CVE-2018-16841 in Samba
Summary
by MITRE
Samba from version 4.3.0 and before versions 4.7.12, 4.8.7 and 4.9.3 are vulnerable to a denial of service. When configured to accept smart-card authentication, Samba's KDC will call talloc_free() twice on the same memory if the principal in a validly signed certificate does not match the principal in the AS-REQ. This is only possible after authentication with a trusted certificate. talloc is robust against further corruption from a double-free with talloc_free() and directly calls abort(), terminating the KDC process.
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Analysis
by VulDB Data Team • 06/12/2023
The vulnerability described in CVE-2018-16841 represents a critical denial of service flaw within the Samba implementation of the Key Distribution Center (KDC) service. This issue affects Samba versions from 4.3.0 through 4.7.11, 4.8.6, and 4.9.2, creating a scenario where the KDC process can be terminated through a specific memory management error. The vulnerability specifically manifests when Samba is configured to accept smart-card authentication, which introduces a complex authentication flow involving certificate validation and principal matching. The flaw occurs during the authentication process when a validly signed certificate contains a principal that does not align with the principal specified in the AS-REQ message, creating a condition that leads to improper memory handling.
The technical root cause of this vulnerability lies in the improper handling of memory deallocation within the talloc memory management system. When the principal mismatch occurs during smart-card authentication, the KDC code path executes talloc_free() twice on the same memory block, which represents a classic double-free vulnerability. This particular implementation flaw demonstrates a failure in memory management practices where the system does not properly track or validate memory allocation states before attempting deallocation operations. The talloc library, while designed to be robust against memory corruption, specifically handles double-free conditions by invoking abort() directly, which terminates the process rather than allowing graceful error recovery. This behavior transforms what could be a recoverable error condition into a complete service disruption.
The operational impact of this vulnerability extends beyond simple service interruption, as it creates a vector for attackers to deliberately crash the KDC service and potentially disrupt authentication infrastructure. The requirement for a trusted certificate to initiate this condition means that the attack vector is limited to environments where certificate trust has already been established, but this still represents a significant risk in enterprise environments where Samba serves as a critical authentication component. The vulnerability affects organizations that rely on Samba for Active Directory integration, particularly those implementing smart-card based authentication systems where certificate trust relationships are already in place. This scenario creates a particularly dangerous situation because the attack can be executed by an authenticated user who has already established trust with the system, making it more difficult to detect and prevent compared to external attacks.
Mitigation strategies for this vulnerability should prioritize immediate patching of affected Samba versions to the recommended secure releases including 4.7.12, 4.8.7, and 4.9.3. Organizations should also implement monitoring for abnormal KDC process termination events and establish alerting mechanisms for authentication service disruptions. Network segmentation and access controls should be reviewed to limit exposure of the KDC service to untrusted networks. The vulnerability aligns with CWE-415 which describes improper behavior in memory management, and represents a specific implementation weakness that could be categorized under ATT&CK technique T1499.002 for network denial of service attacks. System administrators should also consider implementing redundant authentication services and establishing proper incident response procedures for authentication service failures. Additionally, organizations should conduct thorough vulnerability assessments to identify other potential memory management issues in their authentication infrastructure and implement comprehensive logging to track authentication attempts that may trigger similar conditions.