CVE-2018-16853 in Samba
Summary
by MITRE
Samba from version 4.7.0 has a vulnerability that allows a user in a Samba AD domain to crash the KDC when Samba is built in the non-default MIT Kerberos configuration. With this advisory the Samba Team clarify that the MIT Kerberos build of the Samba AD DC is considered experimental. Therefore the Samba Team will not issue security patches for this configuration. Additionally, Samba 4.7.12, 4.8.7 and 4.9.3 have been issued as security releases to prevent building of the AD DC with MIT Kerberos unless --with-experimental-mit-ad-dc is specified to the configure command.
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Analysis
by VulDB Data Team • 06/12/2023
CVE-2018-16853 represents a critical vulnerability in Samba Active Directory Domain Controller implementations that specifically affects systems configured with the MIT Kerberos backend instead of the default Heimdal implementation. This vulnerability stems from improper handling of Kerberos authentication requests within the Key Distribution Center component of Samba AD DC when operating under the non-default MIT Kerberos configuration. The flaw manifests as a denial of service condition that can be triggered by authenticated users within the Samba AD domain, causing the KDC service to crash and effectively rendering the authentication infrastructure unavailable to legitimate users. The vulnerability is classified under CWE-248 as an unchecked exception in the Kerberos authentication flow, specifically within the KDC processing logic. From an operational perspective, this issue represents a significant security risk as it allows attackers with domain user credentials to disrupt critical authentication services, potentially leading to service outages that could impact business continuity and availability of network resources.
The technical implementation of this vulnerability occurs when Samba AD DC processes Kerberos tickets under the MIT Kerberos backend configuration, where the system fails to properly validate or handle certain malformed or specially crafted authentication requests. The crash occurs during the ticket validation process within the KDC service, which operates under the MIT Kerberos library rather than the standard Heimdal implementation. This configuration issue creates a path where authenticated users can send specific Kerberos protocol messages that trigger memory corruption or invalid state conditions within the KDC process, resulting in immediate service termination. The vulnerability is particularly concerning because it requires minimal privileges for exploitation, as any user within the domain can potentially trigger the crash, making it a vector for both disruption and potential reconnaissance activities.
The operational impact of CVE-2018-16853 extends beyond simple service disruption to encompass broader security implications within enterprise environments that rely on Samba AD DC implementations. Organizations using the MIT Kerberos configuration for their Samba deployments face the risk of sustained service availability issues that could compromise their authentication infrastructure. The vulnerability aligns with ATT&CK technique T1499.004 which covers network disruption attacks, and T1566 which involves phishing with malicious attachments that could be used to gain initial access before exploiting this vulnerability. Security teams must consider that the crash condition can be used as a stepping stone for more sophisticated attacks, as the service disruption could mask other malicious activities or create opportunities for privilege escalation. The vulnerability affects Samba versions 4.7.0 through 4.7.11, 4.8.0 through 4.8.6, and 4.9.0 through 4.9.2, with the Samba team addressing the issue through version releases that explicitly disable building AD DC with MIT Kerberos by default, requiring explicit configuration flags to enable this experimental functionality.
Mitigation strategies for CVE-2018-16853 focus primarily on configuration management and version control within Samba deployments. Organizations should immediately upgrade to Samba versions 4.7.12, 4.8.7, or 4.9.3 which explicitly prevent building AD DC with MIT Kerberos unless the experimental flag --with-experimental-mit-ad-dc is explicitly specified during the configure process. This approach aligns with security best practices outlined in NIST SP 800-53 and ISO 27001 controls for system hardening and patch management. The Samba team's decision to classify the MIT Kerberos build as experimental reflects the inherent risks associated with using non-default configurations in production environments, particularly when such configurations have known vulnerabilities that are not fully supported. Security administrators should conduct thorough inventory audits to identify all Samba AD DC instances configured with MIT Kerberos and ensure proper patching or reconfiguration to use the supported Heimdal backend. Additionally, network monitoring should be enhanced to detect unusual authentication patterns that might indicate exploitation attempts, while implementing proper access controls to limit user privileges and reduce the potential impact of successful exploitation attempts.