Common Node.js security vulnerabilities in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the main risk of using unsanitized user input in Node.js?

Consider a Node.js app that directly uses user input in database queries without cleaning it. What is the main security risk?

A Cross-Site Scripting (XSS) attack

B Man-in-the-Middle (MitM) attack

C SQL Injection attack

D Denial of Service (DoS) attack

💡 Hint

Think about attacks that manipulate database commands.

Using unsanitized input in database queries can allow attackers to inject malicious SQL commands, known as SQL Injection.

❓ component_behavior

intermediate

What happens if you use eval() on user input in Node.js?

Given a Node.js server that runs eval(userInput), what is the likely security consequence?

A It can lead to remote code execution vulnerabilities

B It safely executes user code without risks

C It prevents injection attacks automatically

D It only runs code in a sandboxed environment

💡 Hint

Consider what eval() does with arbitrary code strings.

Using eval() on user input allows attackers to run any code on the server, causing remote code execution risks.

📝 Syntax

advanced

Which code snippet properly prevents prototype pollution in Node.js?

Prototype pollution happens when attackers modify object prototypes. Which snippet blocks this?

A obj['__proto__'] = value; // directly assign

B delete obj['constructor']; obj[key] = value;

C Object.assign(obj, userInput); // blindly assign

D if (key === '__proto__') throw new Error('Pollution attempt'); obj[key] = value;

💡 Hint

Check which option blocks dangerous keys before assignment.

Checking for '__proto__' and throwing an error prevents prototype pollution by blocking prototype modification.

🔧 Debug

advanced

Why does this Node.js code cause a denial of service (DoS)?

Examine the code below:

const http = require('http');
http.createServer((req, res) => {
  while(true) {}
  res.end('Hello');
}).listen(3000);

What is the problem?

Node.js

const http = require('http');
http.createServer((req, res) => {
  while(true) {}
  res.end('Hello');
}).listen(3000);

A The server leaks memory causing slowdowns

B The infinite loop blocks the event loop causing DoS

C The server crashes due to missing response headers

D The server responds with wrong content type

💡 Hint

Think about what an infinite loop does in Node.js single-threaded model.

The infinite loop blocks the event loop, preventing any other requests from being handled, causing denial of service.

❓ state_output

expert

What is the output of this Express.js middleware chain with a missing next() call?

Consider this Express.js code:

const express = require('express');
const app = express();

app.use((req, res, next) => {
  console.log('First middleware');
  // missing next()
});

app.use((req, res, next) => {
  console.log('Second middleware');
  res.send('Done');
});

app.listen(3000);

What happens when a request is made?

Node.js

const express = require('express');
const app = express();

app.use((req, res, next) => {
  console.log('First middleware');
  // missing next()
});

app.use((req, res, next) => {
  console.log('Second middleware');
  res.send('Done');
});

app.listen(3000);

A Only 'First middleware' logs; request hangs without response

B 'First middleware' and 'Second middleware' both log; response sent

C Only 'Second middleware' logs; response sent immediately

D Server crashes with an error about missing next()

💡 Hint

Think about what happens if next() is not called in middleware.

Without calling next(), the request does not proceed to the next middleware, so it hangs and no response is sent.

Practice

1. Which of the following is a common security vulnerability in Node.js applications?

easy

A. Memory Leak

B. SQL Injection

C. Slow Network

D. Syntax Error

Solution

1. Step 1: Understand common vulnerabilities

   SQL Injection is a well-known security risk where attackers inject malicious SQL code.

2. Step 2: Identify relevant options

   Memory leaks and slow networks are performance issues, syntax errors are coding mistakes, not security vulnerabilities.

