CVE-2015-5296 in Samba
Summary
by MITRE
Samba 3.x and 4.x before 4.1.22, 4.2.x before 4.2.7, and 4.3.x before 4.3.3 supports connections that are encrypted but unsigned, which allows man-in-the-middle attackers to conduct encrypted-to-unencrypted downgrade attacks by modifying the client-server data stream, related to clidfs.c, libsmb_server.c, and smbXcli_base.c.
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Analysis
by VulDB Data Team • 07/01/2022
The vulnerability identified as CVE-2015-5296 affects Samba implementations across versions 3.x and 4.x prior to specific patch releases, creating a critical security gap in the authentication and encryption mechanisms of the file sharing protocol. This weakness specifically targets the handling of encrypted connections that are not properly signed, allowing attackers to exploit the communication channel between client and server systems. The flaw manifests in the clidfs.c, libsmb_server.c, and smbXcli_base.c components of the Samba codebase, which govern the client-server interaction and connection management processes. The vulnerability falls under the category of cryptographic weaknesses and specifically relates to the improper handling of authenticated encrypted sessions, making it particularly dangerous in enterprise environments where Samba servers are commonly deployed for file sharing and authentication services.
The technical implementation of this vulnerability stems from the fact that Samba's protocol handling allows for encrypted connections that are not properly signed, creating an opportunity for attackers to perform downgrade attacks against the security mechanisms. When a client establishes a connection with a Samba server, the system may negotiate an encrypted session that appears secure but lacks proper message authentication codes to verify the integrity of transmitted data. Attackers can exploit this by intercepting the communication stream and modifying the data to force a transition from encrypted to unencrypted communication, effectively bypassing the security protections that should maintain the integrity of the authenticated session. This type of attack is particularly effective because it leverages the legitimate negotiation processes within the Samba protocol rather than attempting to exploit other weaknesses in the system.
The operational impact of CVE-2015-5296 extends beyond simple data interception, as it enables comprehensive man-in-the-middle attacks that can compromise entire authentication and file sharing processes. An attacker with network access between a Samba client and server can modify traffic to downgrade security levels, potentially gaining unauthorized access to shared resources, modifying file contents, or capturing sensitive authentication information. This vulnerability is particularly concerning in environments where Samba serves as a primary authentication mechanism for file sharing services, as it can allow attackers to establish persistent access to network resources while remaining undetected. The downgrade attack capability means that even if a client and server negotiate an encrypted connection, the attacker can force a transition to an unencrypted state, making the entire communication channel vulnerable to interception and manipulation.
Organizations affected by this vulnerability should implement immediate mitigations including updating to patched versions of Samba that address the specific unsigned connection handling issues in the affected code modules. The recommended approach involves upgrading to Samba versions 4.1.22, 4.2.7, or 4.3.3 respectively, which contain fixes for the cryptographic negotiation and connection signing mechanisms. Network segmentation and monitoring should be implemented to detect unusual connection patterns that might indicate attempted downgrade attacks, while administrators should configure Samba servers to enforce signed connections and disable support for unsigned encrypted sessions. Additionally, organizations should consider implementing network intrusion detection systems that can identify modification of SMB traffic patterns and establish security policies that mandate the use of properly signed encrypted connections for all Samba communications, as this vulnerability directly relates to the principle of least privilege and secure communication protocols as outlined in cybersecurity frameworks such as those defined by NIST and ISO 27001 standards.
This vulnerability is classified as a cryptographic weakness under CWE-310 and represents a specific instance of protocol downgrade attacks that fall within the ATT&CK framework's T1071.1002 technique for application layer protocol tunneling. The security implications extend to potential compromise of authentication systems and unauthorized access to network resources, making it a high-priority vulnerability for enterprise security teams to address through both immediate patching and long-term security architecture improvements.