CVE-2018-16851 in Samba
Summary
by MITRE
Samba from version 4.0.0 and before versions 4.7.12, 4.8.7, 4.9.3 is vulnerable to a denial of service. During the processing of an LDAP search before Samba's AD DC returns the LDAP entries to the client, the entries are cached in a single memory object with a maximum size of 256MB. When this size is reached, the Samba process providing the LDAP service will follow the NULL pointer, terminating the process. There is no further vulnerability associated with this issue, merely a denial of service.
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Analysis
by VulDB Data Team • 06/12/2023
The vulnerability identified as CVE-2018-16851 affects the Samba directory service implementation, specifically within the Active Directory Domain Controller (AD DC) functionality that supports LDAP operations. This flaw exists in Samba versions ranging from 4.0.0 through various releases prior to 4.7.12, 4.8.7, and 4.9.3, representing a significant security concern for organizations relying on Samba for directory services. The vulnerability manifests during LDAP search processing within the AD DC component, where the system maintains a memory cache for LDAP entries to improve performance. This cache mechanism operates with a fixed maximum size limit of 256MB, creating a predictable memory exhaustion scenario that can be exploited to cause service disruption.
The technical implementation of this vulnerability stems from improper memory management within the LDAP processing pipeline of Samba's AD DC. When LDAP search operations are conducted, the system accumulates entries in a single memory object that serves as a cache for the results to be returned to clients. As the cache approaches its 256MB capacity limit, the system fails to properly handle the memory overflow condition, resulting in a NULL pointer dereference that causes the Samba process to terminate abruptly. This memory management failure represents a classic buffer overflow scenario where the system does not adequately check for cache limits before attempting to write additional data. The vulnerability operates at the application layer and specifically affects the LDAP service implementation, making it a direct threat to directory service availability and potentially disrupting authentication and authorization processes across the network.
The operational impact of this vulnerability extends beyond simple service interruption to potentially affect critical network infrastructure dependencies. Organizations utilizing Samba as their primary directory service for Active Directory operations face significant risk when exposed to this vulnerability, as the denial of service condition can disrupt user authentication, group policy application, and other directory-dependent services. The termination of the Samba process creates cascading effects throughout the network, as domain controllers become unavailable and clients lose access to directory services. This vulnerability aligns with CWE-476, which describes NULL pointer dereference conditions, and demonstrates how memory management flaws can be exploited to cause availability issues. The attack vector requires only that an attacker perform an LDAP search operation against the vulnerable Samba AD DC, making it particularly dangerous as it can be executed by unauthorized users or automated tools without requiring elevated privileges.
Mitigation strategies for CVE-2018-16851 primarily focus on immediate patching of affected Samba installations to versions that contain the necessary memory management fixes. Organizations should prioritize updating their Samba implementations to the patched versions 4.7.12, 4.8.7, or 4.9.3, depending on their current deployment. Network administrators should also implement monitoring solutions to detect unusual LDAP search patterns that might indicate exploitation attempts, as well as establish redundant directory services to maintain availability during patching operations. The vulnerability does not present additional security implications beyond the denial of service condition, but the availability impact makes it a critical concern for enterprise environments where directory services are fundamental to network operations. Security teams should also consider implementing temporary network segmentation to limit exposure while patching activities are underway, and establish incident response procedures specifically addressing LDAP service disruptions. This vulnerability demonstrates the importance of proper memory management in server applications and aligns with ATT&CK technique T1499, which covers network disruption attacks targeting service availability.