3. Final Answer:

   SQL Injection -> Option B

4. Quick Check:

   Common Node.js security risk = SQL Injection [OK]

Hint: Security risks often involve data injection or unauthorized access [OK]

Common Mistakes:

• Confusing performance issues with security vulnerabilities
• Thinking syntax errors are security problems
• Ignoring injection attacks

2. Which syntax correctly prevents Cross-Site Scripting (XSS) in a Node.js Express app when rendering user input in a template?

easy

A. res.send(`${escape(userInput)}`);

B. res.render('page', { data: userInput });

C. res.send(`${userInput}`);

D. res.send(userInput);

Solution

1. Step 1: Identify safe rendering method

   Escaping user input before inserting into HTML prevents XSS attacks.

2. Step 2: Compare options

   Only res.send(`
   ${escape(userInput)}`); uses an escape function to sanitize input before sending.

3. Final Answer:

   res.send(`
   ${escape(userInput)}
   `); -> Option A

4. Quick Check:

   Escape user input to prevent XSS = res.send(`
   ${escape(userInput)}
   `); [OK]

Hint: Always escape user input before rendering in HTML [OK]

Common Mistakes:

• Sending raw user input directly in HTML
• Assuming template rendering auto-escapes input
• Not using escape functions

3. What will be the output of this Node.js code snippet vulnerable to prototype pollution?

const obj = {};
const payload = JSON.parse('{"__proto__": {"polluted": "yes"}}');
Object.assign(obj, payload);
console.log({}.polluted);

medium

A. Throws an error

B. undefined

C. null

D. "yes"

Solution

1. Step 1: Understand prototype pollution

   Assigning __proto__ property modifies the prototype of all objects.

2. Step 2: Analyze code behavior

   Object.assign merges payload into obj, changing Object prototype to include polluted property.

3. Step 3: Check console.log output

   Accessing {}.polluted reads from prototype, outputs "yes".

4. Final Answer:

   "yes" -> Option D

5. Quick Check:

   Prototype pollution sets property globally = "yes" [OK]

Hint: Prototype pollution changes all objects' properties unexpectedly [OK]

Common Mistakes:

• Expecting undefined instead of polluted value
• Not recognizing __proto__ effect
• Assuming error thrown on prototype pollution

4. Identify the security flaw in this Express middleware code:

app.use((req, res, next) => {
  const token = req.headers['authorization'];
  if (token) {
    req.user = jwt.verify(token, 'secret');
  }
  next();
});

medium

A. Not checking token expiration

B. Using synchronous jwt.verify instead of async

C. Missing error handling for invalid token

D. Not sanitizing user input

Solution

1. Step 1: Review token verification

   jwt.verify can throw if token is invalid, but no try-catch is used.

2. Step 2: Identify missing error handling

   Without try-catch, invalid tokens cause app crash or unhandled exceptions.

3. Final Answer:

   Missing error handling for invalid token -> Option C

4. Quick Check:

   Always catch errors from jwt.verify [OK]

Hint: Wrap jwt.verify in try-catch to handle invalid tokens [OK]

Common Mistakes:

• Ignoring exceptions from jwt.verify
• Assuming synchronous call is insecure
• Confusing input sanitization with token validation

5. You want to secure a Node.js app against outdated package vulnerabilities. Which approach is best to keep dependencies safe?

hard

A. Run 'npm audit' regularly and update vulnerable packages

B. Ignore warnings and only update major versions yearly

C. Manually edit package-lock.json to remove vulnerabilities

D. Use only packages without any dependencies

Solution

1. Step 1: Understand package vulnerability management

   npm audit scans for known vulnerabilities in dependencies.

2. Step 2: Choose best practice

   Regularly running npm audit and updating packages fixes security issues promptly.

3. Step 3: Evaluate other options

   Ignoring warnings risks attacks, manual edits are error-prone, and avoiding dependencies is impractical.

4. Final Answer:

   Run 'npm audit' regularly and update vulnerable packages -> Option A

5. Quick Check:

   Use npm audit and update often = Run 'npm audit' regularly and update vulnerable packages [OK]

Hint: Use npm audit and update packages regularly [OK]

Common Mistakes:

• Ignoring audit warnings
• Editing lock files manually
• Avoiding dependencies completely