Header Injection in Flask with Firestore

Header Injection occurs when user-controlled data is placed into HTTP response headers without validation or encoding. In a Flask application that uses Google Cloud Firestore as a backend, this typically happens when values from Firestore documents are forwarded into headers such as Location, X-Request-ID, or custom headers without sanitization. Because Firestore often stores dynamic metadata like redirect URLs, state tokens, or user-supplied labels, these values can become sources for injection if they are later used in Flask’s response mechanisms.

For example, if a Flask route retrieves a document containing a redirect_url field and uses it in a Location header via redirect(), an attacker who can influence the Firestore document can inject new header lines. This can lead to header manipulation, session fixation, or open redirects. The unauthenticated scan capabilities of middleBrick test such unauthenticated attack surfaces, including how data from external sources like Firestore flows into headers, which may reveal injection points.

Flask’s make_response and direct header assignment via response.headers do not inherently validate or encode values from external databases. If a Firestore document contains newline characters (e.g., %0a or encoded newline bytes) or other control characters, these can be used to inject additional headers. Because Firestore rules typically focus on read/write permissions rather than content validation, it is the application’s responsibility to treat all stored data as potentially untrusted when used in headers. The risk is compounded when the application uses Firestore metadata for security-sensitive headers such as X-Content-Type-Options or Strict-Transport-Security.

middleBrick’s 12 security checks, including Input Validation and Property Authorization, are designed to detect scenarios where external data reaches headers without proper sanitization. The scanner examines the unauthenticated attack surface and maps findings to frameworks such as OWASP API Top 10, highlighting risks like open redirects or response splitting that may originate from Firestore-backed data. Since Firestore rules do not sanitize content for header context, the scanner emphasizes reviewing how data is used rather than how it is stored.

To prevent Header Injection when using Firestore with Flask, you must validate and sanitize any Firestore field that reaches an HTTP header. This includes rejecting or encoding newlines, validating URLs strictly before redirects, and avoiding direct use of document fields in security-sensitive headers. Below are concrete, Firestore-aware remediation examples.

1. Safe redirect using a validated allowlist

Instead of using a Firestore field directly in a redirect, maintain an allowlist of permitted paths or use a signed token to reference intended destinations. If you must store a redirect URL in Firestore, validate it rigorously before use.

from flask import Flask, redirect, request
import re
from google.cloud import firestore

app = Flask(__name__)
db = firestore.Client()

# Approved domains for redirects
ALLOWED_REDIRECT_DOMAINS = {"app.example.com", "dashboard.example.com"}

def is_safe_url(url: str) -> bool:
    if not url:
        return False
    # Basic structural validation
    if not re.match(r"^https?://[a-zA-Z0-9.-]+(\/.*)?$", url):
        return False
    # Ensure the domain is in the allowlist
    domain = url.split("://")[1].split("/")[0]
    return domain in ALLOWED_REDIRECT_DOMAINS

@app.route("/go")
def go():
    doc_ref = db.collection("redirects").document("home")
    doc = doc_ref.get()
    if not doc.exists:
        return "Not found", 404
    target = doc.to_dict().get("url", "")
    if is_safe_url(target):
        return redirect(target, code=302)
    return "Invalid redirect target", 400

2. Sanitizing custom headers derived from Firestore

If you must set custom headers from Firestore values (for example, a x-request-tags header), ensure you strip or encode control characters and avoid values that contain newlines or colons that could enable injection.

from flask import Flask, make_response
from google.cloud import firestore

app = Flask(__name__)
db = firestore.Client()

def sanitize_header_value(value: str) -> str:
    # Remove newlines and carriage returns to prevent header injection
    return value.replace("\n", "").replace("\r", "").strip()

@app.route("/data")
def data():
    doc_ref = db.collection("config").document("headers")
    doc = doc_ref.get()
    response = make_response({"status": "ok"})
    if doc.exists:
        tags = doc.to_dict().get("tags", "")
        safe_tags = sanitize_header_value(tags)
        if safe_tags:
            response.headers["X-Document-Tags"] = safe_tags
    return response

3. Avoid using Firestore metadata in security headers

Do not use Firestore fields to set security headers such as Content-Security-Policy or Strict-Transport-Security. These should be defined explicitly in your Flask configuration. If dynamic values are required, derive them from authenticated and validated sources rather than directly from Firestore documents.