system: You are an expert security code reviewer. You will be shown a source code file along with a multiple-choice question listing several function or class names from that file. Your task is to identify which one contains the specified CWE vulnerability.

OUTPUT FORMAT: Respond with the single letter of the correct option, wrapped in \boxed{...}. Example: `\boxed{B}`. Output the boxed letter ONLY -- no explanation, no other text.

user: ```python
import errno
import getpass
import logging
import os

import paramiko
from libsubmit.channels.errors import *
from libsubmit.utils import RepresentationMixin

logger = logging.getLogger(__name__)


class SSHChannel(RepresentationMixin):
    ''' SSH persistent channel. This enables remote execution on sites
    accessible via ssh. It is assumed that the user has setup host keys
    so as to ssh to the remote host. Which goes to say that the following
    test on the commandline should work :

    >>> ssh <username>@<hostname>

    '''

    def __init__(self, hostname, username=None, password=None, script_dir=None, envs=None, **kwargs):
        ''' Initialize a persistent connection to the remote system.
        We should know at this point whether ssh connectivity is possible

        Args:
            - hostname (String) : Hostname

        KWargs:
            - username (string) : Username on remote system
            - password (string) : Password for remote system
            - script_dir (string) : Full path to a script dir where
              generated scripts could be sent to.
            - envs (dict) : A dictionary of environment variables to be set when executing commands

        Raises:
        '''

        self.hostname = hostname
        self.username = username
        self.password = password
        self.kwargs = kwargs

        self.ssh_client = paramiko.SSHClient()
        self.ssh_client.load_system_host_keys()
        self.ssh_client.set_missing_host_key_policy(paramiko.AutoAddPolicy())

        if script_dir:
            self._script_dir = script_dir
        else:
            self._script_dir = "/tmp/{0}/scripts/".format(getpass.getuser())

        self.envs = {}
        if envs is not None:
            self.envs = envs

        try:
            self.ssh_client.connect(
                hostname,
                username=username,
                password=password,
                allow_agent=True
            )
            t = self.ssh_client.get_transport()
            self.sftp_client = paramiko.SFTPClient.from_transport(t)

        except paramiko.BadHostKeyException as e:
            raise BadHostKeyException(e, self.hostname)

        except paramiko.AuthenticationException as e:
            raise AuthException(e, self.hostname)

        except paramiko.SSHException as e:
            raise SSHException(e, self.hostname)

        except Exception as e:
            raise SSHException(e, self.hostname)

    @property
    def script_dir(self):
        return self._script_dir

    def prepend_envs(self, cmd, env={}):
        env.update(self.envs)

        if len(env.keys()) > 0:
            env_vars = ' '.join(['{}={}'.format(key, value) for key, value in env.items()])
            return 'env {0} {1}'.format(env_vars, cmd)
        return cmd

    def execute_wait(self, cmd, walltime=2, envs={}):
        ''' Synchronously execute a commandline string on the shell.

        Args:
            - cmd (string) : Commandline string to execute
            - walltime (int) : walltime in seconds, this is not really used now.

        Kwargs:
            - envs (dict) : Dictionary of env variables

        Returns:
            - retcode : Return code from the execution, -1 on fail
            - stdout  : stdout string
            - stderr  : stderr string

        Raises:
        None.
        '''

        # Execute the command
        stdin, stdout, stderr = self.ssh_client.exec_command(
            self.prepend_envs(cmd, envs), bufsize=-1, timeout=walltime
        )
        # Block on exit status from the command
        exit_status = stdout.channel.recv_exit_status()
        return exit_status, stdout.read().decode("utf-8"), stderr.read().decode("utf-8")

    def execute_no_wait(self, cmd, walltime=2, envs={}):
        ''' Execute asynchronousely without waiting for exitcode

        Args:
            - cmd (string): Commandline string to be executed on the remote side
            - walltime (int): timeout to exec_command

        KWargs:
            - envs (dict): A dictionary of env variables

        Returns:
            - None, stdout (readable stream), stderr (readable stream)

        Raises:
            - ChannelExecFailed (reason)
        '''

        # Execute the command
        stdin, stdout, stderr = self.ssh_client.exec_command(
            self.prepend_envs(cmd, envs), bufsize=-1, timeout=walltime
        )
        # Block on exit status from the command
        return None, stdout, stderr

    def push_file(self, local_source, remote_dir):
        ''' Transport a local file to a directory on a remote machine

        Args:
            - local_source (string): Path
            - remote_dir (string): Remote path

        Returns:
            - str: Path to copied file on remote machine

        Raises:
            - BadScriptPath : if script path on the remote side is bad
            - BadPermsScriptPath : You do not have perms to make the channel script dir
            - FileCopyException : FileCopy failed.

        '''
        remote_dest = remote_dir + '/' + os.path.basename(local_source)

        try:
            self.sftp_client.mkdir(remote_dir)
        except IOError as e:
            if e.errno is None:
                logger.info(
                    "Copying {0} into existing directory {1}".format(local_source, remote_dir)
                )
            else:
                logger.exception("Pushing {0} to {1} failed".format(local_source, remote_dir))
                if e.errno == 2:
                    raise BadScriptPath(e, self.hostname)
                elif e.errno == 13:
                    raise BadPermsScriptPath(e, self.hostname)
                else:
                    logger.exception("File push failed due to SFTP client failure")
                    raise FileCopyException(e, self.hostname)

        try:
            self.sftp_client.put(local_source, remote_dest, confirm=True)
            # Set perm because some systems require the script to be executable
            self.sftp_client.chmod(remote_dest, 0o777)
        except Exception as e:
            logger.exception("File push from local source {} to remote destination {} failed".format(
                local_source, remote_dest))
            raise FileCopyException(e, self.hostname)

        return remote_dest

    def pull_file(self, remote_source, local_dir):
        ''' Transport file on the remote side to a local directory

        Args:
            - remote_source (string): remote_source
            - local_dir (string): Local directory to copy to


        Returns:
            - str: Local path to file

        Raises:
            - FileExists : Name collision at local directory.
            - FileCopyException : FileCopy failed.
        '''

        local_dest = local_dir + '/' + os.path.basename(remote_source)

        try:
            os.makedirs(local_dir)
        except OSError as e:
            if e.errno != errno.EEXIST:
                logger.exception("Failed to create script_dir: {0}".format(script_dir))
                raise BadScriptPath(e, self.hostname)

        # Easier to check this than to waste time trying to pull file and
        # realize there's a problem.
        if os.path.exists(local_dest):
            logger.exception("Remote file copy will overwrite a local file:{0}".format(local_dest))
            raise FileExists(None, self.hostname, filename=local_dest)

        try:
            self.sftp_client.get(remote_source, local_dest)
        except Exception as e:
            logger.exception("File pull failed")
            raise FileCopyException(e, self.hostname)

        return local_dest

    def close(self):
        return self.ssh_client.close()

```

Which of the following functions or classes contains a `CWE-295: Improper Certificate Validation` vulnerability?

  A) script_dir
  B) prepend_envs
  C) push_file
  D) __init__
Answer: D