CVE-2024-6409 – A race condition vulnerability in OpenSSH's ssh... (How to Fix)

Q: What is the severity of CVE-2024-6409?

CVE-2024-6409 has a CVSS score of 7.0 (HIGH). A race condition vulnerability in OpenSSH's sshd server allows remote attackers to potentially execute code as an unprivileged user. The vulnerability occurs when SIGALRM signals are handled asynchronously during authentication timeouts, calling non-async-signal-safe functions like syslog(). This affects systems running vulnerable versions of OpenSSH server.

📋 TL;DR

A race condition vulnerability in OpenSSH's sshd server allows remote attackers to potentially execute code as an unprivileged user. The vulnerability occurs when SIGALRM signals are handled asynchronously during authentication timeouts, calling non-async-signal-safe functions like syslog(). This affects systems running vulnerable versions of OpenSSH server.

💻 Affected Systems

Products:

OpenSSH

Versions: Specific versions not provided in CVE description, but Red Hat advisories indicate affected versions in RHEL distributions.

Operating Systems: Linux distributions using vulnerable OpenSSH versions, particularly RHEL-based systems

Default Config Vulnerable: ⚠️ Yes

Notes: Affects sshd server configurations where authentication timeout handling is enabled (default).

⚠️ Manual Verification Required

This CVE does not have specific version information in our database, so automatic vulnerability detection cannot determine if your system is affected.

Why? The CVE database entry doesn't specify which versions are vulnerable (no version ranges provided by the vendor/NVD).

🔒 Custom verification scripts are available for registered users. Sign up free to download automated test scripts.

Recommended Actions:

Review the CVE details at NVD
Check vendor security advisories for your specific version
Test if the vulnerability is exploitable in your environment
Consider updating to the latest version as a precaution

⚠️ Risk & Real-World Impact

🔴

Worst Case

Remote code execution as the unprivileged user running sshd, potentially leading to system compromise.

🟠

Likely Case

Service disruption or denial of service due to memory corruption or crashes from improper signal handling.

🟢

If Mitigated

Limited impact if proper network segmentation and authentication controls prevent unauthorized access attempts.

🌐 Internet-Facing: HIGH - SSH servers exposed to the internet are directly vulnerable to remote exploitation attempts.

🏢 Internal Only: MEDIUM - Internal SSH servers are still vulnerable but require attacker access to internal networks.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: HIGH - Race conditions require precise timing and conditions to exploit successfully.

Exploitation requires triggering the SIGALRM handler during authentication timeout without successful authentication.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Check specific Red Hat advisories for patched versions (RHSA-2024:4457, RHSA-2024:4613, etc.)

Vendor Advisory: https://access.redhat.com/errata/RHSA-2024:4457

Restart Required: Yes

Instructions:

1. Update OpenSSH packages using your distribution's package manager. 2. For RHEL: 'yum update openssh' or 'dnf update openssh'. 3. Restart sshd service: 'systemctl restart sshd'.

🔧 Temporary Workarounds

Reduce authentication timeout

linux

Shorten LoginGraceTime to minimize window for race condition exploitation

Edit /etc/ssh/sshd_config
Set LoginGraceTime to 30s or lower
Restart sshd: systemctl restart sshd

Disable password authentication

linux

Require key-based authentication only to reduce attack surface

Edit /etc/ssh/sshd_config
Set PasswordAuthentication no
Restart sshd: systemctl restart sshd

🧯 If You Can't Patch

Implement network controls to restrict SSH access to trusted sources only
Monitor for authentication timeout events and failed login attempts

🔍 How to Verify

Check if Vulnerable:

Check OpenSSH version: 'ssh -V' and compare with patched versions in Red Hat advisories

Check Version:

ssh -V 2>&1 | head -1

Verify Fix Applied:

Verify updated OpenSSH version and check that authentication timeout handling functions properly

📡 Detection & Monitoring

Log Indicators:

Multiple authentication timeout events
Unexpected sshd crashes or restarts
Failed login attempts followed by service disruption

Network Indicators:

Multiple SSH connection attempts without successful authentication
Unusual timing patterns in SSH traffic

SIEM Query:

source="sshd" AND ("authentication timeout" OR "Connection closed by authenticating user" OR "Received signal")

📊 Metadata

CVE ID: CVE-2024-6409

CVSS v3 Score: 7.0 (HIGH)

CVSS Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H

CWE: CWE-364

Published: July 8, 2024

Last Updated: November 21, 2024

🔗 References

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