The reason SMB deserves careful attention isn't that it shares files — it's that a single misconfigured or outdated SMB service has been behind some of the most damaging cyberattacks of the last decade. WannaCry spread across hundreds of thousands of machines through an SMB flaw. That's the tension worth understanding up front: SMB is one of the most useful protocols in any office network and one of the most exploited. Knowing how it works is inseparable from knowing how it gets attacked.
This walks through what SMB is, how the client-server exchange actually functions, how the protocol evolved from a genuinely insecure early version to a hardened modern one, and — the part most overviews rush past — the concrete security risks and the practical steps that keep an SMB deployment from becoming an entry point.
What the SMB Protocol Is
Server Message Block (SMB) is a client-server network protocol that lets a computer access files, printers, and other resources on a remote machine as if they were local. When you open a shared folder on a company network drive and edit a document that physically lives on a server down the hall — or in another country — SMB is the protocol making that feel seamless.
Originally created at IBM in the mid-1980s and developed extensively by Microsoft thereafter, SMB became the backbone of Windows file and printer sharing. Its defining trait is transparency: a remote file behaves, from the user's perspective, like one sitting on their own hard drive. Multiple people can access, open, and edit shared resources in the same location, from anywhere they can reach the server.
It operates as a request-response protocol. The client asks; the server answers. That simple pattern, repeated many times, is what sustains a working file share — and understanding it is the key to understanding both the protocol's usefulness and its exposure.
How SMB Actually Works
The exchange follows a consistent sequence. The client initiates by sending a request to the server — to connect, to authenticate, to open a file, to read or write data. The server receives each request, processes it, and responds. This back-and-forth establishes and maintains the session through which all file operations flow.
Walk through a typical interaction:
Connection and negotiation. The client contacts the server and the two negotiate which SMB dialect (version) they'll both use — the highest both support. This step matters for security, and we'll return to it.
Authentication. The client proves who it is, typically against Windows credentials or a domain controller. The server decides what that user is permitted to access.
Tree connect. The client connects to a specific shared resource (a "share") — a folder, a drive, a printer.
File operations. Open, read, write, close, lock. Each is a request-response pair. SMB includes file locking so two users editing the same file don't overwrite each other.
Disconnection. The session closes when the client is done.
Modern SMB (over TCP port 445) runs directly over TCP/IP, which is why a remote worker can reach an office share across the internet, not just across the local network. That reach is the feature — and, when the port is exposed carelessly, the vulnerability.
The Evolution of SMB Dialects
SMB's version history is not trivia — each dialect changed what the protocol could do and, critically, how safe it was. The single most important security decision you can make about SMB is which version you allow.
Dialect | Introduced | What it changed | Security posture |
SMB 1.0 | Mid-1980s (IBM); Microsoft-extended early 1990s | Original file sharing for DOS/early Windows | Dangerously outdated — should be disabled everywhere |
CIFS | 1996 (Windows 95/NT) | A dialect of SMB 1.0; larger files, more features; often used loosely as a synonym for SMB 1 | Same fundamental weaknesses as SMB 1 |
SMB 2.0 | 2006 (Windows Vista / Server 2008) | Major rewrite; far fewer commands, much better performance over higher-latency links | Meaningfully improved over SMB 1 |
SMB 2.1 | 2010 (Windows 7 / Server 2008 R2) | Performance and locking improvements | Improved |
SMB 3.0 | 2012 (Windows 8 / Server 2012) | End-to-end encryption, secure dialect negotiation | Strong — encryption introduced |
SMB 3.1.1 | 2015 (Windows 10 / Server 2016) | AES-128 encryption, pre-authentication integrity, mandatory secure negotiation | Current best practice |
Two points the short version of this history usually omits:
CIFS is not "better than SMB" — it's old SMB. People sometimes treat CIFS as a distinct, modern protocol. It's a dialect of SMB 1.0 and carries the same fundamental weaknesses. Seeing "CIFS" in a configuration is a flag to check what you're actually running.
SMB 3.1.1's security additions are the reason to upgrade. AES encryption protects data in transit; pre-authentication integrity (secure negotiation) stops an attacker from forcing a downgrade to a weaker, exploitable dialect during the negotiation step. That downgrade defense directly closes an attack class that earlier versions left open.
The Security Problem You Have to Take Seriously
This is where a file-sharing protocol becomes a security topic. SMB has been the vector for some of the most destructive malware in recent memory, and understanding why is more useful than a vague "it can be attacked."
The blast radius is the network, not one machine. SMB connects devices, so an SMB-borne exploit doesn't stop at the machine it lands on — it spreads laterally to every connected device. A single compromised endpoint can seed an entire network. This is precisely what made SMB worms so devastating: they self-propagated across shares.
