6 Critical Web Backend Security Risks and How to Avoid Them

As web developers, we store and process sensitive user data in our backends. This includes information like financial details, healthcare records, personal identifiers that require strict protections. Unfortunately, attackers are continuously probing for backend vulnerabilities to steal data and hijack user accounts.

In this comprehensive guide, we‘ll dive deep on the critical backend security risks you need to be aware of and how to avoid them in your code. I‘ll share techniques used by attackers along with code examples and expert-recommended protections. My goal is to equip you with the knowledge to build secure, hardened backends that protect your users and business from compromise.

What is the Backend and Why Secure It?

But first, what exactly is the backend? It‘s easier to secure what you understand:

Server-side code – This includes languages like Python, Ruby, Java, C# that contain the core application logic and interact with the database.
Database – Stores structured data like users, accounts, transactions, content. SQL databases like Postgres and MySQL are most common.
Application services – Functionality like authentication, notifications, payment processing, APIs, caching, rate limiting.
File storage – Where uploaded files like images, documents, etc. are stored. Often using cloud object storage.
Web server – The interface between the frontend and backend, handling request/response flow. Popular ones include Nginx, Apache, IIS.

The frontend is what users directly interact with in the browser, comprised of HTML, CSS, and JavaScript.

So why is backend security so crucial? The backend is where sensitive data is concentrated, processed, and managed. Any vulnerabilities can result in data loss, financial fraud, account takeovers, regulatory penalties, and reputational damages.

According to 2022 research from IBM:

The average data breach costs a company $4.35 million as of 2022, up 13% from 2021.
Over 40% of reported breaches originate from web application attacks like SQL injection or cross-site scripting.

Given the immense risks, locking down your backend is imperative. Next we‘ll explore the top backend attack vectors and proven methods to protect against them.

#1 SQL Injection

SQL injection has infamously topped the OWASP Top 10 list for over a decade. This attack enables malicious users to modify backend SQL queries by injecting new SQL syntax or operators.

For example, consider this Python backend code querying a database:

username = request.args.get(‘username‘) 
password = request.args.get(‘password‘)

query = f"SELECT * FROM users WHERE username = ‘{username}‘ AND password = ‘{password}‘"

An attacker can manipulate the password parameter to terminate the string and append a OR 1=1 condition:

‘ OR 1=1--

Resulting in:

SELECT * FROM users WHERE username = ‘amy‘ AND password = ‘‘ OR 1=1--‘

This allows the attacker to bypass authentication and gain unauthorized access. The query will always return all records.

How can you prevent SQL injection vulnerabilities?

Use prepared statements or stored procedures instead of dynamic SQL. These separate data from commands.
Carefully validate and sanitize all user input. Whitelist allowable characters if feasible.
Use the principle of least privilege – restrict database permissions. Read-only roles limit damage from successful attacks.
Adopt an Object Relational Mapper (ORM) like SQLAlchemy that provides abstraction from SQL.
Utilize static and dynamic application security testing tools that scan for SQL injection flaws.
Perform thorough code reviews focused on injection risks before deploying to production.

Let‘s secure our code above using prepared statements:

stmt = "SELECT * FROM users WHERE username = ? AND password = ?" 
params = (username, password)
result = db.execute(stmt, params)

This properly separates input data from the query logic, preventing injection.

According to Verizon‘s 2022 DBIR report, injection attacks like SQLi account for over 40% of reported breaches! But proper coding techniques prevent this devastating flaw.

#2 Broken Authentication

Authentication mechanisms are the first line of defense when securing our web applications. But flaws like weak credentials, brute forcing vulnerabilities, and poor session management place accounts at risk of takeover.

Some common authentication vulnerabilities include:

Permitting simple or default passwords through poor configuration.
Not rate limiting or locking accounts after repeated failed login attempts.
Relying solely on password authentication without multi-factor.
Storing passwords improperly without sufficient hashing or salting.
Failing to regenerate session IDs after login or privilege changes.

So how can we code defensively against authentication bypasses?

Enforce password complexity rules and minimum 12 character lengths.
Implement multi-factor authentication using an additional factor like biometrics or hardware keys.
Salt and hash stored credentials using bcrypt, scrypt or PBKDF2 with high work factors.
Require email verification before enabling new accounts.
Lock accounts for 15 minutes after 10 failed login attempts to deter brute forcing.
Regenerate session tokens after authentication and privilege changes.
Configure short session expiration times of 20-40 minutes.
Secure password recovery flows against enumeration and token cracking.

Let‘s look at some Python code to securely hash a password:

from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC

password = input("Please enter a password: ")
salt = os.urandom(16)
kdf = PBKDF2HMAC(algorithm=hashes.SHA256(), length=32, salt=salt, iterations=100000)
password_hash = kdf.derive(password.encode())

This uses PBKDF2 with a random salt and 100,000 iterations to compute a secure password hash. Proper credential storage and handling protects against many common authentication attacks.

#3 Broken Access Controls

Access controls restrict which users are allowed to access various application functions and data. Authentication tells you who a user is, access control tells you what they can do.

Some examples of access control schemes:

Role based access control (RBAC) – Permissions based on roles like admin, manager, guest.
Attribute based access control (ABAC) – Use attributes like time, location, department for dynamic control.
Mandatory access control (MAC) – Central authority determines controls, not users.

When access controls are misconfigured or missing, attackers can act outside their intended permissions. For example, an anonymous user accessing admin-only pages, or users viewing data they aren‘t authorized for.

Some common access control vulnerabilities include:

Forceful browsing – Guessing or brute forcing URLs, like incrementing IDs.
Insecure direct object references – Accessing objects via guessable parameters like /object?id=123.
Metadata manipulation – Tampering with fields like is_admin or user_id in requests.
CORS misconfiguration – Allows API access from unauthorized domains.

