OpenSSH regreSSHion (CVE-2024-6387): A Blast from the Past with Critical Repercussions

OpenSSH, a crucial tool in secure communications, has recently been impacted by a critical vulnerability identified as CVE-2024-6387, also known as “regreSSHion.” This blog will provide an overview of the vulnerability, its exploitation methods, and strategies for mitigation. 

Introduction to OpenSSH

OpenSSH (Open Secure Shell) is a suite of secure networking utilities based on the Secure Shell (SSH) protocol. It is widely used for remote login, secure file transfer, and network administration. Its importance in maintaining secure communication channels over unsecured networks cannot be overstated, making any vulnerability in OpenSSH a significant concern.

Understanding CVE-2024-6387: regreSSHion

What is CVE-2024-6387?

CVE-2024-6387 is a vulnerability found in OpenSSH versions 8.8 and later. This flaw arises from improper handling of user authentication credentials, potentially leading to remote code execution (RCE). An attacker can exploit this vulnerability by sending specially crafted requests to the OpenSSH server, allowing them to execute arbitrary code with the same privileges as the SSH service.

Here’s the Python script to check if the OpenSSH version on the target is vulnerable to CVE-2024-6387. 

import socket

vulnerable_versions = [
    'SSH-2.0-OpenSSH_8.5p1',
    'SSH-2.0-OpenSSH_8.6p1',
    'SSH-2.0-OpenSSH_8.7p1',
    'SSH-2.0-OpenSSH_8.8p1',
    'SSH-2.0-OpenSSH_8.9p1',
    'SSH-2.0-OpenSSH_9.0p1',
    'SSH-2.0-OpenSSH_9.1p1',
    'SSH-2.0-OpenSSH_9.2p1',
    'SSH-2.0-OpenSSH_9.3p1',
    'SSH-2.0-OpenSSH_9.4p1',
    'SSH-2.0-OpenSSH_9.5p1',
    'SSH-2.0-OpenSSH_9.6p1',
    'SSH-2.0-OpenSSH_9.7p1'
]


def check_vulnerability(ip, port=22):
    try:
        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
            s.settimeout(5)
            s.connect((ip, port))
            s.sendall(b'SSH-2.0-Test\r\n')
            response = s.recv(1024).decode('utf-8')

            for version in vulnerable_versions:
                if version in response:
                    print(f'Target {ip}:{
                          port} is vulnerable with version {version}')
                    return
            print(f'Target {ip}:{port} is not vulnerable.')

    except socket.timeout:
        print(f'Connection to {ip}:{port} timed out.')
    except Exception as e:
        print(f'An error occurred: {e}')


if __name__ == '__main__':
    import argparse

    parser = argparse.ArgumentParser(
        description='Check if OpenSSH version is vulnerable to CVE-2024-6387')
    parser.add_argument('ip', help='Target IP address')
    parser.add_argument('--port', type=int, default=22,
                        help='Target port (default is 22)')

    args = parser.parse_args()
    check_vulnerability(args.ip, args.port)

To use this script, save it to a file (e.g., check_openssh_vulnerability.py ) and run it from the command line:

python check_openssh_vulnerability.py --port

Replace <target_ip> with the IP address of the target and with the port number if it’s different from the default port 22. If the port is 22, you can omit the –port argument

Technical Details

The regreSSHion vulnerability is rooted in a logic flaw within the authentication module of OpenSSH. Specifically, the issue lies in the auth-passwd.c file, where user-supplied input is not properly sanitized before processing. This can result in a buffer overflow, enabling an attacker to overwrite critical memory segments and execute arbitrary code.

Here is a simplified representation of the vulnerable code:

int auth_password(struct ssh *ssh, const char *password) {
    char buf[256];
    if (strlen(password) >= sizeof(buf)) {
        log("Password too long");
        return 0;
    }
    strcpy(buf, password);  // Vulnerable line
    // Further processing
    return 1;
}

In this example, the strcpy function is used without appropriate bounds checking, making it susceptible to buffer overflow attacks.

Why is regreSSHion concerning?

The critical nature of regreSSHion lies in its potential impact. Here’s why it demands immediate attention:

Unauthenticated Access: Unlike traditional attacks requiring valid credentials, regreSSHion allows attackers to bypass authentication altogether. This significantly expands the attack surface for potential exploitation.

Root Privilege Escalation: Successful exploitation grants root access, the highest level of control within a Linux system. This empowers attackers to manipulate files, install malware, and establish persistence for long-term access.

Widespread Impact: glibc-based Linux systems, encompassing a vast majority of Linux distributions, are vulnerable. This broadens the potential attack landscape.

A Regressed Vulnerability: A Blast from the Past

The discovery of regreSSHion is a stark reminder that vulnerabilities can resurface even in well-established software. This specific race condition was supposedly fixed in OpenSSH versions released in 2006. Its reappearance highlights the importance of ongoing vigilance and proactive security measures.

Indicators of Compromise (IOCs) – Potential Signs of Exploitation

While definitive IOCs for regreSSHion exploits might not be readily available yet, here are some general indicators that could suggest suspicious activity:

• Unusual SSH Login Attempts: A surge in failed SSH login attempts, particularly from unknown IP addresses, could warrant investigation.
• Unexpected Processes: The presence of unauthorized or unidentified processes running on the system, especially with root privileges, is a red flag.
• File Modifications: Unexplained changes to system files, particularly those related to SSH configuration or user accounts, could indicate tampering.

