation. With minimal system knowledge, attackers can gain unauthorized access to critical systems, potentially leading to data breaches, ransomware attacks, and operational disruptions. It’s strongly encouraged to take proactive measures, such as upgrading to NTLMv2 or other more secure authentication protocols, to mitigate these risks and protect critical systems.

References

[1] Boaz, S. (2025, January 28). “CVE-2025-21311 Explained: Critical Windows NTLM Vulnerability and How to Protect Your Organization.” Linkedin.com. https://www.linkedin.com/pulse/cve-2025-21311-explained-critical-windows-ntlm-how-protect-shunami-t5wpf

[2] CVE. (2025, January 14). “CVE-2025-21311.” Cve.org. https://www.cve.org/CVERecord?id=CVE-2025-21311

[3] Microsoft. (2017, April 19). “Network security: LAN Manager authentication level.” Learn.microsoft.com. https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-10/security/threat-protection/security-policy-settings/network-security-lan-manager-authentication-level

[4] Microsoft Security Response Center. (2025, January 13). “Windows NTLM V1 Elevation of Privilege Vulnerability.” Mscr.microsoft.com. https://msrc.microsoft.com/update-guide/vulnerability/CVE-2025-21311

[5] MITRE. (n.d.). “CWE-303: Incorrect Implementation of Authentication Algorithm.” Cwe.mitre.org. https://cwe.mitre.org/data/definitions/303.html

[6] National Vulnerability Database. (2025, January 14). “CVE-2025-21311.” Nvd.nist.gov. https://nvd.nist.gov/vuln/detail/CVE-2025-21311#range-16298965