A recent pentest of an Active Directory environment turned into a struggle to uncover an avenue for escalating privileges. The client in question diligently adhered to best practices, including LDAP signing being enabled, multi-factor authentication(MFA) being used on domain administrator's accounts, and passwords with complexity that kept us from cracking the hashes. Simply put, we were at a loss on how to continue. Encountering repeated obstacles from security measures can be frustrating for pentesters.
In an effort to stop banging our heads on the proverbial wall, we decided to perform a closer review of the Certificate Authority (CA) and certificate template settings stored in Active Directory. We were able to identify two certificates, SCOMCertificate and SCOMCertificatev2, that allowed any authenticated user to request certificates for any user in the domain. These certificates could then be used to login as the user who requested the certificate, not the user ID that actually requested the certificate. So why did this work and how can organizations effectively mitigate the risk associated with certificates and Active Directory?
In order to fully understand why this worked and how to prevent it we first need to look at Active Directory Certificate Services, commonly referred to as AD CS. This is Microsoft’s implementation of a Public Key Infrastructure (PKI) framework. PKI involves the issuance and management of digital certificates, which serve as cryptographic keys to secure communication, authenticate users, and protect sensitive data.
As AD CS is a Windows server role, it tightly integrates with Microsoft's Active Directory. This integration simplifies authorization and allows for centralized management of certificates, certificate revocation, and trust relationships. However, the ease at which AD CS can be put in place often means that some settings are overlooked and organizations are actually weakening their security.
Several of the common AD CS exploits don’t target the Certificate Authorities, but the certificate templates available on the CAs. Certificate templates are an easy area to overlook when configuring AD CS. Templates serve as blueprints for creating certificates and defining their attributes and permissions. If not configured securely, they can be used to create certificates with excessive privileges and give potential attackers access to sensitive data. Depending on the template, an attacker with a standard user account could request and automatically be issued a certificate that allows them to log in as any user they want. To make matters worse, the attacker can continue to log in with that certificate even if the user they are pretending to change their password. The only way to stop the attacker in this scenario is by certificate expiration or revoking the certificate manually. Additionally, attackers may exploit misconfigured templates to generate fraudulent certificates, which can be used for phishing attacks or man-in-the-middle attacks.
Another potential risk lies in misconfigurations in certificate enrollment and revocation. Proper enrollment and revocation processes are fundamental for maintaining the security of the Public Key Infrastructure (PKI). Misconfigurations can introduce vulnerabilities. Unauthorized certificate enrollment, for example, can occur if attackers exploit flaws in the enrollment process to obtain certificates without proper authentication, leading to unauthorized access. Inadequate certificate revocation practices, on the other hand, can leave security gaps, as compromised or expired certificates may not be promptly revoked.
Additionally, weaknesses in key management can pose a serious threat. Proper key management is essential for PKI security, as private keys are the cornerstone of cryptographic security. If attackers gain access to private keys, they can decrypt encrypted data or impersonate certificate owners. Key misuse by malicious insiders or compromised accounts can also lead to unauthorized activities.
Lastly, AD CS servers themselves can become targets for attackers seeking privilege escalation within a network. Unauthorized access to certificate stores on AD CS servers could potentially be used by attackers to impersonate other users or devices. Manipulation of trust relationships established through certificates can also lead to unauthorized access and privilege escalation.
To address these risks, organizations should prioritize meticulous configuration, continuous monitoring, and proactive security measures to protect their AD CS infrastructure. Vigilance and adherence to best practices are paramount in safeguarding against these potential vulnerabilities.
Several real-world examples illustrate the severity of AD CS exploitation risks. Notable incidents include:
- The Flame malware, which leveraged fraudulent certificates to sign malicious code, gaining unauthorized access to systems.
- The Stuxnet worm, which utilized stolen digital certificates to propagate and target critical infrastructure.
These incidents highlight the potential impact of AD CS exploitation on organizations, ranging from data breaches to infrastructure disruption.
Mitigating exploitation risks in Active Directory Certificate Services (AD CS) requires a proactive approach and the implementation of several best practices. Here are key strategies to safeguard your AD CS infrastructure:
Regular Patching and Updates:
Keeping your AD CS components up to date with the latest security patches and updates is critical. Vulnerabilities in software are often patched through updates released by Microsoft. Regularly applying these patches ensures that known security weaknesses are addressed promptly. This not only helps prevent exploitation but also keeps your AD CS infrastructure resilient against emerging threats.
Proper Configuration and Monitoring:
Secure configurations are the foundation of AD CS security. It's imperative to configure AD CS components according to industry best practices and your organization's specific security requirements. Regularly review and audit these configurations to identify any deviations or potential vulnerabilities. Continuous monitoring and detection are crucial components of AD CS security. Employ advanced threat detection tools, such as intrusion detection systems (IDS) and security information and event management (SIEM) solutions, to identify and respond to potential exploitation attempts.
Least Privilege Access:
Limiting access to AD CS to only authorized personnel and following the principle of least privilege is crucial. Grant access rights and permissions on a need-to-know basis. This minimizes the attack surface and reduces the chances of unauthorized users or malicious insiders gaining control over AD CS resources. Regularly review and update access permissions to align them with the changing roles and responsibilities within your organization.
Auditing and Logging:
Implement robust auditing and logging mechanisms within your AD CS infrastructure. This includes monitoring and recording all relevant activities, events, and changes. In the event of a security incident or unauthorized access attempt, detailed logs can provide essential information for forensic analysis and incident response. Configure audit policies to track certificate issuance, revocation, and changes to certificate templates. Ensure that logs are securely stored and regularly reviewed to detect and respond to potential security threats.
The above list outlines the most basic steps organizations should take to properly secure their organization. By adhering to these best practices, you can significantly enhance the security of your AD CS infrastructure. These measures collectively contribute to the prevention and early detection of exploitation risks, ultimately strengthening the overall cybersecurity posture of your organization.
Securing Active Directory Certificate Services is vital in maintaining a resilient and secure network environment. By understanding the exploitation risks, implementing best practices, and maintaining vigilance through monitoring and detection, organizations can mitigate these risks and protect their critical assets.
Remember that AD CS is just one piece of the larger cybersecurity puzzle, and a holistic approach to security is essential. Stay informed, adapt to evolving threats, and prioritize the protection of your digital certificates to maintain trust and security within your organization.
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