How can we code defensively to avoid broken access controls?

Deny all access by default, requiring explicit allowance.
Reconfirm permissions when accessing sensitive records, don‘t just check on initial page load.
Disable auto-increment primary keys and instead use UUIDs.
Enforce same access controls both server and client side.
Rate limit API requests to block automated probing for flaws.
Revoke user access tokens immediately after logout.

Here‘s some example Python code implementing access control:

# Import protection classes 
from permissions import admin_only, manager_only  

# Secure view by applying protections
@app.route(‘/reports‘)
@admin_only 
def sales_reports():
   # return reports

@app.route(‘/accounts‘)
@manager_only
def view_accounts():
   # return account data

Leveraging frameworks and libraries designed for access control makes implementation far easier.

#4 Sensitive Data Exposure

Web applications frequently mishandle sensitive data by not adequately protecting it in transit and at rest. Attackers love targeting these juicy stores of personal and financial information.

Some common data exposure risks include:

Failing to encrypt sensitive data in transit over the network or at rest in databases/storage.
Using weak or insecure cryptographic algorithms and insecure key storage.
Unnecessary retention of sensitive data without data minimization policies.
Detailed error handling that leaks system details, like stack traces.
Browser caching and autocomplete that expose private user data.

So how do we properly protect sensitive data?

Encrypt data in transit using TLS 1.2+ and use HTTPS exclusively.
Encrypt data at rest according to a defined policy – encrypt entire databases, or fields like SSN, passwords, history.
Hash or encrypt credit card numbers following industry standards like PCI DSS.
Adopt key management best practices on key generation, storage, rotation.
Scrub error handling of any stack traces or overly detailed messages.
Follow data minimization principles – delete when no longer required, anonymize where possible.
Evaluate browser protections – disable autocomplete on sensitive fields, secure cookies.

Here‘s some sample code implementing TLS web encryption in Node.js:

const tls = require(‘tls‘);
const fs = require(‘fs‘);

const options = {
  key: fs.readFileSync(‘key.pem‘),
  cert: fs.readFileSync(‘cert.pem‘)
};

https.createServer(options, function (req, res) {
  // Secure traffic only
}).listen(8000);

Enforcing encryption, key management, and data minimization provides multilayered protection against catastrophic data exposures.

#5 Security Misconfiguration

Flawed configurations are a common cause of breaches. Attackers will probe for weaknesses like exposed admin panels, verbose error handling, outdated software, and more.

Some examples include:

Unpatched frameworks and dependencies with disclosed vulnerabilities.
Unprotected admin panels, debug modes, or sensitive endpoints exposed.
Overly verbose error handling disclosing system details.
Missing security headers and protections – CORS, XSS, CSRF, referrer policies, etc.

How can we prevent security misconfigurations?

Establish and enforce hardened configuration baselines for all systems.
Harden OS permissions, ports, services, application defaults.
Continuously scan for misconfigurations using SAST, DAST tools.
Promptly patch systems when vulnerabilities are reported.
Scrub errors of sensitive details and implement sane request logging.
Remove unused artefacts like debug modes, commented out code and configuration before release.
Load and stress test to surface any flaws.

Here‘s a security misconfiguration exposed in Nginx:

server {
  listen 80 default_server;
  server_name example.com;

  # Flawed - allows directory listings
  autoindex on;  

  location / {
    root /var/www/example.com; 
  }
}

Disabling directory listings protects against leaking sensitive files:

  autoindex off;

Regular audits and patching establishes defense in depth against missteps in config.

#6 Cross-Site Scripting (XSS)

Cross-site scripting continues to rank highly on OWASP Top 10. XSS flaws enable attackers to inject malicious JavaScript into vulnerable pages viewed by victims.

Reflected XSS involves injecting malicious code into URL parameters that gets reflected unsanitized in the HTML response. Stored XSS involves persistent JavaScript getting inserted into databases that later renders in victim browsers.

Some key defenses include:

Validating, sanitizing, and escaping all untrusted data before outputting – especially from inputs like forms.
Enabling XSS protections in frameworks like React, Angular, and Vue.
Enforcing Content Security Policy and disabling eval() which executes arbitrary JavaScript.
Limiting use of innerHTML and DOM manipulating functions which can introduce XSS if unescaped.
Scrubbing all user-controllable content of unapproved HTML tags, encoded characters like <script>, <img src=x onerror=alert(1)> etc.
Setting conservative content-type headers like application/json which prevents embedded HTML.

Let‘s properly sanitize user input in our code to prevent XSS vulnerabilities:

from html import escape

@app.route(‘/comment‘) 
def post_comment():
  comment = request.args.get(‘comment‘) 

  # Escape on output to prevent XSS
  sanitized_comment = escape(comment)

  return ‘‘‘<div>‘‘‘ + sanitized_comment + ‘‘‘</div>‘‘‘

Escaping and validating untrusted input defangs XSS injection before it can occur.

In Summary

We covered the 6 most critical risks plaguing web application backends – SQL injection, authentication flaws, access control misconfigurations, data exposure, security missteps, and cross-site scripting.

While this list is not exhaustive, following secure coding best practices around these risks will drastically increase your defensive posture:

Validate and sanitize all user-controllable input and output.
Secure credentials and sessions, implement role-based access controls.
Encrypt sensitive data end to end. Reduce unnecessary data retention.
Harden configurations, patch frequently, disable verbose modes.
Scan continuously for vulnerabilities, code review before release.

No application will be 100% secure, but adopting these backend security principles will help protect your data, users and organization from threats. Implement these tips today to start building more hardened, defensible backends.