CVE-2018-16857 in Samba
Summary
by MITRE
Samba from version 4.9.0 and before version 4.9.3 that have AD DC configurations watching for bad passwords (to restrict brute forcing of passwords) in a window of more than 3 minutes may not watch for bad passwords at all. The primary risk from this issue is with regards to domains that have been upgraded from Samba 4.8 and earlier. In these cases the manual testing done to confirm an organisation's password policies apply as expected may not have been re-done after the upgrade.
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Analysis
by VulDB Data Team • 06/12/2023
CVE-2018-16857 represents a critical authentication bypass vulnerability affecting Samba Active Directory Domain Controller implementations. This flaw exists within Samba versions 4.9.0 through 4.9.2, specifically impacting systems that have upgraded from Samba 4.8 or earlier versions. The vulnerability stems from improper handling of password policy enforcement mechanisms, particularly the monitoring of failed authentication attempts within specified time windows. The technical root cause lies in the configuration management system where password policy enforcement logic fails to properly validate or enforce restrictions when the configured time window exceeds three minutes. This issue manifests as a complete breakdown in the password brute force protection mechanism, effectively removing all restrictions on failed authentication attempts. The vulnerability maps directly to CWE-307, which addresses inadequate password policy enforcement, and aligns with ATT&CK technique T1110.003 for Brute Force, as it removes critical defensive controls that would normally prevent rapid password guessing attempts. Organizations operating Samba AD DC configurations with upgraded systems are particularly at risk since the manual validation processes that would normally confirm password policy enforcement are often not re-executed after upgrades, leading to prolonged periods where authentication attempts remain unrestricted. The operational impact extends beyond simple credential theft, as attackers can leverage this vulnerability to conduct extended brute force campaigns against user accounts, potentially compromising entire domain environments. This vulnerability specifically affects domains that have undergone major version upgrades from Samba 4.8 or earlier, where the upgrade process does not automatically carry forward or properly reconfigure the password policy enforcement mechanisms. The risk is compounded by the fact that administrators may be unaware of the vulnerability's existence until after exploitation has occurred, as the system continues to operate normally while providing no actual password protection. The flaw demonstrates a classic configuration drift issue where automated upgrade processes fail to maintain security controls, creating a false sense of security for administrators who assume their systems continue to enforce proper authentication policies. Organizations should immediately implement patch management procedures to upgrade to Samba 4.9.3 or later versions, while simultaneously verifying that all password policy configurations are properly enforced and revalidated after any major system upgrades. Security teams must also implement monitoring for unusual authentication patterns that could indicate exploitation attempts, as the vulnerability creates a window where authentication attempts can proceed without any rate limiting or account lockout mechanisms. The vulnerability underscores the critical importance of maintaining security controls during system upgrades and highlights the need for comprehensive post-upgrade validation processes that ensure all security mechanisms continue to function as intended.