Patched code sample

import subprocess
import shlex

def run_fixed_merge_driver(driver_command_template, file_path):
    """
    Demonstrates the fixed approach to running a merge driver, which avoids
    shell injection vulnerabilities (like CVE-2026-42563).

    The fix involves two key changes from the vulnerable implementation:
    1. The command string is tokenized into a list using `shlex.split()`.
    2. `subprocess.run()` is called with this list, and `shell=True` is NOT used.
       (The default `shell=False` is the secure option).

    This ensures that the `file_path`, even if it contains shell metacharacters,
    is treated as a single, literal argument to the command, rather than being
    interpreted by the shell.
    """
    # 1. Split the command template into a list of arguments.
    command_list = shlex.split(driver_command_template)

    # 2. Safely substitute the placeholder with the file path.
    # Each item in the list is processed, so the path becomes one argument.
    final_command = [arg.replace('%P', file_path) for arg in command_list]

    print(f"Attempting to execute with safe method: {final_command}")

    # 3. Execute the command using the list of arguments.
    # shell=False is the default and is secure. The OS handles the arguments
    # safely, preventing the injection from being executed.
    # We use 'echo' to safely demonstrate the principle.
    try:
        result = subprocess.run(final_command, check=True, capture_output=True, text=True)
        print("--- Execution Output ---")
        print(result.stdout.strip())
        print("------------------------")
        print("Success: The command executed, but the malicious payload was not interpreted by the shell.")
        print("The injected command was treated as a literal string argument.")
    except (FileNotFoundError, subprocess.CalledProcessError) as e:
        print(f"Execution failed as expected or command not found: {e}")


if __name__ == '__main__':
    # The merge driver command template, as might be configured in Git.
    # We use 'echo' as a safe, illustrative command.
    driver = "echo File path is %P"

    # A malicious file path crafted by an attacker.
    # The intent is to inject the 'id' command.
    malicious_path = "harmless_name.txt; id"

    print("--- Demonstrating the fix for CVE-2026-42563 ---")
    run_fixed_merge_driver(driver, malicious_path)