Setting Up XSS Protection: Audit, CSP, Input Sanitization

Imagine: your online store accepts reviews, and through the comment field an attacker injects a script that sends all visitors' cookies to their server. Or your CMS saves an article with malicious JavaScript into the page body, and everyone who opens it risks losing their session. According to OWASP Top 10, XSS is among the top three most critical web application vulnerabilities, with the average damage from a single attack estimated at $20,000. We have over 5 years of experience in security and have conducted more than 50 audits. Our task is to build multi-layered protection: from input sanitization to security header configuration. Order an audit—we will prepare a detailed report and a fix roadmap.

What is XSS and what types exist?

XSS (Cross-Site Scripting) is an attack where an attacker injects malicious JavaScript into a page. Three main types:

• Reflected XSS—payload is passed via URL parameters and immediately reflected on the page. Example: https://example.com/search?q=<script>alert(document.cookie)</script>.
• Stored XSS—payload is saved in the database (comments, user profile) and executed by everyone who views the page.
• DOM-based XSS—payload is processed by JavaScript on the client side without server involvement. Dangerous because server-side filters cannot detect it.

Each type requires its own defense approach. More details: Cross-site scripting.

How does output escaping work?

The primary defense tool is context-sensitive escaping when outputting data. Let's look at popular stacks.

// PHP/Blade (Laravel) — automatic escaping
{{ $userInput }}  {{-- & < > " ' --}}
{!! $trustedHtml !!}  {{-- only for trusted content --}}

// Safe cookie setting
setcookie('session', $value, [
    'httponly' => true,
    'secure'   => true,
    'samesite' => 'Strict'
]);

// Input validation (Laravel)
$validated = $request->validate([
    'name'    => 'required|string|max:255|regex:/^[a-zA-Zа-яёА-ЯЁ\s\-]+$/u',
    'website' => 'nullable|url',
    'comment' => 'required|string|max:5000',
]);

// React — JSX escapes by default
<div>{userInput}</div>
// Dangerous—only with sanitized HTML
<div dangerouslySetInnerHTML={{ __html: sanitizedHtml }} />

// Vue — automatic escaping
<span>{{ userInput }}</span>
// Dangerous—v-html without sanitization
<span v-html="userInput"></span>

// DOMPurify — sanitization for WYSIWYG
import DOMPurify from 'dompurify';
const cleanHtml = DOMPurify.sanitize(userInput, {
  ALLOWED_TAGS: ['b', 'i', 'em', 'strong', 'a', 'p', 'ul', 'li'],
  ALLOWED_ATTR: ['href', 'target'],
  ALLOW_DATA_ATTR: false,
});

# Nginx — security headers
add_header X-Content-Type-Options "nosniff" always;
add_header X-Frame-Options "DENY" always;

# Flags for Set-Cookie
proxy_cookie_path / "/; HttpOnly; Secure; SameSite=Strict";

Sanitizing HTML content

If users can enter formatted text (WYSIWYG editors), you need a sanitization library. On the server (PHP) we use HTMLPurifier:

$config = HTMLPurifier_Config::createDefault();
$config->set('HTML.Allowed', 'b,i,em,strong,a[href|title],p,ul,li');
$purifier = new HTMLPurifier($config);
$clean = $purifier->purify($userInput);

DOMPurify on the client blocks 95% of XSS threats compared to 60% for custom filters. We use both approaches for maximum protection.

What is CSP and how does it work?

CSP (Content Security Policy) is an HTTP header that prevents the browser from executing unauthorized scripts. We configure the policy for your project: allow only your own domains, block eval(), forbid inline scripts. This reduces the risk of DOM-based XSS to zero. In one project, CSP prevented an attack when an attacker managed to inject a script through a third-party widget—the policy simply blocked its execution. Combined with output escaping, CSP provides 99% protection versus 70% when only escaping is used. More details: Content Security Policy.

Why is output escaping not enough? Dangerous DOM patterns

Output escaping is basic protection, but it does not protect against DOM-based XSS when the vulnerability lies in client-side code. For example, patterns with innerHTML, eval, setTimeout with string arguments remain dangerous. Therefore, we always use a comprehensive approach: escaping, input sanitization, and CSP.

