Patched code sample

import os
import sys
import signal
import multiprocessing
# This code requires the 'python-seccomp' library.
# Install it with: pip install python-seccomp
try:
    import seccomp
except ImportError:
    print("This demonstration requires the 'python-seccomp' library.", file=sys.stderr)
    print("Please install it using: pip install python-seccomp", file=sys.stderr)
    sys.exit(1)


def malicious_payload():
    """
    This function represents the exploit attempt. It tries to remap stdout (fd=1)
    to a new file 'pwned.txt' and then write to it. In a vulnerable system,
    this would succeed.
    """
    try:
        # The goal is to create and write to 'pwned.txt'.
        target_file = 'pwned.txt'
        print(f"[PAYLOAD] Attempting to open '{target_file}' for writing...")
        
        # Open an arbitrary file, which returns a new file descriptor (e.g., fd=3).
        fd_new = os.open(target_file, os.O_WRONLY | os.O_CREAT)
        print(f"[PAYLOAD] Opened '{target_file}' with fd={fd_new}. Now trying to remap stdout.")

        # THE VULNERABLE STEP: Use fcntl or dup2 to remap stdout (fd=1)
        # to the file descriptor of 'pwned.txt'.
        # The CVE describes fcntl, but dup2 is a more direct way to do this.
        # A robust fix must block both.
        os.dup2(fd_new, 1)
        
        # This write *appears* to go to stdout (fd=1), which might be allowed
        # by a simple seccomp rule like 'allow write(fd=1)'.
        # However, due to the remapping, it now writes to 'pwned.txt'.
        print("[PAYLOAD] Remapping successful. Writing to sys.stdout...")
        sys.stdout.write("Vulnerability exploited!\n")
        sys.stdout.flush()
        
        print("[PAYLOAD] File 'pwned.txt' should now exist with content.")
        os.close(fd_new)

    except Exception as e:
        # In the fixed environment, the seccomp filter will kill the process
        # before this exception handler is ever reached.
        print(f"[PAYLOAD] An unexpected error occurred: {e}", file=sys.stderr)

def secure_sandbox_worker(payload_function):
    """
    This function represents the secured sandbox environment. It applies a
    seccomp filter to block the syscalls necessary for the exploit before
    executing the payload.
    """
    print("[SANDBOX] Configuring seccomp filter to fix the vulnerability...")

    # Initialize a seccomp filter. Default action is to allow all syscalls.
    # We will then create a blacklist of dangerous syscalls.
    filt = seccomp.SyscallFilter(defaction=seccomp.ALLOW)

    # --- THE FIX ---
    # The vulnerability is caused by remapping file descriptors. The syscalls
    # responsible are `fcntl`, `dup`, `dup2`, and `dup3`.
    # By adding rules to kill the process if it attempts to use these
    # syscalls, we prevent the stdout remapping and fix the vulnerability.
    print("[SANDBOX] FIX: Blocking fcntl, dup, dup2, and dup3 syscalls.")
    filt.add_rule(seccomp.KILL, "fcntl")
    filt.add_rule(seccomp.KILL, "dup")
    filt.add_rule(seccomp.KILL, "dup2")
    filt.add_rule(seccomp.KILL, "dup3")

    # Load the seccomp filter into the kernel.
    # From this point on, any attempt to call a blacklisted syscall will
    # result in the process being terminated by the kernel with a SIGSYS signal.
    filt.load()
    print("[SANDBOX] Seccomp filter loaded. Executing payload...")
    
    # Execute the potentially malicious payload.
    payload_function()


if __name__ == "__main__":
    # Clean up any file from previous failed runs.
    if os.path.exists("pwned.txt"):
        os.remove("pwned.txt")

    # We run the sandboxed code in a separate process to demonstrate that
    # the seccomp filter kills the child process without affecting the parent.
    print("[MAIN] Starting the secure sandbox as a new process.")
    process = multiprocessing.Process(
        target=secure_sandbox_worker,
        args=(malicious_payload,)
    )
    process.start()
    process.join()

    # The exit code of a process killed by a signal is -SIGNAL_NUMBER.
    # The seccomp.KILL action sends SIGSYS (signal 31).
    expected_exit_code = -signal.SIGSYS

    print("\n--- VERIFICATION ---")
    print(f"[MAIN] Sandbox process finished with exit code: {process.exitcode}")

    if process.exitcode == expected_exit_code:
        print("[MAIN] SUCCESS: Process was terminated by the seccomp filter as expected.")
        print("[MAIN] The `dup2` syscall was blocked, preventing the exploit.")
    else:
        print("[MAIN] FAILED: Process was not terminated by the seccomp filter.", file=sys.stderr)

    if not os.path.exists("pwned.txt"):
        print("[MAIN] SUCCESS: The file 'pwned.txt' was not created.")
        print("\n[CONCLUSION] The vulnerability has been fixed.")
    else:
        print("[MAIN] VULNERABILITY PRESENT: The file 'pwned.txt' was created.", file=sys.stderr)
        os.remove("pwned.txt")