CVE-2016-2124 in Samba
Summary
by MITRE • 02/18/2022
A flaw was found in the way samba implemented SMB1 authentication. An attacker could use this flaw to retrieve the plaintext password sent over the wire even if Kerberos authentication was required.
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Analysis
by VulDB Data Team • 09/14/2026
The vulnerability identified in Samba regarding its implementation of Server Message Block version 1 (SMB1) authentication represents a critical failure in cryptographic protocol enforcement and legacy support hygiene. SMB1, also known as CIFS, is an outdated network file sharing protocol that has been deprecated by Microsoft due to numerous security weaknesses, including susceptibility to man-in-the-middle attacks and the inability to provide strong encryption for data in transit. The core issue lies in how Samba handled authentication requests when both legacy NTLMv1 capabilities and modern Kerberos mechanisms were potentially involved or misconfigured within the same environment. Specifically, the flaw allowed an attacker who could intercept network traffic between a client and the Samba server to extract plaintext credentials even if the system was ostensibly configured to require Kerberos authentication for certain operations or users. This indicates that the fallback logic or negotiation phase of the SMB1 protocol implementation did not strictly enforce the highest security level agreed upon, allowing a downgrade attack where an attacker could force the connection into a state using weaker authentication methods like NTLMv1, which are vulnerable to offline cracking and pass-the-hash attacks.
From a technical perspective, this flaw exploits the complexity inherent in maintaining backward compatibility with legacy protocols while attempting to enforce modern security policies. When SMB1 is enabled, it introduces multiple negotiation points where protocol versions and encryption levels can be manipulated by an active network attacker. The specific defect suggests that Samba failed to properly validate or reject authentication attempts that did not meet the strict Kerberos requirements when those requirements were explicitly set in the configuration. Consequently, if a client attempted to authenticate using SMB1, the server might have accepted weaker credentials or allowed the extraction of sensitive data during the handshake process before fully committing to the secure channel established by Kerberos. This behavior effectively nullifies the security benefits of requiring Kerberos for high-value assets, as an attacker positioned on the network path can exploit this inconsistency to capture passwords in a format that is trivially reversible or crackable using standard tools like Hashcat or John the Ripper.
The operational impact of this vulnerability is severe, particularly in environments where SMB1 has not been completely disabled due to compatibility requirements with older devices such as legacy printers, IoT sensors, or outdated operating systems. An attacker within the same network segment can perform a man-in-the-middle attack to intercept authentication traffic. By leveraging this flaw, they can retrieve plaintext passwords that were transmitted during the initial negotiation phase of SMB1 sessions. These credentials can then be used for lateral movement across the domain, privilege escalation, or access to sensitive file shares. The risk is amplified in mixed-environment networks where some systems still rely on SMB1, creating a weak link that compromises the security posture of otherwise well-hardened servers running modern authentication protocols like Kerberos and NTLMv2.
To mitigate this vulnerability, organizations must prioritize the complete removal of SMB1 from their infrastructure wherever possible. This involves disabling the SMB1 protocol at both the client and server levels through operating system configurations and group policy settings. For systems that absolutely require legacy support due to incompatible hardware or software dependencies, strict network segmentation is essential to isolate these devices from general corporate traffic using VLANs or firewalls, thereby preventing attackers from intercepting SMB1 negotiations. Additionally, administrators should ensure that Samba configurations explicitly disable any fallback mechanisms that allow weaker authentication methods when stronger ones are available. Regular auditing of protocol usage and monitoring for unexpected SMB1 traffic can help detect attempts to exploit this flaw before credentials are compromised. Keeping Samba updated to the latest stable release ensures that such implementation errors in legacy code paths are addressed by upstream developers who continuously patch security regressions in older protocols.
This vulnerability aligns with CWE-326, Inadequate Encryption Strength, as it involves the use of weak cryptographic mechanisms for authentication and data protection during the session setup phase. It also relates to CWE-918, Server-Side Request Forgery (SSRF), if considered through the lens of protocol negotiation manipulation, but more accurately fits CWE-20, Improper Input Validation, regarding how the server processes incoming SMB1 negotiation packets without strictly enforcing the configured security policy. In terms of MITRE ATT&CK tactics, this flaw facilitates Credential Access via T1557, Adversary-in-the-Middle, as it allows an attacker to intercept and potentially alter communications between two parties trusting each other for authentication purposes. Furthermore, the exploitation technique falls under Lateral Movement techniques such as T1021, Remote Services, specifically SMB/Windows Admin Shares, if the stolen credentials are used to access additional systems within the network domain.