保護 Web 應用免遭未經授權的 JavaScript 執行的頂級技術

Refer to the following code example.

<script src="https://example.com/script.js" integrity="sha384-oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxqAtD6x48V2aB1xzA7e2h53sF2aAuM" crossorigin="anonymous"></script>

In this example, the integrity attribute contains the hash, and the crossorigin=”anonymous” attribute ensures the resource is fetched with CORS (cross-origin resource sharing).

You can use SRI for stylesheets, as well.

<link rel="stylesheet" href="https://example.com/styles.css" integrity="sha384-oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxqAtD6x48V2aB1xzA7e2h53sF2aAuM" crossorigin="anonymous">

4. Secure JavaScript coding practices

Secure JavaScript coding practices are crucial for developing web apps robust against various attacks, XSS, and other malicious exploits. By following these best practices, developers can ensure their code is secure, maintainable, and less vulnerable to unauthorized execution.

Avoid using eval()

The eval() function is a significant security risk, as it executes a string of code, potentially allowing attackers to inject malicious scripts. Always avoid using eval() and similar functions like setTimeout(string) and setInterval(string).

Why these functions are dangerous:

• Arbitrary code execution: These functions can execute any code passed to them as a string. If an attacker successfully inserts a malicious string, it will operate in the same way as the remaining code of your script.
• Difficulty in code analysis: Using these functions makes it harder to analyze the code for security vulnerabilities. Static analysis tools cannot examine the strings that are passed through such functions.
• Dynamic code injection: Attackers can use these functions to inject and execute code dynamically that was not originally part of the app, bypassing traditional security measures.

Use strict mode

Enabling strict mode in JavaScript helps catch common coding mistakes and unsafe actions, such as assigning values to undeclared variables. This improves the security and stability of your code. To enable strict mode, add “use strict”; at the beginning of a script or a function.

"use strict";

function safeFunction() {
    // Code in strict mode.
    let secureVariable = "Secure";
    console.log(secureVariable);
}

safeFunction();

Advantages and implications of enabling strict mode:

• In strict mode, this is undefined in functions that are not called methods.
• Strict mode will throw an error if a function has duplicate parameter names or an object literal has duplicate property names.
• A with statement is not allowed in the strict mode because it makes code difficult to predict and optimize.

Refer to the following code example.

"use strict";

// Eliminates this coercion.
function showThis() {
    console.log(this); // In non-strict mode, this would be the global object; in strict mode, it's undefined.
}
showThis();

// Disallows duplicate property names or parameter values.
// This will throw an error in strict mode.
const obj = {
    prop: 1,
    prop: 2
};

// Prevents the use of with statement.
// This will throw an error in strict mode.
with (Math) {
    let x = cos(3.14);
}

Avoid inline JavaScript

Inline JavaScript can be significantly vulnerable to XSS attacks because it allows attackers to inject malicious scripts directly into your HTML. Instead, use external scripts to ensure all JavaScript is properly vetted and sanitized.

Avoid inline JavaScript because of:

• Ease of injection: Inline JavaScript is more susceptible to injection attacks because it is part of the HTML content.
• CSP compliance: Content security policies (CSP) can be more effectively enforced when JavaScript is kept in external files. Inline scripts often require the use of the unsafe-inline directive, which weakens CSP’s effectiveness.
• Maintainability: Keeping JavaScript in separate files makes the codebase easier to manage and maintain.

Refer to the following code example.

<!-- Insecure Inline JavaScript -->
<!-- <button onclick="alert('Clicked!')">Click Me</button> -->

<!-- Secure External JavaScript -->
<button id="secureButton">Click Me</button>
<script>
    document.getElementById('secureButton').addEventListener('click', function() {
        alert('Clicked!');
    });
</script>

5. Regular Security Audits and Updates

Regular audits are essential for maintaining the integrity and security of web apps. By continuously assessing your app’s security, you can identify and fix vulnerabilities that could be exploited to execute unauthorized JavaScript or other malicious actions.

如何進行定期安全審計

自動安全掃描

使用 OWASP ZAP 或 Burp Suite 等工具掃描已知漏洞。自動掃描提供了識別常見安全問題的快速方法。

手動程式碼審查

定期手動檢查您的程式碼庫，以發現自動化工具可能遺漏的問題。為此，最好使用經驗豐富的開發人員和安全專家。

滲透測試

聘請滲透測試人員模擬對您的應用程式的攻擊，發現其他方法可能無法偵測到的漏洞。

更新依賴項

保持您的依賴更新，以修復庫和框架中的已知漏洞。使用 NPM 或 pip 等套件管理器來管理更新。

安全訓練

持續訓練您的開發團隊了解最新的安全實務和常見漏洞。這將確保您的團隊有能力編寫安全的程式碼。

結論性想法

感謝您閱讀這篇文章。我們希望這 5 種技術能夠增強您的應用程式對未經授權的 JavaScript 執行的防禦能力。透過實施這些策略，您可以降低攻擊風險並確保為使用者提供更安全的 Web 應用程式。請記住，在安全措施方面保持主動和警惕是保護您的數位資產的關鍵。

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