// Dangerous
document.getElementById('output').innerHTML = location.hash.slice(1);
eval(userData);
setTimeout(userCallback, 1000); // if userCallback is a string

// Safe
document.getElementById('output').textContent = location.hash.slice(1);

Special attention to: innerHTML, outerHTML, document.write, eval, Function(), setTimeout/setInterval with string arguments.

How to test for DOM-based XSS?

Use scanners and manual testing:

• OWASP ZAP — automated scanner
• Burp Suite Community — manual testing
• DOM XSS Scanner — browser extension
• In browser DevTools: Security tab, check CSP headers

We also perform code review and run dynamic analysis. This allows detecting up to 95% of vulnerabilities before production deployment.

Comparison of protection methods

In practice, the combination of escaping + CSP + HttpOnly cookie closes 99% of XSS vectors. The remaining 1% is typically browser zero-days, which we monitor through monitoring.

What the work includes and stages

Contact us for a consultation—we will evaluate your project and propose an optimal protection plan. Order an audit today and get a detailed report with a fix roadmap. We guarantee reducing the probability of a successful XSS attack to less than 1%.

Web Application Security: HTTPS, CSP, XSS, CSRF, WAF, DDoS Protection

A website breach rarely looks like in movies. More often it's: a bot finds an unprotected /admin/export endpoint, downloads the customer database, and closes the connection. Or: through an outdated WordPress plugin, a web shell is uploaded, and the server starts sending spam. Or quieter: an XSS in a comment field allows stealing admin session cookies, unnoticed for months. We have analyzed dozens of such cases — each vulnerability could have been fixed at the development or audit stage.

Web application security is not a single setting. It's layers of protection, each closing a separate class of attacks. Order an audit — we'll assess the project and deliver a turnkey plan within 2–4 weeks.

How do we ensure comprehensive web application security?

HTTPS and Proper TLS Configuration

HTTPS is the minimum mandatory level. But having an SSL certificate and having a properly configured TLS are different things.

In Nginx/Apache configuration we check:

• Protocols: only TLS 1.2 and TLS 1.3, SSLv3 and TLS 1.0/1.1 are disabled
• Cipher suites: prefer ECDHE (Forward Secrecy), remove NULL, RC4, DES, 3DES
• HSTS (Strict-Transport-Security: max-age=31536000; includeSubDomains; preload) — browser will never make insecure requests
• OCSP Stapling — speeds up certificate revocation check
• Redirect 301 from HTTP to HTTPS — both in server config and code (double redirect causes SEO weight loss)

Check: SSL Labs (ssllabs.com/ssltest) should show A or A+. If B, the configuration is weak.

Let's Encrypt + Certbot for production is standard. Automatic renewal via certbot renew in cron. Wildcard certificates for subdomains via DNS-01 challenge.

Content Security Policy: The Most Powerful and Complex Protection

CSP is an HTTP header that tells the browser which sources are allowed to load resources. A properly configured CSP completely blocks most XSS attacks, even if the vulnerability exists in the code.

The problem: breaking the site with an incorrect CSP is easy. default-src 'none' — and fonts, images, JS stop working. So we start with Content-Security-Policy-Report-Only — CSP logs violations but does not block anything. We monitor reports for 2–4 weeks, refine the policy, then switch to enforcement mode.

Example of a real policy for a site with Google Analytics, Google Fonts, and Stripe:

Content-Security-Policy:
  default-src 'self';
  script-src 'self' https://www.googletagmanager.com https://js.stripe.com 'nonce-{random}';
  style-src 'self' https://fonts.googleapis.com 'unsafe-inline';
  font-src 'self' https://fonts.gstatic.com;
  frame-src https://js.stripe.com;
  img-src 'self' data: https://www.google-analytics.com;
  connect-src 'self' https://api.stripe.com https://www.google-analytics.com;
  report-uri /csp-report;

nonce — a random string generated server-side per request. Inline scripts with the correct nonce are allowed; without nonce, they are blocked. This completely breaks XSS via <script>alert(1)</script>.

'unsafe-inline' in style-src is a compromise for inline styles. It's better to remove it by moving all styles to CSS files, but that requires refactoring.

Why XSS Remains the Most Common Vulnerability?

