Email Injection in Flask with Api Keys

Email Injection occurs when user-controlled data is improperly handled in email headers or message bodies, enabling attackers to inject additional headers (such as CC, BCC, or newlines) and potentially exfiltrate data or trigger unwanted email actions. In Flask applications that integrate third‑party email services via API keys, this risk arises when the app builds email messages using unsanitized inputs (e.g., user-supplied names or email addresses) and then passes them to an email-sending library or service. The API key itself does not cause injection, but its presence changes the attack surface: developers may store the key in environment variables and log or echo request details for debugging, inadvertently exposing the key in error messages or logs if injection manipulates output formatting.

When an email endpoint accepts user input for headers like To, From, or Subject, missing validation or escaping can allow newline characters ( ) to inject extra headers. In Flask, if you use a library such as requests to call an external email API, the API key is typically included in an Authorization header. A crafted payload that injects newlines can create a header smuggling scenario, where an injected header like CC: [email protected] is appended, and the API request may still succeed if the server’s parsing is lenient. Moreover, if the Flask app logs the raw request or API responses—including the API key—attackers can harvest the key from log files or error pages, leading to credential misuse.

Consider a Flask route that forwards user input to an email API without proper sanitization:

import requests
from flask import Flask, request

app = Flask(__name__)
EMAIL_API_KEY = "sk_live_abc123"  # stored securely, but exposed via logs if injection occurs

@app.route("/send", methods=["POST"])
def send_email():
    name = request.form.get("name", "")
    email = request.form.get("email", "")
    # Unsafe concatenation may allow newline injection in name or email
    payload = {
        "to": email,
        "subject": f"Contact from {name}",
        "body": f"Name: {name}\nEmail: {email}"
    }
    headers = {"Authorization": f"Bearer {EMAIL_API_KEY}"}
    resp = requests.post("https://api.emailservice.com/send", json=payload, headers=headers)
    return resp.text, resp.status_code

An attacker could submit name as Test CC: [email protected], potentially adding an unintended recipient if the downstream parser processes the injected newline. The API key remains in the Authorization header, but the log or response may reveal more context, aiding further exploitation. This illustrates why input validation and output encoding are essential even when using API keys for authentication.

To mitigate Email Injection in Flask while handling API keys securely, adopt strict input validation, avoid direct concatenation of user input into email headers, and ensure API keys are never exposed in logs or error messages. Use a robust email-sending library that handles header encoding safely, and configure your Flask app to treat API keys as sensitive configuration managed outside the request lifecycle.

First, validate and sanitize all user inputs that may appear in email headers. Reject or encode newline characters and control characters. For the email body, prefer plain text or HTML escaping rather than raw concatenation. Second, keep API keys in environment variables or a secrets manager, and access them via os.getenv without logging them. Third, structure your request so that user data never contaminates header construction; rely on the email service’s SDK or a well-maintained library to build headers correctly.

Here is a secure Flask example that incorporates these practices:

import os
import re
import requests
from flask import Flask, request, jsonify

app = Flask(__name__)
EMAIL_API_KEY = os.getenv("EMAIL_API_KEY")
if not EMAIL_API_KEY:
    raise RuntimeError("EMAIL_API_KEY environment variable is required")

def is_valid_email(value: str) -> bool:
    # Basic RFC 5322 local-part and domain validation; adjust as needed
    pattern = r"^[A-Za-z0-9.!#$%&'*+/=?^_`{|}~-]+@[A-Za-z0-9](?:[A-Za-z0-9-]{0,61}[A-Za-z0-9])?(?:\.[A-Za-z0-9](?:[A-Za-z0-9-]{0,61}[A-Za-z0-9])?)*$"
    return re.match(pattern, value) is not None

@app.route("/send", methods=["POST"])
def send_email():
    name = request.form.get("name", "")
    email = request.form.get("email", "")

    # Validate inputs
    if not name or not email or not is_valid_email(email):
        return jsonify({"error": "Invalid input"}), 400

    # Ensure no newline injection by stripping control chars
    safe_name = re.sub(r"[\r\n\x00-\x1F]", "", name)

    # Build body as plain text; avoid injecting user data into headers
    body = f"Name: {safe_name}\nEmail: {email}"

    payload = {
        "to": email,
        "subject": f"Contact from {safe_name}",
        "body": body
    }

    headers = {"Authorization": f"Bearer {EMAIL_API_KEY}"}
    resp = requests.post("https://api.emailservice.com/send", json=payload, headers=headers, timeout=10)
    resp.raise_for_status()
    return jsonify({"status": "sent", "provider_status": resp.status_code})

This approach keeps the API key isolated in the Authorization header, validates and sanitizes user input, and avoids constructing headers with user-controlled strings. For production, consider using an email service SDK that further abstracts header handling and integrates with Flask’s configuration management.