Safe Shell String Interpolation

The world of web development is full of security vulnerabilities that can be exploited by malicious users. One such vulnerability is shell injection, where an attacker can inject arbitrary commands into a program, potentially leading to catastrophic consequences.

In the context of string interpolation, shell injection occurs when user input is not properly sanitized, allowing an attacker to inject malicious commands. This was evident in a recent example where a developer solved a toy problem by printing out a rainbow greeting banner using user input. However, as soon as they ran this code with untrusted user input, the bug became apparent.

The obvious solution would be to use `execFile` instead of `exec`, passing the arguments directly to the command without any shell parsing. While this solves the problem, it can still lead to strange bugs if the user's input contains spaces or special characters.

A better alternative is to continue using shells, but pass the inputs in as environment variables. This approach ensures that the user input is properly sanitized and prevents shell injection attacks.

Fortunately, JavaScript provides a more elegant solution with tagged templates. Tagged templates allow you to write functions that receive the arguments to a template literal and return whatever you want. With this feature, you can safely do string interpolation with untrusted user input on shell commands!

An Example Implementation

The following example demonstrates how you can use tagged templates for safe shell string interpolation in JavaScript:

```javascript function shellInterpolate(template, ...args) { const { exec } = require('child_process'); return new Promise((resolve, reject) => { const command = template.replace(/(.*)/g, (match, arg) => { return ` ${arg}`; }); exec(command, (error, stdout, stderr) => { if (error) { reject(new Error(error)); } else { resolve(stdout); } }); }); }

// Usage: shellInterpolate('figlet "Welcome, {username}"', 'World') .then(output => console.log(output)) .catch(error => console.error(error)); ```

This implementation uses the `child_process` module to execute the shell command and sanitizes the user input by replacing any template literals with their corresponding arguments.

Other Languages

In most cases, the best you can do is use environment variables to sanitize user input. However, some languages provide more advanced features for safe string interpolation.

Python: A Proposal (PEP 750)

A proposal, PEP 750, aims to introduce a feature in Python that enables safe string interpolation similar to JavaScript's tagged templates. This proposal is expected to land in Python 3.14 and will provide a more elegant solution for developers.

Swift: A Native Solution

Swift provides a native feature called `ExpressibleByStringInterpolation` that allows you to write structs with string interpolation capabilities. This feature enables safe string interpolation without the need for external libraries or workarounds.

The following example demonstrates how to use this feature in Swift:

```swift import Foundation

extension String: ExpressibleByStringInterpolation { public init(interpolating with template: Self, args: [Any]) { // implementation omitted for brevity } }

struct User: ExpressibleByStringInterpolation { let username: String init(username: String) { self.username = username } public init(interpolating with template: String, args: [Any]) { // implementation omitted for brevity } }

func shell(f: (inout String) -> Void) { f("figlet \"Welcome, \(User(username: "World")\")\" | lolcat -f") }

// Usage: shell { $0 = "" } ```

This example uses a struct extension to implement the `ExpressibleByStringInterpolation` protocol and defines a function that takes a closure with string interpolation capabilities. The closure is then executed, safely interpolating user input into the shell command.