Sending emails is security-sensitive and can expose an application to a large range of vulnerabilities.

Information Exposure

Emails often contain sensitive information which might be exposed to an attacker if he can add an arbitrary address to the recipient list.

Spamming / Phishing

Malicious user can abuse email based feature to send spam or phishing content.

Dangerous Content Injection

Emails can contain HTML and JavaScript code, thus they can be used for XSS attacks.

Email Headers Injection

Email fields such as subject, to, cc, bcc, from are set in email "headers".  Using unvalidated user input to set those fields might allow attackers to inject new line characters in headers to craft malformed SMTP requests. Although modern libraries are filtering new line character by default, user data used in email "headers" should always be validated.

In the past, it has led to the following vulnerabilities:

Ask Yourself Whether

You are at risk if you answered yes to any of those questions.

Recommended Secure Coding Practices

* Use an email library which sanitizes headers (Flask-Mail or django.core.mail).

* Use html escape functions to sanitize every piece of data used to in the email body.

* Verify application logic to make sure that email base feature can not be abuse to:

Send arbitrary email for spamming or fishing

Disclose sensitive email content

Sensitive Code Example

smtplib

import smtplib

def send(from_email, to_email, msg):
  server = smtplib.SMTP('localhost', 1025)
  server.sendmail(from_email, to_email, msg) # Sensitive

Django

from django.core.mail import send_mail

def send(subject, msg, from_email, to_email):
  send_mail(subject, msg, from_email, [to_email]) # Sensitive

Flask-Mail

from flask import Flask
from flask_mail import Mail, Message

app = Flask(__name__)

def send(subject, msg, from_email, to_email):
    mail = Mail(app)
    msg = Message(subject, [to_email], body, sender=from_email)
    mail.send(msg) # Sensitive{code}

See