What Is SSH? Understanding Network Protocols By Wirex

SSH: Network Protocol Explained

SSH (Secure Shell) is a cryptographic network protocol that provides secure communication channels over unsecured networks. It is widely used for secure remote access and management of network devices, servers, and applications. SSH offers various functionalities, including secure file transfer, remote command execution, and remote system management.

SSH is to provide a secure and encrypted communication channel between two devices over an insecure network. By offering strong encryption, integrity checking, and authentication, SSH allows users to securely access remote systems, execute commands, and transfer files without exposing sensitive data to potential eavesdropping, man-in-the-middle, or other types of attacks.

To mitigate these security concerns, organizations should follow best practices for SSH configuration and key management, use strong authentication mechanisms, and regularly monitor and update their SSH implementations.

What is SSH

SSH, or Secure Shell, is a cryptographic network protocol that provides secure communication channels over unsecured networks. It is designed to enable secure remote access and management of network devices, servers, and applications. SSH ensures the confidentiality, integrity, and authentication of data transmitted between two devices by using strong encryption and various authentication mechanisms.

In addition to remote access and management, SSH is commonly used for secure file transfers through protocols like SCP (Secure Copy Protocol) and SFTP (SSH File Transfer Protocol). It can also be used for secure tunneling of other network protocols, port forwarding, and creating encrypted VPN (Virtual Private Network) connections.

SSH was developed to replace insecure remote access protocols like Telnet and Rlogin, which transmitted data in plaintext and were vulnerable to eavesdropping, man-in-the-middle attacks, and other security threats. By providing a secure alternative to these protocols, SSH has become the standard for remote system administration and secure communication over unsecured networks.

The Purpose of SSH

The primary purpose of SSH is to provide a secure and encrypted communication channel between two devices over an insecure network. It is designed to protect the confidentiality, integrity, and authenticity of data transmitted between a client and a server, ensuring that sensitive information is not exposed to eavesdropping, man-in-the-middle attacks, or other security threats.

SSH serves multiple purposes, including:

  1. Secure remote access: SSH allows users to securely access and manage remote systems, such as servers, routers, or switches, from a local device. With strong encryption and various authentication mechanisms, SSH ensures that only authorized users can access the remote systems and execute commands.
  2. Secure file transfer: SSH enables secure file transfer between devices using protocols like SCP (Secure Copy Protocol) and SFTP (SSH File Transfer Protocol). These protocols leverage SSH’s encryption capabilities to transfer files securely over unsecured networks.
  3. Secure tunneling and port forwarding: SSH can be used to create secure tunnels for other network protocols, effectively encrypting their traffic and protecting them from eavesdropping or tampering. SSH port forwarding allows users to securely forward local ports to remote systems or vice versa.
  4. VPN (Virtual Private Network) capabilities: SSH can be used to create encrypted VPN connections, allowing users to securely access remote networks or resources as if they were connected locally.

By providing a secure and versatile solution for remote access, file transfer, and encrypted communication, SSH has become an essential tool for network administrators, developers, and IT professionals.

Benefits Of SSH

SSH offers several benefits that make it a popular choice for secure communication and remote access over unsecured networks. Some of the key benefits of SSH include:

  1. Security: SSH provides strong encryption algorithms to protect the confidentiality and integrity of data transmitted between devices. This ensures that sensitive information is not exposed to eavesdropping, man-in-the-middle attacks, or other security threats.
  2. Authentication: SSH supports various authentication mechanisms, such as password-based, public key, or multi-factor authentication. This helps ensure that only authorized users can access remote systems and execute commands.
  3. Versatility: SSH can be used for a wide range of applications, including remote system administration, secure file transfer (using SFTP or SCP), port forwarding, and secure tunneling for other protocols. This makes it a flexible solution for many different use cases and requirements.
  4. Platform-independent: SSH is supported by most operating systems and platforms, including Windows, macOS, Linux, and various networking devices. This makes it a widely compatible solution for secure remote access and communication across diverse environments.
  5. Extensibility: SSH can be extended with additional functionality, such as VPN capabilities or custom encryption algorithms, to meet specific security requirements or adapt to unique use cases.
  6. Open standard: SSH is an open standard protocol, which means that it is not tied to any specific vendor or proprietary technology. This allows for greater interoperability and flexibility in choosing the best tools and implementations for a given environment.

By offering a secure, versatile, and widely supported solution for remote access and encrypted communication, SSH has become an essential tool for network administrators, developers, and IT professionals working with unsecured networks.

