Understanding the ‘regreSSHion’ OpenSSH Vulnerability (CVE-2024-6387)

Source: Qualys report on CVE-2024-6387

Cybersecurity researchers from the Qualys Threat Research Unit (TRU) have uncovered a critical flaw in OpenSSH, dubbed ‘regreSSHion’ (CVE-2024-6387), marking a significant threat to the security of Linux-based systems worldwide. This article provides an in-depth exploration of the technical intricacies, impact assessment, and recommended mitigation strategies concerning this vulnerability.

Understanding ‘regreSSHion’

‘RegreSSHion’ is classified as an unauthenticated remote code execution (RCE) vulnerability within OpenSSH’s server (sshd) on glibc-based Linux systems. This flaw allows attackers to exploit a signal handler race condition in sshd, triggered when a client fails to authenticate within the specified LoginGraceTime (typically 120 seconds, or 600 in older versions). Upon expiration, sshd’s SIGALRM handler asynchronously invokes functions like syslog(), which are not async-signal-safe. This asynchronous invocation creates a window of opportunity for attackers to inject malicious code and potentially gain full root access to the affected system.

Historical Context and Regression

The term ‘regreSSHion’ derives from its nature as a regression bug, reintroducing a vulnerability (CVE-2006-5051) that was previously patched. This regression occurred due to changes or updates in OpenSSH’s codebase, inadvertently undoing prior security fixes. Such regressions underscore the challenges in maintaining secure software development practices over time, highlighting the critical need for thorough regression testing and ongoing security scrutiny.

Impact Assessment

Qualys’ research estimates that over 14 million OpenSSH server instances are potentially vulnerable, with approximately 700,000 exposed directly to the internet. The severity of ‘regreSSHion’ lies in its ability to grant unauthenticated attackers full root privileges, enabling them to execute arbitrary commands, install malware, manipulate data, and establish persistent backdoors. This capability poses significant risks to both enterprise environments and individual users, potentially leading to widespread system compromise and unauthorized access.

Exploit Scenario

An attacker can exploit this vulnerability by sending a carefully crafted request to the vulnerable OpenSSH server. By causing a buffer overflow through improper input validation, the attacker can manipulate memory contents and potentially execute arbitrary code with the privileges of the sshd process, typically running with elevated permissions.

CVSS v3 Vector Breakdown:

  • Attack Vector (AV): Network (AV) – Attackers can exploit the vulnerability remotely.
  • Attack Complexity (AC): High (AC) – The exploit requires conditions beyond attacker control, such as timing and system configuration.
  • Privileges Required (PR): None (PR) – The vulnerability can be exploited without needing privileges.
  • User Interaction (UI): None (UI) – Exploitation does not require user interaction.
  • Scope (S): Unchanged (S) – The vulnerability’s impact is limited to the vulnerable system.
  • Confidentiality (C): High (C), Integrity (I): High (I), Availability (A): High (A) – Successful exploitation can result in full compromise of confidentiality, integrity, and availability.

Impact Assessment

  • Base Score: 8.1 (High) – Calculated severity based on CVSS v3 metrics, reflecting the potential for severe system compromise and data breaches.
  • Vulnerable OpenSSH versions include those prior to 4.4p1 and from 8.5p1 up to, but not including, 9.8p1. Linux-based systems utilizing these versions are particularly at risk.

Industry Response

The cybersecurity community has responded swiftly with advisories urging immediate patching and implementation of mitigating controls. Organizations and vendors have issued alerts, emphasizing the criticality of addressing this vulnerability due to its potential widespread impact.

Mitigation and Remediation

Patch Management:

  • Organizations are strongly advised to update affected OpenSSH installations to version 9.8 or later, which includes fixes for regreSSHion. Patching closes the vulnerability by addressing the improper input validation and signal handling issues.

