SMB

SMB. The Server Message Block protocol is a network communication standard primarily used for providing shared access to files, printers, and serial ports between nodes on a network. It functions as an application-layer protocol and is most famously implemented in the Microsoft Windows operating system family, where it is known as Microsoft SMB Protocol. The protocol has evolved through numerous versions, with its development now overseen by Microsoft in collaboration with the wider technology industry.

Overview

SMB operates on a client-server model, allowing programs on a client computer to read, write, and request services from server programs in a computer network. It is integral to the functionality of Windows Server for file sharing and print services, and forms the backbone of the Common Internet File System (CIFS) dialect. Beyond Microsoft ecosystems, the protocol is widely implemented in other operating systems, such as through the Samba (software) suite on Unix-like systems including Linux and macOS, enabling cross-platform interoperability. Core operations facilitated by SMB include browsing shared resources, opening, reading, writing, and locking files, all while managing user authentication and authorization.

Protocol details

The protocol has undergone significant evolution, with major versions including SMB 1.0 (or CIFS), SMB 2.0, SMB 2.1, SMB 3.0, SMB 3.02, and SMB 3.1.1. Each iteration introduced improvements; for instance, SMB 2.0, introduced with Windows Vista and Windows Server 2008, reduced command complexity and added support for pipelining and durable handles. SMB 3.0, launched with Windows 8 and Windows Server 2012, brought major advancements like SMB Direct (RDMA), SMB Multichannel, and end-to-end encryption. The protocol traditionally used NetBIOS over TCP/IP on ports 137-139 but now operates directly over TCP on port 445. It relies on underlying transport protocols and can use Kerberos or NTLM for security negotiation.

Security

Early versions, particularly SMB 1.0/CIFS, were criticized for weak security, often transmitting credentials in plaintext and being vulnerable to attacks like man-in-the-middle. Modern versions have integrated robust security features, with SMB 3.0 and above supporting strong encryption via AES-CCM or AES-GCM algorithms. The protocol supports secure dialect negotiation and pre-authentication integrity to prevent protocol downgrade attacks. Security is managed through features like SMB signing to ensure packet authenticity and integration with Active Directory for centralized access control. Notable vulnerabilities, such as those exploited by the EternalBlue exploit used in the WannaCry ransomware attack, have prompted the deprecation of older protocol versions.

Implementations

The primary implementation is the Microsoft SMB Protocol stack in all modern Windows and Windows Server releases. The open-source Samba (software) project provides a compatible server and client for Linux, macOS, and other Unix-like operating systems, allowing them to participate in Windows domain networks. Other implementations include Sun Microsystems' SunOS, IBM's OS/2, and the FreeNAS and TrueNAS storage platforms. Various network-attached storage (NAS) devices from vendors like Synology and QNAP utilize SMB for file services. Client libraries are also available for programming languages to enable application-level access to SMB shares.

History and development

The protocol was originally created by Barry Feigenbaum at IBM in 1983 for use in IBM PC Network software. Microsoft later heavily adopted and extended the protocol, integrating it into its LAN Manager product and subsequent Windows for Workgroups and Windows NT lines. For years, the protocol was standardized under the name Common Internet File System (CIFS) by The Open Group. Since SMB 2.0, development has been driven primarily by Microsoft, with the protocol becoming a cornerstone of Azure file services. Industry efforts, including those by the Storage Networking Industry Association (SNIA), continue to work on its standardization and interoperability, while Microsoft has actively deprecated the older, less secure SMB 1.0 protocol in modern systems.

Category:Network protocols Category:Application layer protocols Category:File sharing protocols Category:Microsoft protocols