Limitations Of SSH

While SSH is a highly secure and versatile protocol, it does have some limitations and drawbacks that users should be aware of:

  1. Increased resource usage: SSH encryption and authentication mechanisms can consume more resources (CPU and memory) than unencrypted protocols like Telnet or Rlogin. This increased resource usage may impact the performance of the devices involved, particularly when multiple SSH connections are active or when devices have limited resources.
  2. Complexity: Setting up and managing SSH keys, authentication mechanisms, and configurations can be complex, especially for large organizations with numerous devices and users. In addition, understanding and managing the various encryption algorithms and security options can be challenging for some users.
  3. Key management: Proper SSH key management is essential for maintaining security, but it can be a time-consuming and cumbersome process. Users need to generate, distribute, and maintain unique SSH keys for each device and user, and securely store and rotate keys as needed.
  4. Vulnerabilities: Like any software, SSH implementations can have vulnerabilities or security flaws that can be exploited by attackers. Regularly updating and patching SSH software is essential for mitigating these risks, but it can be a time-consuming task.
  5. Latency: Due to the encryption and decryption processes involved in SSH communication, there can be increased latency compared to unencrypted protocols. This may be noticeable when transmitting large amounts of data or in high-latency network environments.
  6. Insecure configurations: SSH can be highly secure, but it requires proper configuration to ensure maximum protection. Insecure configurations, such as using weak ciphers, default passwords, or not disabling unused features, can leave systems vulnerable to attacks.

Despite these limitations, SSH remains a widely used and trusted protocol for secure communication and remote access. By understanding and addressing these limitations and implementing best practices for SSH configuration and key management, users can effectively leverage the security and versatility of SSH while minimizing potential risks.

How Does SSH Work

SSH, is a cryptographic network protocol used for securely accessing and managing network devices, servers, and other remote systems over an unsecured network, such as the internet. It is widely used by system administrators for managing, configuring, and transferring files securely between a local machine and a remote server. SSH works by providing a secure channel over which encrypted data can be exchanged.

Here’s an overview of how SSH works:

  1. Key pair generation: SSH uses public key cryptography to authenticate users and establish a secure connection. Each user has a pair of keys, a public key and a private key. The public key can be shared with anyone, while the private key must be kept secret.
  2. Public key distribution: The user’s public key is uploaded to the remote server and stored in an authorized_keys file. This file is located in the user’s .ssh directory on the remote server.
  3. Establishing a connection: When a client wants to establish a secure connection with a remote server, it sends a connection request to the server’s SSH port (default is port 22).
  4. Server authentication: The server sends its public key to the client. The client then verifies the server’s identity using the known_hosts file, which contains a list of trusted server public keys. This step prevents man-in-the-middle attacks by ensuring the client is connecting to the intended server.
  5. User authentication: After server authentication, the client needs to authenticate itself to the server. This can be done using a password, public key authentication, or other methods like Kerberos. In the case of public key authentication, the server checks whether the client’s public key is present in its authorized_keys file.
  6. Key exchange: Once both parties are authenticated, they negotiate a shared session key using a key exchange algorithm, like Diffie-Hellman. This session key is used to encrypt and decrypt data during the communication.
  7. Encrypted communication: With the session key established, the client and server can now exchange data securely, as all communication is encrypted using symmetric encryption algorithms like AES.
  8. Closing the connection: When the session is finished, the client and server close the connection by sending termination messages, and the session key is discarded.

By using public key cryptography, strong encryption algorithms, and secure key exchange methods, SSH ensures that data transmitted between the client and server is protected from eavesdropping, tampering, and other forms of cyberattacks.

Security Concerns Of SSH

While SSH is designed to provide a secure channel for communication, there are still some potential security concerns and vulnerabilities that need to be addressed. Here are some key security concerns associated with SSH:

  1. Weak passwords: Using weak passwords for SSH authentication can make the system susceptible to brute force attacks, where an attacker systematically tries different password combinations to gain unauthorized access.
  2. Outdated software: Running outdated versions of SSH software can expose systems to known vulnerabilities and exploits. It’s crucial to keep SSH software up-to-date with the latest security patches.
  3. Public key management: Mismanaging public keys, such as using the same key pair across multiple systems or failing to remove old or compromised keys, can lead to unauthorized access.
  4. Man-in-the-middle (MITM) attacks: Although SSH is designed to prevent MITM attacks by authenticating both the client and the server, an attacker who has compromised the known_hosts file on the client side or the authorized_keys file on the server side can still perform MITM attacks.
  5. Privilege escalation: If an attacker gains access to a low-privileged user account, they might attempt to exploit system vulnerabilities to elevate their privileges and gain full control over the server.
  6. SSH agent forwarding: While agent forwarding can simplify authentication when using multiple SSH connections, it can also expose the private keys to an attacker who has gained access to an intermediate server.
  7. Port forwarding: SSH can be used to create encrypted tunnels for other protocols (e.g., TCP). However, improper configuration or unauthorized use of port forwarding can expose internal services or bypass network security policies.

