Mail Command Injection

python.mail_command_injection

Severity

critical

Resource

Injection

Language

Python

Description

Improper neutralization of CRLF sequences sent to an SMTP, POP3, or IMAP mail server ('Mail Injection').

Rationale

Mail Command Injection occurs when inputs used to construct mail commands or emails in applications are not properly sanitized or validated. This can allow an attacker to inject additional commands or manipulate mail parameters for malicious purposes, potentially compromising system integrity or confidentiality.

The following example demonstrates a vulnerable implementation:

from flask import Flask
from flask import request
import smtplib

app = Flask(__name__)

@app.route("/send/", methods=['GET', 'POST'])
def send():
    if request.method == "POST":
        # FP, this validation is not properly processed
        to = request.form['to']
        user = request.form['from']

        with smtplib.SMTP('localhost') as smtp:
            smtp.sendmail(to, user, "Hello, world.")  # FLAW

    return "<h1>Done!</h1>"

app.run(debug=True)

In this example, an attacker can craft input in the from or to parameter to inject newline characters and malicious headers. For instance:

from=attacker@example.com%0ACc:victim@example.com

This causes the constructed message to include a new Cc: header, sending the message to unintended recipients.

Remediation

To remediate Mail Command Injection vulnerabilities in applications, follow these practical steps:

Input Validation and Sanitization: Rigorously validate user inputs such as email addresses and subject lines. Ensure they conform to expected patterns and remove any potentially dangerous characters or sequences.
Use Mail API: Rather than constructing mail commands manually, utilize a Mail API for handling email operations. Mail APIs abstract the complexities of mail handling and reduces the risk of command injection by not relying on shell commands:
Escape Shell Inputs: If executing mail-related shell commands is necessary, ensure all user inputs are correctly escaped to prevent injection. However, this is still risky and should be avoided if possible.
Dependency Updates: Ensure that libraries and tools related to email handling in your application are up to date with the latest security patches and recommendations.
Security Reviews and Automated Testing: Incorporate security reviews and SAST into your development lifecycle to identify and address Mail Command Injection vulnerabilities early.

By adopting these practices, you can mitigate the risk of Mail Command Injection in your applications and enhance the security posture of your email handling processes.

Configuration

The detector has the following configurable parameters:

sources, that indicates the source kinds to check.
neutralizations, that indicates the neutralization kinds to check.

Unless you need to change the default behavior, you typically do not need to configure this detector.

References

CWE-93 : Improper Neutralization of CRLF Sequences ('CRLF Injection').
OTG-INPVAL-011