SMB Protocol Explained: Understanding its Security Risks and Best Practices

The Server Message Block (SMB) protocol is a widely used communication protocol enabling users to interact with remote servers, sharing and managing files, as well as utilizing networked resources. While integral for business operations, SMB has also been a frequent target for cybercriminals seeking unauthorized access to sensitive data and systems. In this blog post, we revisit the security risks associated with SMB, discuss notable attacks, and provide updated best practices to safeguard your network.

Notable SMB-Related Attacks

Operating over TCP Port 445, the SMB protocol’s older versions are notorious for weak security, making them prime targets for exploitation. Here are some high-profile cyberattacks that leveraged SMB vulnerabilities:

• WannaCry Ransomware (2017)
  This global ransomware outbreak exploited the EternalBlue vulnerability, which had been developed by the Shadow Brokers group. WannaCry caused widespread damage by encrypting data on affected systems and demanding ransom payments. By 2019, over 12,000 variants of this ransomware were active.
• Emotet Trojan
  Originally designed as a banking trojan in 2014, Emotet evolved into a self-propagating malware strain by 2017, exploiting EternalBlue to spread across networks. It often infiltrated systems via malicious spam emails and delivered ransomware payloads. Emotet’s operations were disrupted in January 2021, but it remains a concerning legacy.
• TrickBot Trojan
  Since 2016, TrickBot has been capable of stealing financial data and credentials. It uses EternalChampion, another SMB-related exploit developed by Shadow Brokers, to move laterally across networks, furthering its ability to steal sensitive information.

How Does SMB Work?

SMB enables file and resource sharing through a client-server model. Here’s a simplified breakdown of the SMB process:

1. SMB Client Request — The client sends a request to the SMB server to access shared resources.
2. Authentication — After a session is initiated, the client sends its credentials to the server for verification.
3. Resource Access — Once authenticated, the client can access files, shared devices, and perform tasks.
4. Session Termination — The session ends when the client no longer needs access, releasing resources.

There are several authentication mechanisms that the SMB protocol uses to ensure secure and authorized access to shared resources.

1. NT LAN Manager (NTLM) — NTLM, an outdated authentication protocol, is used in legacy Windows and SMB versions. It uses a challenge-response system, with the server sending a random challenge to the client. Although compatible with older systems, NTLM is considered less secure and poses security risks due to its susceptibility to attacks.
2. Kerberos — This is a secure authentication protocol using symmetric key cryptography and a Key Distribution Center (KDC). Kerberos is the recommended authentication protocol for ensuring secure SMB communications within Active Directory (AD) domains.
3. SMB2 and SMB3 Authentication Extensions — SMB2 and SMB3, the latest versions of SMB, introduce significant security improvements:

• Pre-Authentication Integrity (SMB2/3): Enhances security by verifying packet integrity during authentication, ensuring data remains secure.
• Secure Dialect Negotiation (SMB3): Ensures secure version and security setting negotiation, aligning the protocol with modern security requirements.

It is crucial to prioritize modern authentication methods for robust data protection and security.

SMB relay attack

SMB relay attacks exploit SMB’s NTLM authentication, potentially allowing attackers to impersonate users and gain unauthorized access. This attack is facilitated by specific prerequisites such as SMB signing disabled on the target, local network access, and user credentials with remote login permissions.

The SMB relay attack sequence is as follows:

1. Identifying vulnerable workstation IPs.
2. Initiating necessary relay attack tools for the attack.
3. Intercepting user hashes, often through events like LLMNR Poisoning.
4. Using the intercepted credentials to gain unauthorized access.

To protect against SMB relay attacks, apply these defensive measures: remove the first SMB version, enable SMB signing on all devices, disable network-wide NTLM authentication, and impose local admin restrictions.

Best practices for safeguarding against SMB attacks

• Use the latest SMB versions — Replace SMB1 with SMB 3.0 or higher. SMB 3.0 and later versions, including SMB 3.1.1, introduced numerous security enhancements. These include end-to-end data encryption, secure dialect negotiation, and pre-authentication integrity, securing data from eavesdropping and Man in the Middle (MitM) attacks.
• Apply regular updates — Regularly update your OS and SMB software with security patches. Vendors release these patches to fix vulnerabilities. Staying up to date prevents known vulnerabilities and keeps your SMB implementation secure.
• Segment your network — Divide your network into subnetworks to limit the impact of breaches and reduce the risk of lateral movement of threat actors.
• Use a firewall with advanced controls to regulate outbound SMB destinations to prevent connections to potentially malicious servers.

The SMB protocol is a key component of network communication, offering critical file-sharing and resource access capabilities. However, its vulnerabilities — particularly in older versions — pose significant security risks. By following best practices, including upgrading to modern SMB versions, applying patches, and enforcing strong authentication mechanisms, organizations can mitigate these risks and continue leveraging the benefits of SMB safely.