CVE-2024-39894: OpenSSH Timing Attack Vulnerability

CVE-2024-39894 Overview

OpenSSH 9.5 through 9.7 before 9.8 sometimes allows timing attacks against echo-off password entry (e.g., for su and Sudo) because of an ObscureKeystrokeTiming logic error. Similarly, other timing attacks against keystroke entry could occur.

Critical Impact

This vulnerability allows potential attackers to perform timing attacks during password entry processes, exploiting a logic error in keystroke timing.

Affected Products

• OpenSSH 9.5
• OpenSSH 9.6
• OpenSSH 9.7

Discovery Timeline

• Not Available - Vulnerability discovered by Not Available
• Not Available - Responsible disclosure to Not Available
• Not Available - CVE CVE-2024-39894 assigned
• Not Available - Not Available releases security patch
• 2024-07-02 - CVE CVE-2024-39894 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2024-39894

Vulnerability Analysis

The vulnerability in OpenSSH versions 9.5 to 9.7 allows for keystroke timing attacks due to a flawed logic in the ObscureKeystrokeTiming implementation. This vulnerability is particularly concerning as it can be exploited over the network without prior authentication, allowing attackers to infer information from timing discrepancies during password entry.

Root Cause

The root cause of this vulnerability lies in a logic error within the function handling keystroke timings, allowing attackers to measure time differences and gain unauthorized access.

Attack Vector

Network-based attack, allowing a remote attacker to capitalize on timing discrepancies in password input.

// Example exploitation code (sanitized)
#include <time.h>

int main() {
    clock_t start, end;
    double cpu_time_used;
    
    start = clock();
    // Simulate password entry
    printf("Enter password: ");
    // Timing measurement of password entry
    end = clock();
    cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;
    printf("Time used: %f\n", cpu_time_used);
    return 0;
}

Detection Methods for CVE-2024-39894

Indicators of Compromise

• Unusual login times
• Multiple failed login attempts
• Sudden increase in CPU usage during authentication

Detection Strategies

Employ network traffic analysis to detect unusual timing patterns associated with login attempts. Utilize audit logs to identify discrepancies and potential abnormal access patterns.

Monitoring Recommendations

Continuous monitoring of SSH access logs and implementation of anomaly detection systems to identify timing anomalies indicative of this vulnerability exploitation.

How to Mitigate CVE-2024-39894

Immediate Actions Required

• Upgrade to OpenSSH version 9.8 immediately
• Implement network layer security controls to detect and block malicious timing patterns
• Enable multi-factor authentication to mitigate unauthorized access

Patch Information

Ensure OpenSSH is updated to version 9.8 where the logic error causing timing attacks has been patched.

Workarounds

As a temporary measure, apply configuration changes to reduce the risk of exploitation by modifying the keystroke timing logic to normalize time across input attempts.

# Configuration example
sed -i 's/ObscureKeystrokeTiming/NormalizedTiming/' /etc/ssh/sshd_config
systemctl restart sshd.service