Top JWT Interview Questions (2023)

Most frequently asked JWT Interview Questions

  1. What experience do you have with JWT?
  2. How would you apply JWT for authentication and authorization?
  3. Could you explain the different parts of a JWT token?
  4. What techniques do you use to ensure secure usage of JWT?
  5. Are you familiar with attack vectors on JWT tokens?
  6. What strategies do you employ to prevent malicious access to JWT tokens?
  7. How do you keep track of JWT tokens after they are issued?
  8. What challenges have you encountered while implementing JWT?
  9. How do you go about debugging JWT tokens?
  10. What implications can arise when JWT tokens are improperly used?
  11. Is it possible to revoke a JWT token?
  12. What other technologies do you believe are beneficial when using JWT?

What experience do you have with JWT?

I have extensive knowledge of the JSON Web Token (JWT) protocol and how it works.
A JWT is an open standard for securely sending information between two parties.
This information can be used to identify the sender or receiver of the message, and can also include authentication credentials and additional data.
A JWT can be encoded and signed using a variety of algorithms such as SHA-256, RSA, or ECDSA.
Using a JWT for authentication provides an extra layer of security by ensuring that the token can't be guessed or spoofed and is structure-agnostic as well.
The code snippet below shows an example of how to create a JWT using Node.js. 
// Create a JSON Web Token 
let jwt = require('jsonwebtoken'); 

// Set your secret key 
let secretKey = 'your_secret_key';  

// Create the JWT payload 
let payload = { 
	userId: 1, 
	name: 'John Doe'
};

// Encode the payload with the secret key 
let token = jwt.sign(payload, secretKey); 

console.log(token);

How would you apply JWT for authentication and authorization?

Authorization and Authentication using JWT (JSON Web Token) can be implemented in several ways.
In this article, I will discuss one such method which is based on the OAuth 2.0 protocol.
In order to implement authorization and authentication with JWT, you need to have an Authorization Server, a Resource Server, and an Authorization Client.
The authorization server provides an access token that can be used by the client application to gain access to the protected resources on the resource server.
The access token is based on the OpenID Connect (OIDC) authorization protocol, which is an OAuth 2.0-based protocol.
To implement authentication and authorization using JWT, the client application must first generate a request for an Access Token.
This request is sent to the Authorization Server, which verifies the identity of the user by authenticating them using an appropriate authentication mechanism (e.g. username/password, multifactor authentication, etc.).
Once the user is authenticated, the Authorization Server issues an Access Token that is signed with a private key and encrypted with a symmetric key.
The Access Token contains information about the user such as their identity, roles, and other claims.
The Access Token can then be used to access protected resources on the Resource Server.
An example of a code snippet to implement authentication and authorization using JWT is provided below.
// Generate a request for an Access Token 
$request = new Request('POST', '/oauth2/token');
$request->setQueryParams([
 'grant_type' => 'password',
 'username' => $username,
 'password' => $password
]);

// Make a request to the Authorization Server 
$response = $client->send($request);

// Decode the response from the Authorization Server
$jwt = $response->getBody()->json()['access_token'];

// Verify the data in the JWT 
$decodedJwt = \Firebase\JWT\JWT::decode($jwt, 'key', ['HS256']);

// Validate the claims in the JWT 
if ($decodedJwt->iss == 'http://authorization.server.com' && 
 $decodedJwt->aud == 'http://resource.server.com') {
 // JWT is valid, proceed to access protected resources
}else {
 // JWT is invalid, return an error
}

Could you explain the different parts of a JWT token?

A JWT (JSON Web Token) is an open standard for securely transferring data between two parties using JSON as the primary data format.
The token consists of a header, payload, and signature.
The header contains information about type of token, signing algorithm used and other meta-information.
It is typically represented as a JSON object and contains two fields - "typ" which indicates the type of token and "alg" which shows the cryptographic algorithm used to generate the signature.
The payload (also known as the claim set) is the data that is being transferred between two parties.
This can include user information such as username, email address, or even an authentication token.
It is also typically represented as a JSON object.
The signature is a hash of the combined header and payload which is used to verify the authenticity and integrity of the token.
It is generated by applying a cryptographic algorithm to the header and payload.
In the code snippet below, we can see an example of how to generate a signature using the HMACSHA256 algorithm: 
String secret = "my-secret";

String headerAndPayload = base64Encode(header) + "." + base64Encode(payload);

String signature = HmacSHA256(headerAndPayload, secret);

What techniques do you use to ensure secure usage of JWT?

To ensure secure usage of JWT, one must follow a set of best practices and security measures.
Some of these include encrypting JWT payloads with AES-256 or RSA encryption, using strong hashing algorithms such as SHA-256 to generate the auth key, setting strong passwords and rotating them on regular intervals, verifying the payload signature and algorithm used for generating the signature, setting expiry time for tokens, and validating the issuer's and audience's identities.
Additionally, it is important to check for suspicious activity in the app and detect any unauthorized access attempts.
To ensure secure usage of JWT, one can implement the following code snippet in their app: 
// Generate a new JWT
let jwtObject = { 
	"iss": "[Insert Issuer]",
	"aud": "[Insert Audience]",
	"exp": "YYYYMMDDHHmmSS",
	"data": {
		"user_id": "somelongstring"
	}
};

// Sign the JWT with the secret key 
let token = jwt.sign(jwtObject,[INSERT SECRET KEY],{algorithm : 'HS256'});

// Encrypt the JWT with AES-256
let encryptedToken = crypto.AES.encrypt(token,'[INSERT ENCRYPTION PASSWORD]');

// Return the encrypted token
return encryptedToken;

Are you familiar with attack vectors on JWT tokens?

