Cleartext Transmission of Sensitive Information in the smtplib Module

PY016

cleartext_transmission

CWE-319

⛔️ Error

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The Python module smtplib provides a number of functions for accessing SMTP servers. However, the default behavior of the module does not provide utilize secure connections. This means that data transmitted over the network, including passwords, is sent in cleartext. This makes it possible for attackers to intercept and read this data.

The Python module smtplib should only in a secure mannner to protect sensitive data when accessing SMTP servers.

Example

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Error

import smtplib


def prompt(prompt):
    return input(prompt).strip()

fromaddr = prompt("From: ")
toaddrs  = prompt("To: ").split()
print("Enter message, end with ^D (Unix) or ^Z (Windows):")

# Add the From: and To: headers at the start!
msg = ("From: %s\r\nTo: %s\r\n\r\n" % (fromaddr, ", ".join(toaddrs)))
while True:
    try:
        line = input()
    except EOFError:
        break
    if not line:
        break
    msg = msg + line

print("Message length is", len(msg))

server = smtplib.SMTP('localhost')
server.login("user", "password")
server.set_debuglevel(1)
server.sendmail(fromaddr, toaddrs, msg)
server.quit()

Remediation

---

fix

If the SMTP protocol must be used and sensitive data will be transferred, it is recommended to secure the connection using SMTP_SSL class. Alternatively, the starttls function can be used to enter a secure session.

import smtplib


def prompt(prompt):
    return input(prompt).strip()

fromaddr = prompt("From: ")
toaddrs  = prompt("To: ").split()
print("Enter message, end with ^D (Unix) or ^Z (Windows):")

# Add the From: and To: headers at the start!
msg = ("From: %s\r\nTo: %s\r\n\r\n" % (fromaddr, ", ".join(toaddrs)))
while True:
    try:
        line = input()
    except EOFError:
        break
    if not line:
        break
    msg = msg + line

print("Message length is", len(msg))

server = smtplib.SMTP_SSL('localhost')
server.login("user", "password")
server.set_debuglevel(1)
server.sendmail(fromaddr, toaddrs, msg)
server.quit()

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 (PY016) or rule category name (cleartext_transmission).

Using rule ID

fix

import smtplib


def prompt(prompt):
    return input(prompt).strip()

fromaddr = prompt("From: ")
toaddrs  = prompt("To: ").split()
print("Enter message, end with ^D (Unix) or ^Z (Windows):")

# Add the From: and To: headers at the start!
msg = ("From: %s\r\nTo: %s\r\n\r\n" % (fromaddr, ", ".join(toaddrs)))
while True:
    try:
        line = input()
    except EOFError:
        break
    if not line:
        break
    msg = msg + line

print("Message length is", len(msg))

# suppress: PY016
server = smtplib.SMTP('localhost')
# suppress: PY016
server.login("user", "password")
server.set_debuglevel(1)
server.sendmail(fromaddr, toaddrs, msg)
server.quit()

Using category name

fix

import smtplib


def prompt(prompt):
    return input(prompt).strip()

fromaddr = prompt("From: ")
toaddrs  = prompt("To: ").split()
print("Enter message, end with ^D (Unix) or ^Z (Windows):")

# Add the From: and To: headers at the start!
msg = ("From: %s\r\nTo: %s\r\n\r\n" % (fromaddr, ", ".join(toaddrs)))
while True:
    try:
        line = input()
    except EOFError:
        break
    if not line:
        break
    msg = msg + line

print("Message length is", len(msg))

# suppress: cleartext_transmission
server = smtplib.SMTP('localhost')
# suppress: cleartext_transmission
server.login("user", "password")
server.set_debuglevel(1)
server.sendmail(fromaddr, toaddrs, msg)
server.quit()

See also

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info

• smtplib — SMTP protocol client
• CWE-319: Cleartext Transmission of Sensitive Information