Exploitation Challenges

While the potential consequences are severe, exploiting regreSSHion in the real world presents challenges:

• Technical Complexity: Crafting a successful exploit requires significant technical expertise and knowledge of the target system’s specific glibc version.
• Time-Consuming: Launching a large-scale attack would be time-intensive, requiring numerous authentication attempts (approximately 10,000 on average) per server.

Countermeasures: Many systems employ brute-force protection and denial-of-service (DoS) mitigation mechanisms that can hinder exploitation attempts.

Exploitation of regreSSHion

Prerequisites for Exploitation

To exploit the regreSSHion vulnerability, an attacker needs:

1. Network access to the OpenSSH server.
2. Knowledge of a valid username on the target system.
3. A specially crafted payload designed to trigger the buffer overflow and achieve code execution.

Steps to Exploit

1. Identify a Target: The attacker scans for systems running vulnerable versions of OpenSSH (8.8 and later).
2. Craft Payload: The attacker creates a malicious payload that, when sent to the target, causes a buffer overflow and executes arbitrary code.
3. Send Payload: The attacker sends the crafted payload to the OpenSSH server during the authentication phase.
4. Gain Access: If successful, the attacker gains the same privileges as the SSH service, which could be root or another privileged user.

Example Exploit Code

Below is a conceptual example of how an attacker might exploit this vulnerability:

import socket

def exploit(target_ip, target_port, username, payload):
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.connect((target_ip, target_port))

    # Send SSH banner
    s.send(b"SSH-2.0-OpenSSH_8.8\r\n")

    # Receive server's banner
    s.recv(1024)

    # Send crafted payload during authentication
    s.send(b"SSH-2.0-OpenSSH_8.8\r\n")
    s.send(f"{username}\0{payload}".encode())

    # Receive response (if any)
    response = s.recv(1024)
    print(response)

    s.close()

# Usage
exploit("192.168.1.10", 22, "user", "A" * 300)  # Example payload

Real-World Impact

Exploitation of CVE-2024-6387 can lead to severe consequences, including:

• Unauthorized access to sensitive information.
• Installation of malware or backdoors.
• Disruption of services.
• Potential lateral movement within the network.

Mitigation Strategies

Immediate Actions

1. Update OpenSSH: The most effective way to address this vulnerability is to upgrade OpenSSH to the latest version where this vulnerability has been resolved. OpenSSH 9.1 includes a fix for CVE-2024-6387.
2. Network Segmentation: Restrict access to the OpenSSH server by implementing network segmentation and access controls. Only allow trusted IP addresses to connect to the SSH service.
3. Strong Authentication: Implement multi-factor authentication (MFA) to add an additional layer of security beyond just username and password.
4. Monitor Logs: Regularly monitor authentication logs for unusual or suspicious activity, such as repeated login attempts or failed authentication requests.

Long-Term Strategies

1. Code Review and Testing: Conduct thorough code reviews and security testing for all deployed software to identify and remediate vulnerabilities.
2. Use Security Tools: Deploy security tools such as intrusion detection systems (IDS) and intrusion prevention systems (IPS) to detect and block malicious activities.
3. User Education: Educate users on the importance of using strong, unique passwords and recognizing phishing attempts that could lead to credential theft.

Example Mitigation Configuration

Here is an example configuration for OpenSSH that enhances security:

# /etc/ssh/sshd_config

# Disable password authentication
PasswordAuthentication no

# Allow only specific users to connect via SSH
AllowUsers admin

# Enable public key authentication
PubkeyAuthentication yes

# Specify the location of authorized keys
AuthorizedKeysFile .ssh/authorized_keys

# Enable logging
LogLevel VERBOSE

# Restrict root login
PermitRootLogin no

# Enable multi-factor authentication (if supported)
AuthenticationMethods publickey,keyboard-interactive

Strobes ASM Response

At Strobes, we prioritize the security of our customers. Our Asset Security Management (ASM) system, Strobes ASM, has promptly detected the regreSSHion vulnerability (CVE-2024-6387) and has alerted all our customers about the vulnerable assets in a timely manner. This proactive approach ensures that our clients are informed and can take immediate actions to mitigate risks. 

For proactive and automated security solutions, subscribe to Strobes ASM.

Conclusion

The discovery of CVE-2024-6387, or regreSSHion, underscores the importance of maintaining robust security practices. As OpenSSH continues to be a critical component in secure communications, it is vital for organizations to stay vigilant, apply updates, and employ strong security measures to protect their infrastructure.

Understanding the nature of this vulnerability, the methods of exploitation, and the steps to mitigate such threats helps organizations safeguard their systems. Regular updates, strong authentication mechanisms, and proactive security measures are key to maintaining a secure environment in the face of evolving cyber threats. By taking these steps, organizations can minimize the risk posed by vulnerabilities like regreSSHion and ensure the continued security of their systems.

For detailed insights, explore the Strobes Vulnerability Intel Portal and stay ahead of potential threats!