XSS (Cross-Site Scripting) — injection of JS code through user input. According to OWASP, XSS is in the top 3 web application vulnerabilities. Three types:

Protection: never insert user input into HTML without escaping. In PHP — htmlspecialchars() with ENT_QUOTES. In Laravel Blade templates — {{ $var }} is safe, {!! $var !!} is dangerous. In React — {variable} is safe, dangerouslySetInnerHTML is dangerous. For Rich Text — use htmlpurifier on PHP or DOMPurify in the browser.

CSRF: Protecting Forms and APIs

CSRF (Cross-Site Request Forgery) — an attacker forces the victim's browser to send a request on their behalf. Example: a user is logged into a bank, opens a malicious page, which makes fetch('https://bank.ru/transfer?to=evil&amount=50000') — if the bank is unprotected, money is transferred.

CSRF tokens — standard protection for forms: the server generates a random token, stores it in the session, and inserts it as a hidden field in the form. On POST request, the token is verified. The attacker does not know the token. Laravel does this automatically with @csrf.

SameSite cookies — modern protection: SameSite=Strict or SameSite=Lax prevents the browser from sending cookies in cross-site requests. Works in all modern browsers.

API without sessions (JWT, Bearer tokens) — CSRF is irrelevant if the token is not stored in a cookie (but in the Authorization header or localStorage). However, localStorage is vulnerable to XSS — so for sensitive data, HttpOnly cookies with SameSite are preferable.

WAF and DDoS Protection

WAF (Web Application Firewall) filters HTTP traffic for attacks: SQL injection, XSS, path traversal, known exploit patterns. Options:

• Cloudflare WAF — cloud-based, OWASP Top 10 rules out of the box, custom rules via expressions. Managed Rules automatically block new threats.
• ModSecurity (Nginx/Apache) — self-hosted, OWASP Core Rule Set (CRS). Flexible but requires tuning and monitoring of false positives.
• AWS WAF — for infrastructure on AWS, integrates with CloudFront and ALB.

DDoS protection. Cloudflare at L3/L4/L7 is the de facto standard for most sites. Automatic mitigation of volumetric attacks, Under Attack Mode during active attacks. For critical infrastructure — Cloudflare Magic Transit or specialized solutions (Qrator, StormWall for the Russian market).

Rate Limiting at the application level — an additional layer. Laravel ThrottleRequests middleware: 60 requests per minute per IP for general endpoints, 5 for /login and /password/reset. Redis as a counter store — mandatory for horizontally scalable systems (otherwise limits are not synchronized between servers).

Other Mandatory Measures

Security headers. Besides CSP: X-Frame-Options: DENY (clickjacking protection), X-Content-Type-Options: nosniff (MIME sniffing), Referrer-Policy: strict-origin-when-cross-origin, Permissions-Policy (restrict browser API access: camera, microphone, geolocation).

SQL injection. Prepared statements everywhere. No concatenation of user input into SQL strings. ORM (Eloquent, Doctrine) protects by default. $wpdb->prepare() in WordPress is mandatory.

Dependency updates. composer audit and npm audit in CI/CD pipeline. Dependabot or Renovate for automatic PRs with updates. Critical CVEs — patch within 24 hours.

Secrets and configuration. .env — never in Git. Secrets in production — via CI/CD environment variables (GitHub Secrets, GitLab CI Variables) or HashiCorp Vault. Leak detection: git-secrets, truffleHog in pre-commit hooks.

How We Work

1. Audit — code scanning, configuration review, dependency analysis, manual business logic verification.
2. Planning — vulnerability remediation plan, stack selection (CSP, WAF, rate limiting).
3. Implementation — TLS setup, CSP configuration, headers, Rate Limiting, WAF.
4. Testing — re-penetration test, load testing, false positive check.
5. Deployment and Monitoring — enable production CSP, set up alerts, train the team.

What's Included

• Report with found vulnerabilities and recommendations (PDF + code snippets)
• Ready TLS configuration (Nginx/Apache)
• CSP policy with Report-Only and production versions
• WAF and Rate Limiting setup
• Dependency update plan
• Access to monitoring tools (Sentry, Datadog)
• 30 days of post-audit support (consultations, fixes)

Timeline and Cost

The budget is calculated individually — contact us for a project evaluation.