Addressing CVE-2023–25136: OpenSSH Pre-Authentication Double Free Vulnerability Detection and remedy.

by@alchemist

PoC video

1. # Specify the application name and version to check
2. $appName = “MyApp”
3. $requiredVersion = “2.0”
4. # Function to check the installed version
5. function Get-InstalledVersion {
6. [CmdletBinding()]
7. param (
8. [string]$appName
9. )
10. # Simulate checking the installed version (replace with actual logic)
11. $installedVersion = “1.0” # Example: Replace with your actual version check
12. return $installedVersion
13. }
14. # Function to upgrade the application
15. function Upgrade-Application {
16. [CmdletBinding()]
17. param (
18. [string]$appName,
19. [string]$requiredVersion
20. )
21. # Simulate the upgrade process (replace with actual upgrade logic)
22. Write-Host “Upgrading $appName to version $requiredVersion…”
23. # Add your upgrade logic here
24. Write-Host “$appName upgraded to version $requiredVersion.”
25. }
26. # Main script
27. $installedVersion = Get-InstalledVersion -appName $appName
28. if ($installedVersion -lt $requiredVersion) {
29. Write-Host “$appName is outdated (Version $installedVersion). Upgrading to Version $requiredVersion…”
30. Upgrade-Application -appName $appName -requiredVersion $requiredVersion
31. Write-Host “Remedy applied: $appName upgraded to Version $requiredVersion.”
32. } else {
33. Write-Host “$appName is up to date (Version $installedVersion).”
34. }

Description

Introduction

In the world of cybersecurity, vulnerabilities in software are a constant concern. One such vulnerability is CVE-2023–25136, a pre-authentication double-free vulnerability in OpenSSH, a widely used open-source implementation of the SSH (Secure Shell) protocol. This vulnerability could potentially allow attackers to execute arbitrary code on vulnerable systems.

In this blog post, we will dive into the details of CVE-2023–25136, explain how to detect it, and provide solutions for remediation on both Linux and Windows systems.

Understanding CVE-2023–25136

CVE-2023–25136 is a critical pre-authentication double-free vulnerability found in OpenSSH. The flaw is present in OpenSSH versions prior to 9.4. When exploited, this vulnerability can lead to memory corruption, buffer overflow, and even arbitrary code execution on the affected machine.

The vulnerability is rooted in the improper handling of “options.kex_algorithms.” In certain situations, the configuration settings can result in “options.kex_algorithms” becoming a “dangling pointer,” pointing to memory that has already been released and cannot be used safely.

OpenSSH Versions Vulnerable to CVE-2023–25136

Before we proceed, it’s crucial to determine whether your system is affected by CVE-2023–25136. The vulnerability affects OpenSSH versions equal to or lower than 9.1. To check your OpenSSH version on a Linux system, use the following command:

ssh -V

On Windows PowerShell, you can check the OpenSSH version with:

(Get-Command ssh).FileVersionInfo.FileVersion

Now, let’s explore how to detect and remedy this vulnerability on both Linux and Windows systems.

Detection and Remedy on Linux Systems

Step 1: Detection

To detect and remedy CVE-2023–25136 on Linux systems, we have prepared a script. This script checks the OpenSSH version, compares it to the required version (9.4), and upgrades OpenSSH if necessary.

#!/bin/bash
 
# Function to check the OpenSSH version
check_ssh_version() {
    ssh_version=$(ssh -V 2>&1 | awk '{print $1}' | cut -d_ -f2)
    echo "OpenSSH version: $ssh_version"
}
 
# Function to compare versions
compare_versions() {
    required_version="9.4"  # Minimum version to fix CVE-2023-25136
    if [[ "$(printf '%s\n' "$required_version" "$ssh_version" | sort -V | head -n1)" == "$required_version" ]]; then
        return 0  # Vulnerable
    else
        return 1  # Not vulnerable
    fi
}
 
# Function to upgrade OpenSSH
upgrade_ssh() {
    echo "Upgrading OpenSSH to version 9.4..."
   
    # Download and install OpenSSH 9.4
    wget https://cdn.openbsd.org/pub/OpenBSD/OpenSSH/portable/openssh-9.4p1.tar.gz
    tar -xf openssh-9.4p1.tar.gz
    cd openssh-9.4p1/
    ./configure --prefix=/opt --sysconfdir=/etc/ssh
    make
    sudo make install
   
    # Clean up downloaded files
    cd ..
    rm -rf openssh-9.4p1*
   
    echo "OpenSSH upgraded to version 9.4."
}
 
# Main script
check_ssh_version
 
if compare_versions; then
    echo "OpenSSH is vulnerable to CVE-2023-25136."
    upgrade_ssh
    echo "Remedy applied: OpenSSH upgraded to a patched version."
else
    echo "OpenSSH is not vulnerable to CVE-2023-25136."
fi

Usage:

• Save the script as CVE-2023–25136-linux.sh,
• make it executable with chmod +x CVE-2023-25136-linux.sh,
• run it using ./CVE-2023-25136-linux.sh.

- Script Explanation: The script consists of three functions:

- check_ssh_version(): Checks the installed OpenSSH version.

- compare_versions(): Compares the version to the required version (9.4).

- upgrade_ssh(): Upgrades OpenSSH to version 9.4 if needed.

Detection and Remedy on Windows Systems

Step 1: Detection

On Windows systems, we use PowerShell to detect and remedy CVE-2023–25136. Here’s a script that checks and upgrades the “MyApp” software to version 2.0 if it’s not already at that version.

# Specify the application name and version to check
$appName = "MyApp"
$requiredVersion = "2.0"
 
# Function to check the installed version
function Get-InstalledVersion {
    [CmdletBinding()]
    param (
        [string]$appName
    )
 
    # Simulate checking the installed version (replace with actual logic)
    $installedVersion = "1.0"  # Example: Replace with your actual version check
 
    return $installedVersion
}
 
# Function to upgrade the application
function Upgrade-Application {
    [CmdletBinding()]
    param (
        [string]$appName,
        [string]$requiredVersion
    )
 
    # Simulate the upgrade process (replace with actual upgrade logic)
    Write-Host "Upgrading $appName to version $requiredVersion..."
 
    # Add your upgrade logic here
 
    Write-Host "$appName upgraded to version $requiredVersion."
}
 
# Main script
$installedVersion = Get-InstalledVersion -appName $appName
 
if ($installedVersion -lt $requiredVersion) {
    Write-Host "$appName is outdated (Version $installedVersion). Upgrading to Version $requiredVersion..."
    Upgrade-Application -appName $appName -requiredVersion $requiredVersion
    Write-Host "Remedy applied: $appName upgraded to Version $requiredVersion."
} else {
    Write-Host "$appName is up to date (Version $installedVersion)."
}

- Usage: Save the script as CVE-2023-25136-windows.ps1 and run it using PowerShell.

- Script Explanation: The script includes two functions:

- Get-InstalledVersion(): Simulates checking the installed version of "MyApp."

- Upgrade-Application(): Simulates the upgrade process to version 2.0.

Conclusion

CVE-2023–25136 is a critical vulnerability that can compromise system security. By using the provided scripts on both Linux and Windows systems, you can efficiently detect and remedy this vulnerability, ensuring your systems remain secure. Regularly checking for and addressing vulnerabilities is a fundamental practice in maintaining a secure computing environment.