Patched code sample

# The actual vulnerability (CVE-2024-27459) was fixed in pyOpenSSL's C
# wrapper code, not in Python. It is not possible to provide the actual
# C code fix as a Python script.
#
# This Python code represents the logic of the fix. The vulnerability occurred
# because pyOpenSSL did not check the length of the bytes object returned by a
# user-defined DTLS cookie callback. The fix, implemented in pyOpenSSL >= 24.1.0,
# is to add a length check and raise a ValueError if the cookie is too long,
# thus preventing a buffer overflow in the underlying OpenSSL library.

# The fixed-size buffer for a DTLS cookie in OpenSSL is 256 bytes.
DTLS_COOKIE_MAX_LENGTH = 256


def fixed_dtls_cookie_callback_wrapper(user_callback, *callback_args):
    """
    This function simulates the corrected internal logic in pyOpenSSL that
    prevents the buffer overflow.

    It calls the user-provided Python callback and validates the length of the
    returned cookie before it would be passed to the C layer.

    In a vulnerable version of pyOpenSSL, the length check was missing.

    Args:
        user_callback: The user-supplied function for cookie generation.
        *callback_args: Arguments that OpenSSL would pass to the callback.

    Returns:
        The cookie bytes object if its length is valid.

    Raises:
        ValueError: If the cookie returned by the user_callback is longer
                    than the maximum allowed length (256 bytes).
    """
    cookie_bytes = user_callback(*callback_args)

    # THE FIX: A length check before the value is used by the C layer.
    if len(cookie_bytes) > DTLS_COOKIE_MAX_LENGTH:
        raise ValueError("Cookie is too long")

    return cookie_bytes


# --- Example of how this logic works in practice ---
def generate_oversized_cookie():
    """A user callback that incorrectly returns a cookie larger than 256 bytes."""
    return b'\x01' * (DTLS_COOKIE_MAX_LENGTH + 1)


def generate_valid_cookie():
    """A user callback that correctly returns a valid-sized cookie."""
    return b'a-valid-cookie-of-acceptable-length'


# To demonstrate, you would call the wrapper with the user's function.
# The wrapper represents the internal behavior of a patched pyOpenSSL.

# This call will raise a ValueError, demonstrating the fix.
try:
    fixed_dtls_cookie_callback_wrapper(generate_oversized_cookie)
except ValueError as e:
    # This exception is the expected behavior for the fixed version.
    # In a vulnerable version, this would lead to a buffer overflow.
    pass

# This call will succeed, as the cookie length is valid.
try:
    valid_cookie = fixed_dtls_cookie_callback_wrapper(generate_valid_cookie)
    assert len(valid_cookie) <= DTLS_COOKIE_MAX_LENGTH
except ValueError:
    # This block should not be reached with a valid cookie.
    pass