Access Control and Monitoring:

  • Implement strict access controls, limiting SSH access to trusted networks and users.
  • Deploy intrusion detection systems (IDS) to monitor for signs of exploitation attempts and anomalous SSH activity.
  • Disable password-based logins where possible and enforce key-based authentication for enhanced security.


The emergence of ‘regreSSHion’ highlights the ongoing challenge of securing critical infrastructure against evolving cybersecurity threats. Despite the robustness of OpenSSH as a secure networking utility, vulnerabilities like CVE-2024-6387 underscore the necessity for continuous vigilance and proactive mitigation strategies. By staying informed, promptly applying patches, and implementing layered security measures, organizations can effectively protect their systems from potential exploitation and safeguard sensitive data.

References and Additional Resources

About OpenSSH

OpenSSH continues to play a pivotal role in enabling secure communication across Unix-like systems. It remains a cornerstone of secure network management, providing robust encryption and authentication mechanisms essential for maintaining confidentiality and integrity in network operations globally. Despite vulnerabilities like ‘regreSSHion,’ OpenSSH’s commitment to security and ongoing community support underscores its critical importance in modern cybersecurity practices.

FAQ: regreSSHion (CVE-2024-6387) Remote Code Execution Vulnerability in OpenSSH

What is regreSSHion (CVE-2024-6387)?

regreSSHion, CVE-2024-6387, is a critical remote code execution vulnerability discovered in OpenSSH’s server (sshd) on glibc-based Linux systems. It allows remote attackers to execute arbitrary code without authentication, potentially leading to system compromise.

How does regreSSHion (CVE-2024-6387) impact OpenSSH users?

regreSSHion affects OpenSSH versions prior to 4.4p1 and versions from 8.5p1 up to, but not including, 9.8p1. Systems running these versions are vulnerable to exploitation if exposed to untrusted networks or the internet, posing significant risks to confidentiality, integrity, and availability.

Has regreSSHion (CVE-2024-6387) been exploited in the wild?

As of the latest reports, there have been no confirmed instances of active exploitation in the wild. However, the nature of the vulnerability and the widespread use of OpenSSH necessitate immediate action to mitigate potential risks.

How can organizations protect themselves against regreSSHion (CVE-2024-6387)?

  • Patch Management: Update affected OpenSSH versions to 9.8 or later, which includes fixes for regreSSHion.
  • Access Control: Limit SSH access to trusted networks and users. Implement strong authentication methods such as key-based authentication.
  • Monitoring: Regularly monitor SSH access logs for unusual activity and deploy intrusion detection systems (IDS) to detect exploitation attempts.

What should I do if I suspect my system is vulnerable to regreSSHion (CVE-2024-6387)?

Immediately apply the latest patches provided by OpenSSH. If patching is not feasible immediately, consider implementing temporary mitigations such as setting LoginGraceTime to 0 in the OpenSSH configuration file to reduce the risk of exploitation.

How Can Netizen Help?

Netizen ensures that security gets built-in and not bolted-on. Providing advanced solutions to protect critical IT infrastructure such as the popular “CISO-as-a-Service” wherein companies can leverage the expertise of executive-level cybersecurity professionals without having to bear the cost of employing them full time. 

We also offer compliance support, vulnerability assessments, penetration testing, and more security-related services for businesses of any size and type. 

Additionally, Netizen offers an automated and affordable assessment tool that continuously scans systems, websites, applications, and networks to uncover issues. Vulnerability data is then securely analyzed and presented through an easy-to-interpret dashboard to yield actionable risk and compliance information for audiences ranging from IT professionals to executive managers.

Netizen is an ISO 27001:2013 (Information Security Management), ISO 9001:2015, and CMMI V 2.0 Level 3 certified company. We are a proud Service-Disabled Veteran-Owned Small Business that is recognized by the U.S. Department of Labor for hiring and retention of military veterans. 

Questions or concerns? Feel free to reach out to us any time –

Copyright © Netizen Corporation. All Rights Reserved.