SMB 1.0 is the standing danger. The most famous SMB attacks — WannaCry and NotPetya in 2017 — exploited a flaw in SMB 1.0 (via an exploit known as EternalBlue). The protocol version millions of machines still had enabled, often needlessly, was the doorway. Microsoft has deprecated SMB 1 and recommends disabling it entirely, yet legacy systems keep it alive. If you run nothing else from this article: find and disable SMB 1.
Exposed port 445 is an open invitation. SMB should almost never be reachable directly from the public internet. An internet-facing port 445 is continuously scanned by attackers. Remote access to shares belongs behind a VPN, not exposed to the world.
The relationship to keep in mind: SMB's usefulness and its risk come from the same property. It gives remote machines transparent access to shared resources — which is exactly what an attacker wants too, if they can get in.
Practical Steps to Secure SMB
Concrete measures, roughly in priority order:
Disable SMB 1.0 / CIFS everywhere. No modern system needs it. This single step eliminates the largest category of SMB attacks. Audit for it first, because it's often silently enabled on older machines.
Enforce SMB 3.1.1 where possible, and require SMB encryption for sensitive shares so data in transit can't be read if intercepted.
Never expose port 445 to the internet. Block it at the firewall. Require a VPN for any remote share access.
Segment the network. Because SMB spreads laterally, network segmentation limits how far a compromise can travel. A breach in one segment shouldn't reach the whole organization.
Enforce least-privilege access. Users and shares should have only the permissions they need. Tight access control shrinks what any single compromised account can touch.
Patch promptly. SMB vulnerabilities get patched, but only patched systems are protected. WannaCry hit machines that hadn't applied an available fix. Keep systems current.
Monitor SMB traffic for unusual patterns — unexpected lateral connections are an early sign of a spreading compromise.
How SMB Differs From Messaging Protocols
Because "protocol" covers a lot of ground, it helps to place SMB against a protocol built for an entirely different job. SMB is a file-sharing protocol — its purpose is transparent access to remote files and resources within a network. A protocol like SMPP, by contrast, exists to move SMS traffic between applications and mobile networks. Both are client-server, both are request-response, but they solve unrelated problems: SMB connects a user to a network drive; SMPP connects a business application to a carrier's messaging infrastructure.
The comparison is useful for one reason — it clarifies that a protocol's security model has to match its exposure. SMB's risk profile comes from sitting inside corporate networks and, when misconfigured, facing the internet. Different protocols carry different threat surfaces, and treating them interchangeably is how misconfigurations happen.
Bringing It Together
SMB is foundational to how organizations share files, and it will keep running quietly in the background of most networks. The mistake is treating it as purely a convenience feature and ignoring the security weight it carries. The two things that determine whether your SMB deployment is a workhorse or a liability are the version you run and whether it's exposed. Disable SMB 1, enforce a modern encrypted dialect, keep port 445 off the public internet, and patch on schedule.
An experienced network team treats SMB as both essential and dangerous — essential because it enables everyday collaboration, dangerous because its lateral reach turns one compromise into many. Respect both halves and it does its job without becoming the story after a breach.
A clear guide to the SMB protocol — how the client-server exchange works, every dialect from SMB 1.0 to 3.1.1, and the security steps that prevent attacks.
FAQ
What is the SMB protocol used for?
SMB (Server Message Block) is a client-server protocol for sharing files, printers, and other resources across a network. It lets users open and edit files stored on a remote server as if they were on their own device, which is why it underpins most Windows file and printer sharing in offices.
Is SMB 1.0 safe to use?
No. SMB 1.0 is outdated and carries serious vulnerabilities — it was the vector for the WannaCry and NotPetya attacks via the EternalBlue exploit. Microsoft has deprecated it and recommends disabling it entirely. Any modern network should identify and turn off SMB 1.0, as no current system genuinely needs it.
What's the difference between SMB and CIFS?
CIFS (Common Internet File System) is a dialect of SMB 1.0 from the late 1990s, not a newer or separate protocol. People sometimes treat it as modern, but it shares SMB 1.0's fundamental weaknesses. Modern deployments should use SMB 3.1.1, which adds AES encryption and secure dialect negotiation.
Which port does SMB use, and should it be open to the internet?
Modern SMB uses TCP port 445 (older setups also used 137–139). It should almost never be exposed directly to the internet — an open port 445 is continuously scanned by attackers. Remote access to SMB shares should go through a VPN, with port 445 blocked at the perimeter firewall.
What is the most secure version of SMB?
SMB 3.1.1, introduced in 2015, is the current best practice. It adds AES-128 encryption for data in transit and pre-authentication integrity, which prevents attackers from forcing a downgrade to a weaker, exploitable dialect during connection negotiation. Enforcing it, with encryption required on sensitive shares, is the recommended baseline.