Email Injection in Flask with Basic Auth

Email Injection occurs when user-controlled data is placed into email headers without proper validation, enabling attackers to inject additional headers or malformed content. In Flask applications that use HTTP Basic Authentication, the combination of direct header manipulation and legacy authentication schemes can amplify risks if email-related functionality reuses request metadata or headers.

Basic Authentication in Flask typically relies on the Authorization header, which is base64-encoded but not encrypted. While this header itself does not directly carry email data, applications may derive email addresses from request metadata (e.g., X-Forwarded-For, Referer, or custom headers) for logging, notification, or identity correlation. If these values are later used in email composition—such as in Reply-To, From, or CC fields—lack of input validation can permit newline characters (\r\n) that break header structure and enable injection.

For example, if a Flask route builds an email using a username derived from authentication or a header value without sanitization, an attacker could supply a crafted payload like attacker%40example.com%0D%0ACc:%20evil@example.com. Decoded, this introduces a second Cc header, redirecting email delivery or exposing internal routing information. This maps to the broader OWASP API Security Testing category of Input Validation and resembles classic header injection issues seen in SMTP and HTTP header handling (CVE-2021-3177, CVE-2020-13677 patterns).

Flask’s default development server does not sanitize headers used in application logic, so developers must explicitly validate any data that flows into email generation. The risk is particularly relevant when integrating with notification services or audit trails that include user-supplied identifiers. Without rigorous validation, the API’s unauthenticated or weakly authenticated endpoints can become vectors for email header manipulation, even when Basic Auth protects certain routes.

middleBrick’s 12 security checks, including Input Validation and Unsafe Consumption, are designed to detect scenarios where headers or user input could corrupt email structures. The scanner tests unauthenticated attack surfaces and cross-references runtime behavior with OpenAPI specifications to highlight missing constraints on string fields that reach email-generation code.

Remediation focuses on strict input validation, avoiding header reuse, and ensuring email-generation routines treat all external data as untrusted. Below are concrete Flask patterns that mitigate Email Injection when Basic Authentication is in use.

1. Validate and sanitize any data used in email headers

Ensure that values such as usernames, emails, or custom headers do not contain newline characters before they reach email logic. Use allowlists and reject sequences like \r and \n.

import re
from flask import Flask, request, abort

app = Flask(__name__)

# Allowlist pattern for a safe email local part
EMAIL_LOCAL_RE = re.compile(r'^[a-zA-Z0-9._%+-]+$')

def safe_email_domain(user_email: str) -> str:
    # Accept only a pre-approved domain to avoid injection via user input
    if not user_email.endswith('@trusted.example.com'):
        raise ValueError('Untrusted email domain')
    local, domain = user_email.split('@', 1)
    if not EMAIL_LOCAL_RE.match(local):
        raise ValueError('Invalid local email part')
    return user_email

@app.route('/notify')
def notify():
    auth = request.authorization
    if not auth or not (auth.username and auth.password):
        abort(401, 'Basic Auth required')
    # Simulate a derived email; in practice, use a verified source
    user_email = safe_email_domain(f'{auth.username}@trusted.example.com')
    # Safe usage: user_email is guaranteed newline-free and domain-valid
    return {'status': 'ok', 'email': user_email}

2. Avoid using authentication or header values directly in email composition

Do not pass raw Authorization or header values into email From or Reply-To. Instead, use a fixed, verified sender address and map identities through your application’s own user model.

from flask import Flask, request, jsonify
import smtplib
from email.message import EmailMessage

app = Flask(__name__)

SENDER = 'no-reply@trusted.example.com'

@app.route('/send-report')
def send_report():
    auth = request.authorization
    if not auth or not valid_credentials(auth.username, auth.password):
        return jsonify({'error': 'Unauthorized'}), 401
    # Do NOT use auth.username in email headers directly
    msg = EmailMessage()
    msg['Subject'] = 'Your report'
    msg['From'] = SENDER
    msg['To'] = 'user@example.com'  # resolved server-side, not from client input
    msg.set_content('Report content here')
    with smtplib.SMTP('localhost') as server:
        server.send_message(msg)
    return jsonify({'sent': True})

def valid_credentials(username: str, password: str) -> bool:
    # Replace with secure credential verification
    return username == 'alice' and password == 'correct-basic-auth-password'

3. Enforce content-type and header constraints in OpenAPI

When using an OpenAPI specification, define strict schemas for headers and query parameters to prevent unexpected newline injection through extended metadata. This aligns with middleBrick’s OpenAPI/Swagger analysis, which resolves $ref and cross-references definitions with runtime behavior.

openapi: 3.0.3
info:
  title: Secure API
  version: 1.0.0
paths:
  /notify:
    get:
      summary: Send a notification
      security:
        - basicAuth: []
      parameters:
        - name: X-Custom-Tag
          in: header
          required: false
          schema:
            type: string
            pattern: '^[A-Za-z0-9-]+$'
      responses:
        '200':
          description: OK
components:
  securitySchemes:
    basicAuth:
      type: http
      scheme: basic

By combining these practices—validating inputs, isolating email generation from authentication metadata, and tightening OpenAPI constraints—you reduce the surface for Email Injection while retaining Basic Auth where appropriate. middleBrick’s Pro plan supports continuous monitoring and CI/CD integration to catch regressions before deployment, and the CLI allows quick scans from the terminal using middlebrick scan <url>.