Improper Certificate Validation Using ftplib

The Python class ftplib.FTP_TLS by default creates an SSL context that does not verify the server's certificate if the context parameter is unset or has a value of None. This means that an attacker can easily impersonate a legitimate server and fool your application into connecting to it.

If you use ftplib.FTP_TLS without a context set, you are opening your application up to a number of security risks, including:

• Man-in-the-middle attacks
• Session hijacking
• Data theft

Example

import ftplib


with ftplib.FTP_TLS("ftp.us.debian.org") as ftp:
    ftp.cwd("debian")
    ftp.retrlines("LIST")

Remediation

Set the value of the context keyword argument to ssl.create_default_context() to ensure the connection is fully verified.

import ftplib
import ssl


with ftplib.FTP_TLS(
    "ftp.us.debian.org",
    context=ssl.create_default_context(),
) as ftp:
    ftp.cwd("debian")
    ftp.retrlines("LIST")

See also

• ftplib — FTP protocol client
• ssl — TLS_SSL wrapper for socket objects
• CWE-295: Improper Certificate Validation

New in version 0.3.14