CVE-2019-1040 in Windows
Summary
by MITRE
A tampering vulnerability exists in Microsoft Windows when a man-in-the-middle attacker is able to successfully bypass the NTLM MIC (Message Integrity Check) protection, aka 'Windows NTLM Tampering Vulnerability'.
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Analysis
by VulDB Data Team • 05/21/2025
The CVE-2019-1040 vulnerability represents a critical security flaw in Microsoft Windows operating systems that specifically targets the NTLM authentication protocol. This vulnerability enables man-in-the-middle attackers to bypass the Message Integrity Check mechanism that is designed to protect NTLM authentication messages from tampering. The flaw exists within the Windows authentication stack where the system fails to properly validate the integrity of NTLM messages, allowing attackers to modify authentication exchanges without detection. This represents a fundamental breakdown in the security assurances that NTLM authentication is supposed to provide, particularly in environments where legacy authentication protocols are still in use.
The technical implementation of this vulnerability stems from a weakness in how Windows handles NTLM message integrity verification. When NTLM authentication occurs, the protocol includes a Message Integrity Check field that should prevent attackers from modifying authentication messages during transmission. However, the vulnerability allows attackers to manipulate this check value in a way that bypasses the validation process entirely. This occurs because Windows does not properly validate the MIC field when certain conditions are met, specifically when the authentication occurs over unencrypted channels or when specific authentication negotiation sequences are followed. The flaw is particularly dangerous because it operates at the protocol level rather than at the application level, making it difficult to detect and mitigate through traditional application security measures.
The operational impact of CVE-2019-1040 is severe and far-reaching across enterprise environments that rely on legacy authentication systems. Organizations using Windows domains that have not fully migrated away from NTLM authentication are particularly vulnerable, as this flaw can enable attackers to perform credential theft, relay attacks, and privilege escalation without detection. The vulnerability can be exploited in various scenarios including network interception attacks, where attackers position themselves between communicating parties to capture and modify authentication messages. This allows for unauthorized access to systems and resources that would normally be protected by proper authentication mechanisms, potentially leading to complete domain compromise. The attack vector is particularly concerning because it requires minimal privileges and can be executed from any location where network traffic can be intercepted.
Mitigation strategies for CVE-2019-1040 should focus on both immediate patching and architectural changes to reduce reliance on vulnerable authentication protocols. Microsoft released security updates that address the specific validation flaw in the NTLM implementation, and organizations should prioritize applying these patches across all affected systems. However, long-term security requires moving away from NTLM authentication entirely and implementing more secure alternatives such as Kerberos authentication, which provides better protection against the types of attacks this vulnerability enables. Network segmentation and monitoring solutions should be deployed to detect anomalous authentication patterns that might indicate exploitation attempts. The vulnerability aligns with CWE-310, which covers cryptographic issues related to message integrity, and represents a significant concern under ATT&CK technique T1550.001 for use of valid accounts and T1550.002 for use of stolen credentials. Organizations should also implement network-level protections such as SMB signing enforcement and disable weak authentication protocols where possible to reduce the attack surface.