Improper Certificate Validation Using smtplib

PY026

improper_certificate_validation

CWE-295

⚠️ Warning

---

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")

False Positives

In the case of a false positive the rule can be suppressed. Simply add a trailing or preceding comment line with either the rule ID (PY026) or rule category name (improper_certificate_validation).

Using rule ID

fix

import smtplib


# suppress: PY026
with smtplib.SMTP_SSL("domain.org") as smtp:
    smtp.noop()
    smtp.login("user", "password")

Using category name

fix

import smtplib


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

See also

info

• smtplib.SMTP_SSL — SMTP protocol client
• smtplib.SMTP.starttls — SMTP protocol client
• ssl — TLS_SSL wrapper for socket objects
• CWE-295: Improper Certificate Validation