CVE-2021-20251 in Samba
Summary
by MITRE • 03/07/2023
A flaw was found in samba. A race condition in the password lockout code may lead to the risk of brute force attacks being successful if special conditions are met.
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Analysis
by VulDB Data Team • 01/23/2026
The vulnerability identified as CVE-2021-20251 represents a critical race condition flaw within the samba authentication system that directly impacts password lockout mechanisms. This issue resides in the samba software suite which serves as a widely deployed file sharing and authentication solution across enterprise environments and operating systems including linux and windows platforms. The race condition specifically manifests in the password lockout code path where concurrent authentication attempts can create unpredictable states that bypass intended security controls. This flaw operates at the intersection of authentication security and concurrent processing, making it particularly dangerous in environments where samba serves as a primary authentication gateway for network resources.
The technical implementation of this vulnerability stems from improper synchronization mechanisms within the samba password lockout subsystem. When multiple authentication requests occur simultaneously against the same user account, the race condition allows for a scenario where the lockout counter may not be properly incremented or checked before subsequent authentication attempts are processed. This creates a window where brute force attackers can exploit the timing gap to make additional password attempts without triggering the intended lockout mechanism. The flaw requires specific conditions to be met including concurrent access patterns and timing windows that align with the vulnerable code path. This behavior aligns with CWE-362, which describes race conditions in concurrent programming where the order of execution affects program behavior and security outcomes.
The operational impact of CVE-2021-20251 extends beyond simple authentication bypass to potentially enable credential stuffing attacks and password spraying techniques against samba-managed systems. Organizations relying on samba for authentication services face significant risk as attackers can systematically attempt multiple password combinations against user accounts without triggering the expected lockout protections. This vulnerability particularly affects environments where samba serves as a primary authentication mechanism for file servers, domain controllers, or network access points. The risk is amplified in scenarios where samba is configured with default or weak password policies that may not adequately protect against automated attack vectors. This vulnerability directly relates to ATT&CK technique T1110.003, which covers credential brute force attacks using password spraying techniques that can exploit weak lockout mechanisms.
Mitigation strategies for this vulnerability require immediate patching of affected samba versions as provided by the vendor, which addresses the underlying race condition in the password lockout implementation. Organizations should also implement additional security controls including stronger password policies, account lockout thresholds, and monitoring of authentication patterns for suspicious activity. Network segmentation and access controls should be reviewed to limit exposure of samba services to unauthorized networks. Security teams should implement continuous monitoring of authentication logs to detect potential exploitation attempts and establish alerting for unusual authentication patterns that may indicate brute force attacks. The vulnerability demonstrates the importance of proper concurrent programming practices in security-critical code paths and highlights the necessity of thorough testing for race conditions in authentication systems. Organizations should also consider implementing multi-factor authentication as a compensating control to reduce the impact of potential credential compromise even if the race condition is exploited.