Crlf Injection in Flask with Cockroachdb

Crlf Injection occurs when an attacker can inject CRLF sequences (\r\n) into HTTP headers or other protocol boundaries. In a Flask application that uses Cockroachdb as the backend database, the risk arises when user-controlled input is reflected into headers, cookies, or redirect URLs without proper sanitization before the response is committed. Flask’s development server and common WSGI setups do not inherently sanitize these inputs, and Cockroachdb—while secure—does not protect against application-layer header manipulation.

Consider a Flask route that reads a redirect target or a UI theme from a Cockroachdb row and uses it directly in a response header:

import sqlite3
from flask import Flask, request, make_response

app = Flask(__name__)

@app.route('/redirect')
def redirect_user():
    target = request.args.get('target', '/home')
    # Example: fetch a user preference for redirect from Cockroachdb
    conn = sqlite3.connect('/cockroach/cockroach-data/attrs.db')
    cur = conn.cursor()
    cur.execute('SELECT redirect_url FROM user_prefs WHERE user_id = ?', (request.cookies.get('user_id'),))
    row = cur.fetchone()
    if row:
        target = row[0]
    resp = make_response('Redirecting...')
    resp.headers['Location'] = target  # Crlf Injection risk if target contains \r\n
    resp.set_cookie('landing', target)  # Also vulnerable if target is reflected here
    return resp

If an attacker controls the target query parameter or can influence the Cockroachdb-stored redirect_url, they can inject \r\n sequences. For example, https://api.example.com/redirect?target=%0D%0ASet-Cookie:%20session=evil can append a new cookie or header, leading to session fixation or header smuggling. Cockroachdb stores the untrusted value as-is; the vulnerability is in how Flask uses that value in protocol-sensitive headers without validation or encoding.

Another scenario involves HTTP response splitting via cookies or custom headers when data from Cockroachdb is reflected. For instance, a Flask route that renders a user’s display name (stored in Cockroachdb) into a cookie without sanitization:

@app.route('/profile')
def profile():
    conn = sqlite3.connect('/cockroach/cockroach-data/profiles.db')
    cur = conn.cursor()
    cur.execute('SELECT display_name FROM users WHERE id = ?', (request.cookies.get('uid'),))
    name = cur.fetchone()[0]
    resp = make_response(f'Hello {name}')
    resp.set_cookie('display', name)  # If name contains \r\n, it can inject headers
    return resp

In both cases, Cockroachdb is merely the data source; the risk stems from treating stored data as safe for protocol-level placement. Attack patterns like CVE-2023-30861 illustrate how header injection can bypass security controls when frameworks do not enforce strict header validation.

Remediation focuses on ensuring that any data sourced from Cockroachdb—and any other user input—is sanitized before being used in HTTP headers, cookies, or redirects. Use strict allowlists, avoid reflecting untrusted data into protocol-sensitive locations, and prefer framework helpers that handle encoding.

1. Validate and restrict redirect targets

Do not use raw values from Cockroachdb for redirects. Instead, maintain an allowlist or validate against a safe base URL:

from urllib.parse import urlparse, urljoin
from flask import Flask, request, make_response, redirect

app = Flask(__name__)

def is_safe_url(url, allowed_hosts):
    parsed = urlparse(url)
    return parsed.scheme in ('http', 'https') and parsed.hostname in allowed_hosts

@app.route('/redirect')
def safe_redirect():
    target = request.args.get('target', '/home')
    # Fetch from Cockroachdb with parameterized query
    conn = sqlite3.connect('/cockroach/cockroach-data/attrs.db')
    cur = conn.cursor()
    cur.execute('SELECT redirect_url FROM user_prefs WHERE user_id = ?', (request.cookies.get('user_id'),))
    row = cur.fetchone()
    if row:
        candidate = row[0]
    else:
        candidate = target
    # Ensure the URL is safe before redirect
    if is_safe_url(candidate, allowed_hosts={'api.example.com', 'app.example.com'}):
        return redirect(candidate, code=307)
    return redirect('/home', code=307)

2. Encode or avoid reflecting data into cookies and headers

When setting cookies or headers using Cockroachdb values, ensure they do not contain CRLF sequences. Use frameworks’ built-in encoding and avoid direct reflection:

@app.route('/profile')
def safe_profile():
    conn = sqlite3.connect('/cockroach/cockroach-data/profiles.db')
    cur = conn.cursor()
    cur.execute('SELECT display_name FROM users WHERE id = ?', (request.cookies.get('uid'),))
    name = cur.fetchone()[0]
    resp = make_response(f'Hello {name}')
    # Use only safe characters for cookie values; avoid raw reflection
    safe_name = name.replace('\r', '').replace('\n', '')
    resp.set_cookie('display', safe_name, httponly=True, samesite='Lax')
    return resp

3. Use framework-level protections and input validation

Leverage Flask’s werkzeug utilities and request validation libraries to enforce constraints on headers and cookies. For Cockroachdb-stored data, treat all fields as untrusted and apply context-specific escaping:

from werkzeug.http import dump_cookie

@app.route('/custom-cookie')
def custom_cookie():
    conn = sqlite3.connect('/cockroach/cockroach-data/cookies.db')
    cur = conn.cursor()
    cur.execute('SELECT value FROM settings WHERE key = ?', ('theme',))
    theme = cur.fetchone()[0]
    # dump_cookie handles encoding and prevents CRLF injection
    header_value = dump_cookie(theme, key='theme')
    resp = make_response('Cookie set')
    resp.headers['Set-Cookie'] = header_value
    return resp

These patterns ensure that even if Cockroachdb contains malicious payloads, they are neutralized before reaching the HTTP layer. Combine these practices with runtime scanning using tools like middleBrick to detect header-injection findings and map them to OWASP API Top 10 and compliance frameworks.