CVE-2021-3671 in Samba
Summary
by MITRE • 10/12/2021
A null pointer de-reference was found in the way samba kerberos server handled missing sname in TGS-REQ (Ticket Granting Server - Request). An authenticated user could use this flaw to crash the samba server.
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Analysis
by VulDB Data Team • 11/17/2024
The vulnerability identified as CVE-2021-3671 represents a critical null pointer dereference flaw within the Samba kerberos server implementation, specifically affecting the Ticket Granting Server component. This issue manifests when the server processes TGS-REQ (Ticket Granting Server - Request) messages that lack the required sname (service name) parameter. The flaw resides in the kerberos server's handling logic where it fails to properly validate the presence of essential service name components before attempting to dereference pointers associated with these missing elements. The vulnerability is particularly concerning as it allows authenticated users to exploit this condition and cause a controlled crash of the Samba server daemon, effectively creating a denial of service scenario that can be leveraged to disrupt legitimate service access for all users relying on the affected kerberos infrastructure.
From a technical perspective, this vulnerability maps directly to CWE-476 which defines NULL Pointer Dereference as a condition where a null pointer is dereferenced, leading to program termination or unpredictable behavior. The flaw operates within the Samba kerberos server's authentication processing pipeline where it receives and validates kerberos ticket requests. When a TGS-REQ message is received without the sname field, the server's processing logic attempts to access memory locations associated with this missing parameter without proper null checks, resulting in an immediate system crash. This type of vulnerability is classified under the ATT&CK technique T1499.004 which covers Network Denial of Service, as it allows an authenticated attacker to cause service disruption through controlled application crashes.
The operational impact of CVE-2021-3671 extends beyond simple service disruption, as it can severely compromise the availability of kerberos-based authentication services within Samba environments. Organizations relying on Samba for file sharing, print services, and domain authentication may experience complete service outages when this vulnerability is exploited, affecting thousands of users who depend on these services for daily operations. The authenticated nature of the exploit means that attackers do not require special privileges to cause damage, as they only need valid credentials to access the Samba server's kerberos service. This makes the vulnerability particularly dangerous in enterprise environments where Samba servers often serve as critical infrastructure components for authentication and resource access across multiple domains and user groups.
Mitigation strategies for CVE-2021-3671 should prioritize immediate patch application from Samba project releases, which typically include proper validation of kerberos message parameters before pointer dereference operations. Network segmentation and access controls should be implemented to limit exposure of vulnerable Samba servers to untrusted networks, while monitoring systems should be deployed to detect unusual crash patterns or authentication request anomalies. Security teams should also consider implementing automated failover mechanisms and redundant authentication services to minimize the impact of potential exploitation attempts. Additionally, regular security audits of kerberos implementations should be conducted to identify similar validation gaps in other authentication components, as this vulnerability demonstrates the importance of proper input validation in security-critical systems. The fix typically involves adding null checks and proper error handling before dereferencing pointers associated with optional kerberos parameters, ensuring that the server gracefully handles malformed requests rather than crashing.