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Secure Coding & OWASP Top 10 for Backend · Aula

Content Security Policy (CSP) para o back-end

Compreenda como as configurações do back-end podem influenciar a Content Security Policy (CSP) para mitigar ataques no lado do cliente, como XSS.

Content Security Policy (CSP) para o back-end é uma aula grátis de Secure Coding & OWASP Top 10 for Backend no CoddyKit. Esta é a aula 3 de 4. Você pode ler a aula completa abaixo gratuitamente — depois pratica ao vivo no navegador com um editor de código integrado e um tutor de IA 24/7. Faz parte do caminho de aprendizado de Secure Coding & OWASP Top 10 for Backend, e seu progresso é sincronizado entre a web e o app CoddyKit. O curso de Secure Coding & OWASP Top 10 for Backend inclui 4 aulas no total.

Partes desta aula ainda não foram traduzidas e aparecem em inglês.

Backend's Role in CSP

Welcome to Content Security Policy (CSP) for Backend! You might think CSP is just for frontend developers, but the backend plays a crucial role.

CSP is a security standard that helps prevent client-side attacks like Cross-Site Scripting (XSS). It does this by telling the browser which resources (scripts, styles, images) it's allowed to load and execute.

The backend is responsible for delivering these rules to the browser.

How Backend Delivers CSP

The backend delivers CSP rules to the browser using a special HTTP response header called Content-Security-Policy. When the browser receives this header, it enforces the rules defined within it.

This means your backend application directly controls the security policy for your frontend. Let's see how a backend might set this header.

Setting the CSP Header

In a backend application, you'd typically add the CSP header to your HTTP response. This example simulates a backend sending a basic CSP header.

The default-src 'self' directive allows resources only from the same origin as the document.

public class BackendCspExample {
  public static void main(String[] args) {
    System.out.println("HTTP/1.1 200 OK");
    System.out.println("Content-Type: text/html");
    System.out.println("Content-Security-Policy: default-src 'self';");
    System.out.println("");
    System.out.println("<!-- Your secure HTML content goes here -->");
  }
}

Key CSP Directives for Backend

As a backend developer, you'll often define directives that control various resource types. Some common ones include:

  • script-src: Specifies valid sources for JavaScript.
  • style-src: Specifies valid sources for stylesheets.
  • img-src: Specifies valid sources for images.
  • connect-src: Restricts URLs that can be loaded using scripting interfaces (e.g., AJAX, WebSockets).

Each directive can have multiple allowed sources, like 'self', https://example.com, or 'unsafe-inline' (generally to be avoided).

Mitigating Inline Scripts with Nonces

One common XSS attack vector is injecting inline scripts. CSP can block these, but sometimes inline scripts are necessary.

The backend can generate a unique, cryptographically secure nonce (Number Used Once) for each request. This nonce is added to both the CSP header and the allowed inline <script> tags.

Backend Nonce Generation

Here's how a backend can generate a nonce. This nonce is then included in the Content-Security-Policy header (e.g., script-src 'nonce-YOUR_NONCE_HERE') and rendered into the HTML script tag.

The browser will only execute inline scripts that have a matching nonce attribute.

import java.util.Base64;
import java.security.SecureRandom;

public class NonceGenerator {
  public static void main(String[] args) {
    SecureRandom random = new SecureRandom();
    byte[] nonceBytes = new byte[16]; // 16 bytes for a good nonce
    random.nextBytes(nonceBytes);
    String nonce = Base64.getEncoder().encodeToString(nonceBytes);
    System.out.println("Generated Nonce: " + nonce);
    System.out.println("\nUse this in your CSP header:");
    System.out.println("Content-Security-Policy: script-src 'self' 'nonce-" + nonce + "';");
    System.out.println("\nAnd in your HTML:");
    System.out.println("<script nonce=\"" + nonce + "\">alert('Hello!');</script>");
  }
}

CSP Reporting: `report-to`

CSP isn't just about blocking; it's also about visibility. The backend can specify a reporting endpoint using the report-to directive (or older report-uri).

If a browser violates the CSP (e.g., tries to load a script from an unauthorized source), it will send a JSON report to this backend endpoint. Your backend can then log and analyze these reports to detect potential attacks or policy misconfigurations.

Integrating CSP with Frameworks

Modern backend frameworks often provide convenient ways to manage CSP headers without manually concatenating strings.

  • Spring Security (Java): Has dedicated configurations for HTTP security headers, including CSP.
  • Helmet (Node.js/Express): A middleware that helps secure Express apps by setting various HTTP headers, including CSP.
  • Django (Python): Can set CSP headers via middleware or specific libraries.

These tools simplify implementation and help ensure best practices.

CSP for XSS Prevention (Backend View)

From a backend perspective, correctly configured CSP adds a powerful layer of defense against XSS. Even if an attacker manages to inject malicious content into your HTML, CSP can prevent the browser from executing it.

By controlling the Content-Security-Policy header, your backend dictates what content is safe, significantly reducing the impact of client-side vulnerabilities.

CSP Header Challenge

Consider a backend service that needs to allow scripts only from its own domain and from cdn.example.com. Which CSP header directive would best achieve this?

Recap: Backend & CSP

You've learned that Content Security Policy (CSP) is a critical security layer delivered by the backend via the Content-Security-Policy HTTP header.

  • Backend sets CSP headers to control resource loading.
  • Directives like script-src define allowed sources.
  • Nonces are backend-generated tokens to safely allow specific inline scripts.
  • Backend can collect violation reports via report-to.
  • Frameworks simplify CSP implementation.

By actively managing CSP, backend developers significantly enhance their application's defense against client-side attacks like XSS.

Perguntas Frequentes

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Todas as aulas deste curso

  1. Técnicas avançadas de SQLi e NoSQLi
  2. Estratégias abrangentes de validação de dados de entrada
  3. Content Security Policy (CSP) para o back-end
  4. Prevenção de injeção de comandos e LDAP
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