Improper Certificate Validation Using smtplib

The Python class smtplib.SMTP_SSL 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 smtplib.SMTP_SSL or starttls 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 smtplib


with smtplib.SMTP_SSL("domain.org") as smtp:
    smtp.noop()
    smtp.login("user", "password")

Remediation

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

import smtplib
import ssl


with smtplib.SMTP_SSL(
    "domain.org",
    context=ssl.create_default_context(),
) as smtp:
    smtp.noop()
    smtp.login("user", "password")

See also

New in version 0.3.14