To mitigate these concerns, system administrators should follow best practices for securing SSH:

  1. Use strong, unique passwords and enable public key authentication to minimize the risk of brute force attacks.
  2. Keep SSH software updated with the latest security patches.
  3. Manage public keys properly by using unique key pairs, removing old or compromised keys, and monitoring access.
  4. Validate server public keys to prevent MITM attacks.
  5. Limit user access and permissions, and monitor the system for any signs of privilege escalation.
  6.  Use SSH agent forwarding judiciously and only when necessary.
  7. Configure port forwarding securely and monitor its usage.

By following these best practices, the security concerns associated with SSH can be significantly reduced, ensuring a more secure communication channel.

Attack Examples using SSH

While specific examples of large-scale SSH-related attacks may not always make headlines, SSH is often exploited by attackers as an entry point or a means to move laterally within a target network. Here are a few examples of cyberattacks in which SSH played a role:

  1. The Sony Pictures Hack (2014): In this high-profile incident, attackers infiltrated the internal network of Sony Pictures Entertainment and stole a vast amount of sensitive data. While the exact method of the initial breach remains unclear, it is widely believed that the attackers utilized SSH to move laterally within the network and gain access to other systems. They also used SSH to exfiltrate stolen data.
  2. The Linux.ProxyM botnet (2017): In this case, attackers exploited SSH servers to create a botnet comprising thousands of Linux devices. The attackers gained unauthorized access to the devices by using a list of common and weak default credentials. Once they had access, the attackers installed the Linux.ProxyM malware, which allowed them to use the compromised devices as proxies for malicious activities, including spamming and hiding their real IP addresses during attacks.
  3. The Ebury malware (2014): This SSH-based malware targeted Linux and Unix systems and was used by cybercriminals to steal SSH login credentials. Once the malware was installed on a system, it intercepted SSH communications and harvested usernames and passwords, giving attackers access to additional systems on the network. The Ebury malware was primarily used to create a large botnet for sending spam emails and performing click fraud operations.

Although these examples do not detail large-scale attacks that solely utilized SSH, they demonstrate that SSH can be a significant component in the overall attack strategy. To prevent such incidents, it is crucial to follow SSH security best practices, monitor SSH traffic, and maintain up-to-date security patches.

WireX Systems NDR can help with SSH Investigation

WireX Systems Ne2ition NDR (Network Detection and Response) is a cybersecurity solution designed to detect and respond to threats and anomalous activities within a network. It uses advanced analytics, machine learning, and artificial intelligence to continuously monitor network traffic and identify potential security incidents. WireX Systems Ne2ition can be helpful in investigating attacks over SSH in several ways:

  1. Traffic analysis: Ne2ition NDR solutions can analyze SSH traffic patterns and detect unusual or unexpected activity, such as multiple failed login attempts, connections from suspicious IP addresses, or unusually high data transfers, which may indicate an ongoing attack or compromise.
  2. Anomaly detection: By creating a baseline of normal SSH activity, Ne2ition can detect deviations from the norm, such as new or unexpected connections, unusual login times, or abnormal user behavior. These anomalies can serve as early warning signs of an attack or intrusion.
  3. Encryption-aware analysis: Ne2ition NDR solutions can analyze encrypted SSH traffic metadata without decrypting the actual data, helping security teams identify potentially malicious activities, such as unauthorized port forwarding or tunneling.
  4. Incident response: Ne2ition solutions can automatically trigger alerts or initiate predefined responses when they detect potential security incidents involving SSH. This allows security teams to quickly react to threats and mitigate any potential damage.
  5. Forensic investigation: WireX Systems Ne2ition NDR solutions can store network traffic metadata and logs, providing valuable information for forensic investigations after an attack. This data can help security teams identify the attacker’s methods, entry points, and lateral movements within the network, as well as any potential data exfiltration.
  6. Threat intelligence: WireX Systems Ne2ition NDR solutions often integrate threat intelligence feeds, which can help identify known malicious IP addresses, domains, or other indicators of compromise (IoCs) related to SSH attacks.
  7. Continuous monitoring: Ne2ition NDR solutions provide continuous, real-time monitoring of network traffic, enabling security teams to detect and respond to threats in a timely manner, reducing the potential impact of an attack.

