On July 1, 2024, the Qualys Threat Research Unit (TRU) disclosed CVE-2024-6387, a critical remote code execution (RCE) vulnerability affecting OpenSSH servers running on glibc-based Linux systems. What makes this vulnerability particularly concerning is that it isn’t new—it’s a case of regression, where an issue that was originally patched back in 2006 has been unintentionally reintroduced in later software updates. This regression, now known as RegreSSHion, has put millions of systems at risk once again.

OpenSSH is one of the most widely used tools for secure data transfer and remote access across businesses, cloud platforms, and critical infrastructure. The impact of a vulnerability like this extends far beyond individual systems. Exploitation is already happening, with reports confirming that attackers are actively scanning for vulnerable servers, sharing exploit tools, and leveraging this weakness to gain unauthorised access. If OpenSSH is part of your infrastructure, now is the time to act.

How RegreSSHion Became a Security Threat Again

The original vulnerability in OpenSSH was discovered and patched in 2006. However, due to the complexity of software development and maintenance, the fix was unintentionally undone in a later update. This highlights an ongoing challenge in cybersecurity: regression testing failures can bring back old vulnerabilities, exposing modern systems to threats that were thought to be resolved years ago.

RegreSSHion is particularly dangerous because it allows unauthenticated remote code execution, meaning an attacker can gain full control over a system without requiring credentials or user interaction. Unlike many cyberattacks that rely on social engineering, phishing, or credential theft, this exploit only requires an unpatched system. If attackers can locate a vulnerable OpenSSH server, they can take control remotely.

The widespread use of OpenSSH makes this even more alarming. From corporate networks to cloud services, OpenSSH is a foundational technology that facilitates secure communication, remote administration, and automated data transfers. A single compromised system can serve as an entry point for a larger network attack, leading to data breaches, ransomware infections, or persistent unauthorised access.

Reports from security researchers confirm that threat actors are already exploiting this vulnerability. Dark web discussions and underground forums have surfaced where attackers are actively sharing lists of affected systems and exploit tools. Automated scanners are being deployed to identify and compromise unpatched servers at scale, making the situation urgent for organisations that have yet to apply security updates.

Additional OpenSSH Vulnerabilities Discovered

RegreSSHion isn’t the only security concern affecting OpenSSH. In early 2025, researchers identified two additional vulnerabilities that further complicate the security landscape. CVE-2025-26465 introduces a Man-in-the-Middle (MitM) risk, allowing attackers to impersonate legitimate servers and intercept sensitive SSH communications. This flaw affects OpenSSH versions 6.8p1 to 9.9p1 and poses a significant risk to organisations relying on SSH for remote administration.

Meanwhile, CVE-2025-26466 exposes both OpenSSH clients and servers to a pre-authentication denial-of-service (DoS) attack, which can cause excessive CPU and memory consumption, leading to system slowdowns or outages. This vulnerability, affecting OpenSSH versions 9.5p1 to 9.9p1, has the potential to disrupt operations, particularly for organisations that depend on high-availability systems.

Both of these vulnerabilities have been patched in OpenSSH version 9.9p2, which is now the recommended update to ensure comprehensive protection.

Who Is at Risk?

Organisations running vulnerable versions of OpenSSH should prioritise updating immediately. Systems running versions 8.5p1 up to (but not including) 9.8p1 are at risk, as well as any system running a version earlier than 4.4p1, unless they have been patched for CVE-2006-5051 and CVE-2008-4109. With mass exploitation already underway, any delay in applying security updates increases the likelihood of compromise.

For businesses that rely on OpenSSH but are unsure of their exposure, now is the time to conduct a security assessment. Given how attackers are automating their efforts to find and exploit vulnerable systems, it is not a matter of if but when an unpatched server will be targeted.

How to Mitigate the Risk

The best course of action is to update OpenSSH to version 9.9p2, which contains the necessary patches to address RegreSSHion and the recently discovered vulnerabilities. For organisations that cannot immediately update, temporary mitigations include disabling async-signal-unsafe code in log.c:sshsigdie, restricting SSH access to trusted IP addresses only, and enforcing multi-factor authentication (MFA) for SSH logins.

Monitoring system logs is also critical at this stage. With active exploitation confirmed, organisations should review SSH access logs for unusual activity, such as unexpected login attempts or connections from unfamiliar sources. Identifying early signs of an attack can help security teams respond before a full-scale breach occurs.

Why This Matters

RegreSSHion is not just another vulnerability—it is a direct reminder of how past security flaws can resurface unexpectedly and put entire infrastructures at risk. The fact that this flaw was patched 18 years ago but has now become an active threat again highlights the importance of continuous security monitoring and timely updates.

This is not just a risk for a few niche systems; OpenSSH is a core component of modern IT infrastructure. The widespread nature of this vulnerability means that organisations of all sizes are potential targets. If left unpatched, attackers could use this weakness to steal data, install malware, or establish long-term access to critical systems.

With the rise of automated exploitation tools, cybercriminals no longer need to manually search for vulnerable systems—they can simply run scripts that scan the internet for exposed OpenSSH instances. The longer a system remains unpatched, the higher the risk of compromise.

What Organisations Should Do Now

If your business relies on OpenSSH, the time to act is now. Updating to version 9.9p2 is the most effective way to mitigate the risk, but for those unable to update immediately, applying temporary mitigations and conducting a security audit should be the next steps. Organisations should also proactively monitor for signs of exploitation, as attackers are moving quickly to take advantage of unpatched systems.

For those unsure of where to start, seeking expert guidance can make a significant difference. At 59 Degrees North, we help businesses assess vulnerabilities, implement security controls, and strengthen their defenses against emerging threats. If you want to ensure your infrastructure is secure, reach out to us today for an assessment.

With the RegreSSHion vulnerability already being exploited in the wild, organisations cannot afford to delay action. The security of your systems depends on what you do next.