CVE-2016-2125 in Samba
Summary
by MITRE
It was found that Samba before versions 4.5.3, 4.4.8, 4.3.13 always requested forwardable tickets when using Kerberos authentication. A service to which Samba authenticated using Kerberos could subsequently use the ticket to impersonate Samba to other services or domain users.
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Analysis
by VulDB Data Team • 10/12/2022
The vulnerability identified as CVE-2016-2125 represents a critical flaw in Samba's Kerberos authentication implementation that persisted across multiple versions of the software. This weakness specifically affects Samba versions prior to 4.5.3, 4.4.8, and 4.3.13, creating a significant security risk in environments where Kerberos authentication is utilized. The flaw stems from Samba's improper handling of Kerberos ticket flags, particularly the forwardable ticket attribute, which should only be requested when explicitly required by the service configuration. When Samba consistently requested forwardable tickets, it inadvertently created a pathway for privilege escalation and lateral movement attacks within domain environments.
The technical nature of this vulnerability lies in Samba's failure to respect the forwardable ticket flag during Kerberos authentication processes. In standard Kerberos implementations, forwardable tickets are explicitly controlled by the service requesting authentication, allowing the ticket to be forwarded to other services on behalf of the authenticated user. However, Samba's implementation ignored this control mechanism and always requested forwardable tickets regardless of the service requirements. This behavior creates a fundamental security misconfiguration where any service that Samba authenticated to using Kerberos could potentially use the ticket to impersonate Samba to other services or domain users, effectively breaking the principle of least privilege.
The operational impact of CVE-2016-2125 extends beyond simple authentication failures, creating a substantial attack surface for malicious actors within Windows domain environments. Attackers who gain access to a system running vulnerable Samba versions can exploit this flaw to perform unauthorized authentication delegation attacks, potentially allowing them to move laterally through the domain infrastructure. This vulnerability directly aligns with attack patterns described in the MITRE ATT&CK framework under the Authentication Tactics category, specifically targeting credential access and privilege escalation techniques. The flaw enables attackers to leverage Kerberos authentication mechanisms to gain unauthorized access to additional resources, as the forwardable tickets essentially provide a backdoor for impersonation attacks against other domain services.
Organizations utilizing Samba in domain environments face significant risk exposure when running vulnerable versions, as the vulnerability can be exploited without requiring elevated privileges or specialized tools. The security implications are particularly severe in environments where Samba serves as a file server or domain controller, as these services often require authentication to multiple domain resources. This vulnerability contributes to the broader class of Kerberos delegation issues categorized under CWE-540 in the Common Weakness Enumeration system, which specifically addresses insecure handling of authentication tokens and delegation mechanisms. The flaw represents a classic example of improper privilege management where the system grants more access rights than necessary, violating fundamental security principles.
Mitigation of CVE-2016-2125 requires immediate deployment of patched Samba versions, with administrators prioritizing updates to versions 4.5.3, 4.4.8, or 4.3.13 depending on their current installation. System administrators should also implement monitoring for Kerberos ticket usage patterns to detect potential exploitation attempts, as the vulnerability creates detectable artifacts in authentication logs. Network segmentation and access control policies should be reviewed to limit the potential impact of successful exploitation, while Kerberos configuration should be audited to ensure that forwardable ticket usage is properly restricted. Additionally, organizations should consider implementing security controls such as Kerberos ticket auditing and monitoring for unusual authentication patterns that might indicate unauthorized delegation attempts. The vulnerability highlights the importance of proper authentication protocol implementation and the critical need for regular security updates to address known weaknesses in authentication systems.