Rocky Linux 9 - CVE-2024-6387: regreSSHion

Neil Hanlon

Update - 2024-07-03 17:00 UTC

Today, RHSA-2024:4312 was released, addressing CVE-2024-6387. The update has been rebuilt for Rocky Linux and is available as openssh-8.7p1-38.el9_4.1 on dl.rockylinux.org. It is currently propagating to the mirror network.

If you have the SIG/Security openssh package installed, it will be upgraded to this new BaseOS package on your next dnf upgrade. You can confirm the installed version of openssh with rpm -q openssh.

If you have applied the LoginGraceTime mitigation, it is recommended to revert this change after upgrading to a patched openssh, and restart sshd again.

The original announcement is below:

CVE-2024-6387: OpenSSH Vulnerability in Rocky Linux 9

A critical vulnerability, identified as CVE-2024-6387, affects OpenSSH server (sshd) on all Enterprise Linux 9 systems (including Rocky Linux 9). This issue involves a signal handler race condition that can lead to a potential remote code execution.

Details

A client failing to authenticate within the LoginGraceTime (120 seconds by default) triggers sshd's SIGALRM handler, which calls non async-signal-safe functions like syslog(). This vulnerability does not affect versions shipped with Enterprise Linux 8, as the problematic code was introduced in later upstream releases.

Risk Potential

While remote code execution is possible, it requires a complex and time-consuming race condition. Most attacks would more likely result in the sshd service crashing. High volumes of connections might be detectable via network monitoring.

Mitigation

To address this issue, you can either update to openssh-8.7p1-38.el9_4.security.0.5 from the SIG/Security repository or configure your ssh server to reduce the LoginGraceTime parameter.

Mitigation using SIG/Security OpenSSH Package

For the SIG/Security OpenSSH package, you may follow these instructions. Please make sure to read the information about this package, including other changes besides this CVE fix, before using this package. Notably, SIG/Security openssh is built without Kerberos authentication support, so care should be taken to ensure this package fits your use case--if not, use the configuration mitigation procedure below.

In addition, SIG/Security's repositories contain additional packages which override the base distribution's: glibc and microcode_ctl. While these packages' changes should be transparent to the system and have been tested, you should review the information about these packages and their specific changes before installing them.

The instructions below will disable the security-common repository so that only openssh from sig-security will be used.

The release package can be installed on other Enterprise Linux distributions. See sig-security wiki for more information.

  1. Install the SIG/Security release file
dnf install rocky-release-security
  1. Disable SIG/Security security-common repo
dnf config-manager --disable security-common
  1. Upgrade openssh
dnf --enablerepo=security-common update openssh\*
  1. Confirm version openssh-8.7p1-38.el9_4.security.0.5 is installed
rpm -q openssh

During the installation of openssh, the service will be automatically restarted.

Mitigation using LoginGraceTime configuration

If you cannot use the SIG/Security override package for any reason, you may apply a configuration mitigation. Note that setting LoginGraceTime to 0 mitigates remote code execution risks but makes the SSH service more susceptible to DoS attacks.

  1. As root, open /etc/ssh/sshd_config.
  2. Add or modify the LoginGraceTime parameter: 
    LoginGraceTime 0
  3. Save and close the file.
  4. Restart the sshd service: 
    systemctl restart sshd.service

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