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The OpenSSH vulnerability described in this headline is CVE-2024-6387, also called regreSSHion: a race condition in the server daemon sshd that may permit remote code execution with root privileges. Qualys researchers developed a working exploit and demonstrated it privately to the OpenSSH project; that does not mean Qualys published exploit code or that attacks are confirmed to be happening in the wild. The upstream fix is OpenSSH 9.8p1, released on July 1, 2024. Administrators should install the corresponding security update from their operating-system or product supplier.
What is CVE-2024-6387?
RegreSSHion is a race-condition vulnerability in OpenSSH’s server daemon, sshd, not a general weakness in the SSH protocol. OpenSSH says the flaw may allow arbitrary code execution with root privileges. It was disclosed on July 1, 2024, and fixed in OpenSSH 9.8p1. OpenSSH 9.8 release notes
The issue concerns how the server handles a signal during a particular unauthenticated connection timeout. Exploiting a race condition requires carefully timed events, so the demonstrated attack was difficult in the tested environment. Difficulty is not the same as safety: an exposed, unpatched server should still be updated.
Which OpenSSH versions and systems are affected?
The main regression affects Portable OpenSSH versions 8.5p1 through 9.7p1 inclusive. The upstream fix is 9.8p1. A version string alone may not settle exposure: operating-system vendors sometimes backport security fixes without changing to the upstream release number. Check the security status of the actual package with its supplier. OpenSSH release notes NCSC-IE security advisory
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| System or version | What the cited sources establish |
|---|---|
| Portable OpenSSH 8.5p1–9.7p1 | OpenSSH identifies these versions, inclusive, as affected by the regression. |
| OpenSSH 9.8p1 | Contains the upstream security correction. |
| Versions earlier than 4.4p1 | Qualys says these may be affected by the related historical signal-handler issue unless patched for CVE-2006-5051 and CVE-2008-4109. |
| Versions 4.4p1–8.4p1 | Qualys describes these as not affected by this regression. |
| OpenBSD | OpenSSH says it is not vulnerable. |
| Windows and macOS | Qualys says Windows installations are not vulnerable; its FAQ describes macOS exploitability as uncertain. Check the relevant product or vendor advisory for a platform-specific conclusion. |
OpenSSH documented successful exploitation in a 32-bit Linux/glibc laboratory setup with ASLR enabled. Its release note said 64-bit exploitation was believed possible but had not been demonstrated, and that non-glibc systems had not been examined. NCSC-IE also frames the remote-code-execution concern around glibc-based Linux systems. These are qualifications about the disclosed testing, not a guarantee that other configurations are safe. OpenSSH release notes NCSC-IE security advisory
What did researchers demonstrate?
Qualys’s Threat Research Unit says it developed a working exploit and demonstrated it to the OpenSSH team during responsible disclosure. Qualys also says it did not release the exploit. OpenSSH’s release notes thank Qualys for discovering and reporting the issue and demonstrating exploitability. The available evidence supports researcher-developed, privately demonstrated exploitability; it does not establish public availability of that exploit or current exploitation in the wild. Qualys regreSSHion advisory OpenSSH release notes
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In the described lab conditions, OpenSSH reported an average of 6–8 hours of continuous connections to achieve exploitation, up to the server’s accepted connection maximum. This figure applies to the demonstrated 32-bit Linux/glibc setup; it is not a general estimate of how long an attack would take on a different system.
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How should administrators fix or mitigate it?
Install the supplier’s security update
- Inventory systems that run OpenSSH, including servers and appliances that may bundle it.
- Check the operating-system or product vendor’s advisory and package status for CVE-2024-6387. A downstream package can include a backported fix even when its version string is older than 9.8p1.
- Install the supported corrective update from that supplier and follow its restart or service-reload instructions.
- Verify the installed package or vendor’s security status after updating. For systems that cannot be identified or updated promptly, prioritize those reachable from untrusted networks and escalate through the supplier’s support channel.
NCSC-IE advises organizations to identify OpenSSH systems, determine whether their versions are vulnerable, and install the supplier’s corrective update. The authoritative remediation path is the package or product supplier’s advisory, not an assumption based solely on an upstream version number. NCSC-IE security advisory
Use LoginGraceTime 0 only as a temporary mitigation
If an update or recompilation cannot be done immediately, OpenSSH and Qualys describe setting LoginGraceTime 0 in the server configuration as a temporary way to prevent the remote-code-execution risk described in the advisory. This changes the denial-of-service risk: unauthenticated connections can occupy the available MaxStartups slots, making it easier to exhaust the server’s capacity for new unauthenticated connections. Use this only with an operational plan, and replace it with the supplier’s fix as soon as possible. OpenSSH release notes Qualys regreSSHion advisory
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Can logs or exposure estimates confirm an attack?
Qualys says repeated “Timeout before authentication” messages in logs can be a clue that exploitation was attempted. The message is not conclusive: a log entry alone does not prove that an attempt exploited the flaw, establish that the server was compromised, or show that the server is safe. Use the logs as an investigative lead alongside your organization’s incident-response process, system telemetry, and vendor guidance. Qualys regreSSHion advisory
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