Email Injection in Django

Email injection in Django typically occurs when user-controlled data is embedded into email headers or message bodies without proper validation or escaping, allowing an attacker to inject additional headers or control message routing. In Django, this most commonly arises when constructing emails via django.core.mail.EmailMessage, send_mail, or EmailMultiAlternatives and passing user input into fields such as to, cc, bcc, subject, or headers.

Attack patterns include newline injection (CRLF, \r\n) to inject extra headers like Cc:, Bcc:, Reply-To:, or Message-ID:. For example, if a view builds an email subject using raw query parameters, an input like test%0D%0ABcc: attacker@example.com can cause the email to be sent to an unintended recipient without the application’s knowledge. Django’s email APIs do not inherently sanitize header values; they pass strings directly to the configured email backend, which means any injected newline and subsequent header lines can be interpreted as part of the email protocol flow.

In a typical vulnerable view, you might see:

from django.core.mail import send_mail
from django.http import HttpRequest

def contact_view(request: HttpRequest):
    name = request.GET.get('name', '')
    email = request.GET.get('email', '')
    message = request.GET.get('message', '')
    # Risk: unsanitized user input used in subject
    subject = f'Reply from {name}'
    send_mail(subject, message, 'noreply@example.com', ['support@example.com'])
    return HttpResponse('Sent')

If an attacker provides email as evil@example.com%0D%0ABcc: victim@example.com, some backends may route the message to the injected Bcc recipient. Another pattern involves the headers parameter with dictionary injection, where user input is placed into a header key or value without validation:

from django.core.mail import EmailMessage

def newsletter_view(request):
    reply_to = request.GET.get('reply_to', 'support@example.com')
    msg = EmailMessage('Newsletter', 'Content', 'noreply@example.com', to=['subscribers@example.com'])
    msg['Reply-To'] = reply_to  # Risk: unsanitized header value
    msg.send()

If reply_to contains a newline, additional headers can be smuggled. Django’s email backends (SMTP, console, file-based) will generally pass these headers as provided, making server-side validation and sanitization essential.

Detecting email injection in Django involves both code review and runtime testing. Look for direct concatenation or formatting of user input into email headers via EmailMessage, message['Header'] = value, or parameters passed to send_mail. Key indicators include missing normalization of newline characters, absence of header validation libraries, and usage of raw request parameters (request.GET, request.POST) in email construction.

Static analysis can flag dangerous patterns such as:

• Assigning request-derived values to msg['Subject'], msg['From'], msg['To'], msg['Cc'], msg['Bcc'], or msg['Reply-To'] without sanitization.
• Using string interpolation or concatenation to build header values without stripping \r and \n.

Runtime detection can be performed by sending crafted payloads that attempt to inject extra headers and inspecting outbound messages or logs. For example, provide subject as Hello%0D%0ABcc: injected@example.com and verify whether the injected recipient receives the message. Because these tests involve crafting newline-based sequences and observing side effects, they align with black-box scanning methodologies that do not require authentication or source code access.

middleBrick can detect email injection by running its 12 parallel security checks, including Input Validation and Unsafe Consumption, against your unauthenticated endpoints. By submitting URLs that trigger email-sending behavior with payloads such as subject=test%0D%0ABcc:honeypot@example.com, middleBrick observes whether injected headers result in unintended delivery paths. Its OpenAPI/Swagger analysis (supporting 2.0, 3.0, and 3.1 with full $ref resolution) cross-references spec definitions with runtime findings to highlight endpoints where user-controlled data reaches email headers. Reports include severity, specific payloads, and remediation guidance without requiring credentials or agents, enabling quick triage in development and CI/CD contexts through the Web Dashboard, CLI (middlebrick scan <url>), GitHub Action, or MCP Server integration.

Remediate email injection in Django by ensuring all user-controlled data used in email headers is sanitized and restricted. The safest approach is to avoid placing raw user input into headers entirely. For email addresses, validate format with Django’s validators and use them only in address lists, not as raw header strings.

Use Django’s django.core.mail.message utilities and strict encoding to prevent newline injection. For example, explicitly set headers using methods that do not allow multiple lines:

from django.core.mail import EmailMessage
from django.core.exceptions import ValidationError
from django.core.validators import validate_email

def safe_newsletter_view(request):
    reply_to = request.GET.get('reply_to', 'support@example.com')
    # Validate email format
    try:
        validate_email(reply_to)
    except ValidationError:
        reply_to = 'support@example.com'
    msg = EmailMessage('Newsletter', 'Content', 'noreply@example.com', to=['subscribers@example.com'])
    msg['Reply-To'] = reply_to
    msg.send()

When constructing subjects or other header-like values, strip carriage returns and line feeds:

import re
from django.core.mail import send_mail

def sanitize_header_value(value: str) -> str:
    # Remove CR and LF characters to prevent header injection
    return re.sub(r'[\r\n]', '', value)

def contact_view(request):
    name = sanitize_header_value(request.GET.get('name', ''))
    email = sanitize_header_value(request.GET.get('email', ''))
    message = request.GET.get('message', '')
    subject = f'Reply from {name}'
    send_mail(subject, message, 'noreply@example.com', ['support@example.com'])

For more robust header handling, use the email.headerregistry module (available in Python 3.6+) to construct headers safely, ensuring proper encoding and no injection of control characters. Also consider using Django’s send_mail with predefined, trusted recipients and avoid dynamically setting to, cc, or bcc from untrusted sources.

Finally, adopt continuous monitoring where applicable. With the Pro plan, you can enable continuous monitoring to periodically rescan endpoints and receive alerts if a regression introduces injection risks. The GitHub Action can fail builds when a scan detects issues, and the Web Dashboard helps track improvements over time.