SMB Relay is a type of cyber attack that targets the Server Message Block (SMB) protocol, which is widely used for network file sharing in Windows environments. This attack exploits the NTLM authentication mechanism, allowing attackers to intercept and relay authentication attempts to gain unauthorized access to network resources.

In essence, SMB Relay attacks take advantage of the inherent trust within the SMB protocol. By capturing and relaying authentication requests, attackers can impersonate legitimate users and access sensitive data or systems without proper authorization. This makes SMB Relay a significant threat in environments where SMB and NTLM are used without adequate security measures.

How does SMB Relay Work?

SMB Relay attacks operate by exploiting the NTLM authentication process within the SMB protocol. Initially, the attacker identifies vulnerable workstations that do not enforce SMB signing. Tools like nmap can be used to scan the network for such targets. Once identified, the attacker sets up a relay attack tool, such as Responder, to intercept authentication requests.

When a user attempts to authenticate, the attack tool captures the NTLM authentication hash. This hash is then relayed to another server, tricking it into believing the attacker is the legitimate user. The attacker uses tools like ntlmrelayx to facilitate this relay, effectively bypassing the need to crack the password.

By relaying the captured credentials, the attacker gains unauthorized access to the target system. This access can be leveraged to perform various malicious activities, such as dumping local SAM hashes or gaining shell access, thereby compromising the network's security.

What are Examples of SMB Relay Attacks?

Examples of SMB Relay attacks often involve sophisticated techniques to exploit network vulnerabilities. One notable instance is the hypothetical scenario involving a user named Frank Castle from the MARVEL.local domain. In this case, an attacker identifies that Frank has local administrator privileges on multiple machines. By leveraging these privileges, the attacker can relay intercepted credentials to gain unauthorized access, demonstrating how SMB Relay attacks can compromise network security.

Another example involves the use of tools like Metasploit and Impacket. Attackers scan the network to identify vulnerable shares and IP addresses, then capture authentication packets. By tricking the server into accepting these packets, they gain access to sensitive systems. This method highlights the technical prowess required to execute SMB Relay attacks and the potential for significant security breaches.

What are the Potential Risks of SMB Relay?

The potential risks of suffering an SMB Relay attack are significant and multifaceted. Here are some of the key risks:

• Unauthorized Access to Sensitive Information: Attackers can impersonate legitimate users, gaining access to confidential data and systems.

• Escalation of Privileges: By relaying credentials of users with higher access levels, attackers can escalate their privileges within the network.

• Compromise of Network Integrity: The attack can facilitate lateral movement, compromising the integrity of the entire network.

• Disruption of Business Operations: Unauthorized access and remote command execution can disrupt critical business functions and cause downtime.

• Financial and Reputational Damage: Data breaches resulting from SMB Relay attacks can lead to financial losses and damage to the company's reputation.

How Can You Protect Against SMB Relay?

To protect against SMB Relay attacks, consider implementing the following measures:

• Enable SMB Signing: Ensure that SMB signing is enabled on all devices to verify the authenticity of communications and prevent unauthorized access.

• Disable NTLM Authentication: Turn off NTLM authentication across the network to eliminate a common attack vector for SMB Relay.

• Use the Latest SMB Versions: Upgrade to SMB 3.0 or higher, which includes enhanced security features like AES-128 encryption and integrity checks.

• Implement Network Segmentation: Divide the network into smaller subnetworks to limit the impact of potential breaches and contain attacks.

• Apply Regular Updates: Keep operating systems and SMB software up to date with the latest security patches to mitigate vulnerabilities.