By leveraging WireX Systems Ne2ition NDR solutions, organizations can gain better visibility into their network activities, identify and respond to SSH-related attacks faster, and improve their overall security posture.

WireX Systems Ne2ition analyzes SSH traffic, extracts and indexes dozens of different attributes including the ones displays below to provide in-depth visibility and context for detection, response, forensics and hunting scenarios over SSH

Client Port Client Software Version Server IP Client  IP
Client: Encryption Algorithm Client: Compression Algorithm S2C Client: MAC Algorithm C2S Client: MAC Algorithm S2C
Client: Protocol Version Server: Protocol Version Compression Algorithm Encryption Algorithm
MAC Algorithm Client: Kex Algorithms Server Software Version Server: Kex Algorithms
Server: Compression Algorithm S2C Server: Encryption Algorithm Server: Compression Algorithm C2S Server: MAC Algorithm C2S
Server: MAC Algorithm S2C Client: Compression Algorithm C2S Server: Server Host Algorithm Client: Server Host Algorithm


These attributes will also help WireX Systems map into the MITRE ATT&CK framework techniques and tactics.


The MITRE ATT&CK framework is a comprehensive, globally accessible knowledge base of cyber adversary tactics and techniques based on real-world observations. When it comes to attacks over SSH, there are several tactics and techniques in the MITRE ATT&CK framework that are relevant. Here’s a list of some of them:

  1. Tactic: Initial Access
    • Technique T1078: Valid Accounts – Attackers may use stolen or compromised SSH credentials (username and password or key pairs) to gain initial access to a target system.
  2. Tactic: Execution
    • Technique T1047: Windows Management Instrumentation (for Windows) or Technique T1059.004: Command and Scripting Interpreter: Unix Shell (for Unix-based systems) – After gaining access through SSH, adversaries can execute commands or scripts on the target system.
  3. Tactic: Persistence
    • Technique T1098: Account Manipulation – Attackers may create new accounts or modify existing accounts on the target system to maintain access via SSH.
  4. Tactic: Privilege Escalation
    • Technique T1078: Valid Accounts – After gaining access through SSH, an attacker may attempt to escalate privileges by exploiting system vulnerabilities or misconfigurations.
  5. Tactic: Defense Evasion
    • Technique T1572: Protocol Tunneling – Attackers may use SSH tunneling to encapsulate malicious traffic or bypass security controls.
    • Technique T1021.001: Remote Services: SSH – Attackers may use SSH to move laterally within a network, evading detection.
  6. Tactic: Credential Access
    • Technique T1552.004: Unsecured Credentials: Private Keys – Attackers may attempt to steal private keys associated with SSH to gain access to additional systems.
  7. Tactic: Lateral Movement
    • Technique T1021.001: Remote Services: SSH – Attackers may use SSH to move laterally within a network after gaining initial access.
  8. Tactic: Command and Control
    • Technique T1572: Protocol Tunneling – Attackers can use SSH tunneling to establish encrypted communication channels for command and control.
  9. Tactic: Exfiltration
    • Technique T1048.003: Exfiltration Over Alternative Protocol: Exfiltration Over Unencrypted/Obfuscated Non-C2 Protocol – Attackers may use SSH to exfiltrate data from the target system, as it is not typically associated with command and control.

These are just some examples of how attacks over SSH can be mapped to the MITRE ATT&CK framework. It is important to note that each attack scenario may involve different tactics and techniques, depending on the attacker’s objectives, tools, and methods.


In conclusion, SSH, or Secure Shell, is a cryptographic network protocol designed for securely accessing and managing network devices, servers, and remote systems over unsecured networks like the internet. It employs a combination of public key cryptography, strong encryption algorithms, and secure key exchange methods to establish a secure communication channel between the client and server.

Despite its robust security features, there are limitations and security concerns associated with SSH. These include weak passwords, outdated software, public key management issues, man-in-the-middle attacks, privilege escalation, SSH agent forwarding, and port forwarding. To mitigate these concerns, it is crucial for system administrators to follow best practices for securing SSH, such as using strong and unique passwords, keeping software updated, managing public keys properly, and monitoring SSH traffic.

While SSH is not immune to exploitation, it remains a critical tool for secure remote access and administration. By understanding the underlying principles of SSH, its potential vulnerabilities, and implementing necessary security measures, organizations can benefit from the secure communication capabilities that SSH provides while minimizing the risk of cyberattacks.

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