In a domain environment, the Domain Controller (DC) plays a pivotal role in authenticating user accounts. However, what happens when a system is offline or unable to communicate with the DC? To address this concern, Windows employs a caching mechanism that stores the last ten logon hashes by default, providing users with the ability to log in even when connectivity to the DC is unavailable. In this blog post, we will explore the intricacies of cached domain credentials, the associated security risks, and effective mitigation strategies to bolster your organization's defense against credential theft.
The Anatomy of Cached Domain Credentials
By default, Windows caches the last ten logon hashes (increased to 25 for Server 2008) to ensure users can log in even without connectivity to the Domain Controller. These credentials persist in the Security registry hive under the SECURITY\Cache key. Admin or System privileges are required to access these saved hashes, which are encrypted in mscash2 format for modern Windows operating systems.
These hashes endure indefinitely in the registry, surviving reboots and potentially exposing sensitive information.
The creddump tool can be utilized to extract these hashes offline, allowing attackers to bypass the risks associated with running common credential dumping binaries on a system.
Risks and Challenges of Cached Credentials
Cached credentials, stored in the registry, pose a considerable security risk. The tools like John the Ripper and hashcat can be used to attempt brute force attacks on these hashes. However, the encryption and salting of mscash2 hashes, coupled with a robust password policy, make decryption a time-consuming process.
Attackers may exploit offline credential extraction tools, including Mimikatz, which requires a process dump of lsass.exe.
Creddump, a reliable Python-based tool, serves as a go-to choice for attackers aiming to extract credential data at a later time without triggering security alerts.
Defense Strategies: Mitigating Credential Theft
1. Limiting Cached Logons:
Organizations can mitigate the risk by reducing the default number of cached logons in the registry key SOFTWARE\Microsoft\Windows NT\Current Version\Winlogon. The default value is 10 for workstations and up to 25 for servers. Some security guidelines recommend setting this value to four or less, or even zero for non-mobile systems, but caution is needed to avoid excluding service accounts and computer account logons.
2. Protected Users Security Group:
Windows 8.1 and above introduce the Protected Users security group, offering non-configurable credential protections. Members of this group have their credentials not cached, providing an additional layer of defense against credential theft. This group is specifically designed for high-value accounts.
3. Periodic Security Audits:
Regularly auditing and monitoring systems for unauthorized access attempts, especially those targeting cached credentials, can help detect and mitigate potential threats.
4. Stay Informed and Updated:
Keeping systems updated with the latest security patches and staying informed about new tools and techniques used by attackers ensures a proactive defense against evolving threats.
In conclusion, understanding the dynamics of cached domain credentials is crucial for building robust defenses against credential theft. By implementing these mitigation strategies, organizations can significantly reduce the risk of compromise and enhance their overall security posture. Stay vigilant, stay secure, and stay ahead of potential threats.
Akash Patel
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