Absolutely! JWT (JSON Web Tokens) are an industry-standard used to securely transmit data between two parties over the web.
In terms of potential attack vectors, JWT tokens are vulnerable to different types of attacks such as cookie hijacking, brute force, and dictionary attacks.
To prevent these attack vectors, it is essential to secure your JWT token using secure algorithms and a strong secret.
Here's an example code snippet to illustrate how you can securely generate and sign a JWT token with the HS256 algorithm: 
const jwt = require('jsonwebtoken');
const secret = 'superSecretKey';
const token = jwt.sign({ name: 'John Doe' }, secret, { algorithm: 'HS256' }); 
console.log(token); // output: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJuYW1lIjoiSm9obiBEb2UifQ.nq3dPAjRUhJnF3Vv8SKXQGD4Vqxwt41aFZcrGAI7_zo

What strategies do you employ to prevent malicious access to JWT tokens?

To prevent malicious access to JWT tokens, there are a few measures that can be employed.
First, token signatures should be enabled in order to authenticate tokens and check for data integrity.
This is done by signing tokens with a secret key before they are passed to the client.
Additionally, timely refreshing of tokens should be enabled in order to invalidate and replace the old tokens.
This is important as to prevent attackers from accessing the token in the event of a successful attack.
To ensure token expiration is enforced properly, tokens should include expiry information that will trigger an invalidation request when accessed after the set time limit.
Additionally, CSRF tokens should be used to verify that requests made to the server are legitimate and coming from trusted sources.
Finally, appropriate access control and authorization policies should be implemented to only give users with valid credentials access to sensitive information.
A code snippet for enabling token signature would look like this: 
// Generate the secret key
String secretKey = "secretKey";

// Create a JwtSignatureValidator object and pass in the secret Key
JwtSignatureValidator jwtSignatureValidator = new JwtSignatureValidator(secretKey);

// Validate the signature on the token using the JwtSignatureValidator
jwtSignatureValidator.validate(token);

How do you keep track of JWT tokens after they are issued?

When issuing a JSON Web Token (JWT) for authentication, it's important to protect the JWT from unauthorized access and tampering.
To do this, there are several steps that can be taken such as:
- Encoding the JWT payload with HMAC or RSA encryption algorithms
- Keeping track of the issuer and the issued time of the JWT
- Validating the signature of the JWT with the client-side code
- Storing the JWT in an encrypted form on the client-side
- Periodically changing the validity period for the token
 In order to keep track of JWTs after they have been issued, we need to maintain a state of the tokens which can be done by storing the token in a database.
This way, when a request is made, the database is queried to check if the token already exists.
If the token does exist, the token can be validated and the request can be authorized.
Apart from simply keeping track of the JWT tokens, it's recommended to also log information related to the JWT used.
This can be done by logging the client IP address, user agent, the scope or level of access granted by the token, the timestamp of the request, etc.
A simple example of how to track JWT tokens using a database would be as follows: 
// Create a database table that will store the JWTs
CREATE TABLE jwt_tokens (
  id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  token_string VARCHAR(255) NOT NULL,
  expiration_date DATETIME NOT NULL,
  user_id INT NOT NULL
);

// Create an index on the 'token_string' column to improve query performance
CREATE INDEX ON jwt_tokens (token_string);

// When a token is issued, add an entry in the 'jwt_tokens' table
INSERT INTO jwt_tokens (token_string, expiration_date, user_id) VALUES (, , );

// When a request is made, validate the token by querying the 'jwt_tokens' table
SELECT * FROM jwt_tokens WHERE token_string = ;

What challenges have you encountered while implementing JWT?

Implementing JSON Web Tokens (JWT) can be a challenge, and there are several common issues that occur in this process.
One issue is related to security, as JWTs are signed but not encrypted, meaning they can be easily decoded.
Additionally, JWT expiration times have to be managed carefully, as expired tokens can lead to authorization failure or session hijacking.
Furthermore, large payloads need to be split up into multiple tokens to reduce decode times and increase performance.
Finally, JWTs should also be validated on the backend to prevent any unauthorized modifications of the token.
The following code snippet illustrates how to implement JWT authentication in NodeJS: 
```
// Create the JWT
var jwt = require('jsonwebtoken');

// Sign the JWT with a secret
var token = jwt.sign({foo: 'bar', exp: Math.floor(Date.now() / 1000) + (60 * 60)}, 'secret');

// Get the decoded JWT
var decoded = jwt.verify(token, 'secret');

// use the decoded JWT 
console.log(decoded);
```

How do you go about debugging JWT tokens?

