How to Safeguard JavaScript Apps from Untrusted Inputs

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I’m in charge of archiving paperwork for an office, and I’ve got this fancy new scanner that uploads documents directly into our database. It’s a real timesaver, but there’s a catch. Just like any system that deals with input, I have to be wary of untrusted documents—those that might contain hidden threats like viruses or incorrect information that could compromise our entire database.

In the world of JavaScript, templating engines are like my scanner. They take input—dynamic content—and integrate it into templates, creating seamless web pages. But here’s the twist: if this input is untrusted, it’s like accepting a suspicious document into my scanner. It could contain harmful scripts that might wreak havoc once processed.

To mitigate these risks, I imagine myself setting up a security checkpoint before the scanner. Each document, or piece of input, is thoroughly inspected. I use a combination of tools, like virus scanners and format validators, to ensure everything is clean and safe. In JavaScript, this is akin to sanitizing and escaping input, preventing malicious code from slipping through.

I also implement strict rules about what kind of documents can be scanned. Similarly, in templating engines, I define what inputs are acceptable, ensuring only trusted and validated data makes it into my web pages.

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Let’s say I’m working with a popular JavaScript templating engine like Handlebars.js. Here’s a simple example of how I ensure my inputs are sanitized:

const Handlebars = require('handlebars');

// Assume userInput is coming from an untrusted source
let userInput = "<script>alert('Hacked!');</script>";

// Handlebars automatically escapes the input
let template = Handlebars.compile("<div>{{userInput}}</div>");
let safeHTML = template({ userInput });

console.log(safeHTML); 
// Output: <div>&lt;script&gt;alert('Hacked!');&lt;/script&gt;</div>

In this example, Handlebars automatically escapes any HTML tags in userInput, rendering them harmless when inserted into the template. It’s like my scanner flagging and neutralizing potential threats before they cause any damage.

However, not every templating engine handles escaping automatically. In those cases, I must be proactive. Here’s how I might handle it with a different approach:

function sanitizeInput(input) {
    return input.replace(/&/g, "&")
                .replace(/</g, "&lt;")
                .replace(/>/g, "&gt;")
                .replace(/"/g, "&quot;")
                .replace(/'/g, "&#039;");
}

let unsafeInput = "<script>alert('Hacked!');</script>";
let safeInput = sanitizeInput(unsafeInput);

console.log(safeInput);
// Output: &lt;script&gt;alert('Hacked!');&lt;/script&gt;

In this function, I manually replace potentially dangerous characters with their safe HTML equivalents, ensuring that any script tags are not executed by the browser.

Key Takeaways:

1. Sanitization is Essential: Whether using built-in features or custom functions, always sanitize input to prevent script injection attacks.
2. Know Your Tools: Understand what your templating engine does automatically and where you need to add extra layers of security.
3. Consistent Vigilance: Just as I constantly check documents in my analogy, regularly review and update security practices to address new threats.