Debugging JWT tokens can be a complex task.
First, it is important to have a basic understanding of what JWT tokens are and how they work.
A JWT token is a JSON Web Token which is used to securely transmit information between two parties.
To debug a JWT token, you must first understand the structure of the token.
A JWT token consists of three components - Header, Payload, and Signature.
The Header contains metadata about the token, such as the version and type.
The Payload contains information about the user, such as user id and other details.
The Signature is created with a secret key by applying a hashing algorithm on the Header and the Payload.
Once you understand the structure of a JWT token, the next step is to validate the token.
This can be done using an online JWT validator, such as jwt.io.
Here, you can enter your JWT token and it will show you the different components of the token.
Examining the contents of the token helps in figuring out any issues or discrepancies.
Next, you need to debug the code which generates the token.
This helps in understanding the logic of the code and finding any errors that might have crept in.
Here, you could add some console.
log statements to print out the values of variables before they are encoded into the token.
You may then compare these values to the actual contents of the token and find out any discrepancies between them.
The last and most important step in debugging JWT tokens is to make sure the signature is correct.
You can use the same JWT validator to check if the signature is valid.
If the signature is not valid, then you would need to review the code again and check whether the Key used for signing is correct or not.
Overall, debugging JWT tokens involves understanding the structure of the token, validating the token, and checking the signature.
With the help of the above steps and tools, you should be able to successfully debug JWT tokens.

What implications can arise when JWT tokens are improperly used?

Improper usage of JWT tokens can result in serious security issues such as increased vulnerability to data breaches, authentication bypass, and increased risk of identity theft.
A JSON Web Token (JWT) is a cryptographic token used to securely transfer information between two parties through a signed representation of data.
It is typically composed of three parts: a header, payload, and signature.
Because JWT tokens are signed and encoded, any changes to them can be detected, preventing tampering or replay attacks.
However, if JWT tokens are not used correctly, it can lead to some dangerous implications.
For example, if an attacker gains access to a JWT token, they can tamper with the contents of the token, leading to authentication bypass.
Additionally, JWTs that are improperly configured without proper encryption, expiration policies, and other security measures can be vulnerable to data breach.
If the JWT is not stored securely on the server side, an attacker can access the token and potentially gain access to sensitive information.
Furthermore, if the token is not set to expire, an attacker could use the same token over and over again, putting the user's data at risk of identity theft and other cyber-related crimes.
To ensure proper use of JWT tokens, developers should take the following steps:
1) cryptographically sign the token using a secure hashing algorithm like SHA-256;
2) encrypt the token using an encryption algorithm like AES;
3) ensure the token expires after a certain amount of time;
4) regularly rotate tokens; and
5) use secure storage mechanismsbr such as KeyVault or a hardware security module (HSM).

Is it possible to revoke a JWT token?

Yes, it is possible to revoke a JWT token.
The typical method of revoking a JWT token involves removing it from the database or blacklisting it, so that it cannot be used for authentication.
In order to do this, a revocation API call must be used to revoke the tokens.
The code snippet below shows how this can be done using the JSON Web Token (JWT) library in Node.js: 
const jwt = require('jsonwebtoken');
 
// blacklisted token array
let revokedTokens = [];
 
// Function to verify if a token is revoked
function verifyToken(token) {
 return revokedTokens.includes(token);
}
 
// Function to revoke a token
function revokeToken(token) {
 revokedTokens.push(token);
}
 
// Verify token
let verified = verifyToken(token);
 
// Revoke token
if (verified) {
 revokeToken(token);
}

What other technologies do you believe are beneficial when using JWT?

JWT (JSON Web Token) is a standard format for securely transmitting information between two parties.
It is often used with web APIs and mobile applications and can be used to securely store user credentials, authentication information, and other sensitive data.
When using JWT, it is important to incorporate other core technologies that can help to ensure a secure environment.
These technologies include:
  1. Secure Sockets Layer (SSL): SSL is an encryption protocol used to establish a secure connection between a client and a server. It is used to encrypt the data sent between the two parties, making it impossible for any third party to access the data.
  2. Access Control Lists (ACLs): An ACL is a set of rules which define who has access to what resources. It is used to restrict access to specific resources or operations to authorized users. This is important when dealing with sensitive data stored in a JWT token.
  3. Single Sign-On (SSO): SSO is a method of authentication which allows users to access multiple applications and services with just one set of credentials. With JWT, users can enjoy a greater level of security as SSO helps reduce the risk of malicious individuals trying to gain access to secure information.
  4. Two-Factor Authentication (2FA): 2FA adds an extra layer of security between the user and the application or service they are accessing. This increases the security of the system and reduces the chances of a malicious actor being able to gain access.
In addition to these core technologies, it is important to ensure that the JWT token is encoded properly and that the necessary validation checks are in place to ensure that the data stored in the token is safe.
Finally, it is important to consider the overall architecture of your application and make sure that it is designed to minimize the risk of attack.
Here is an example code snippet which demonstrates how to create a JWT token in Java: 
String jwtToken = Jwts.builder()
    .setSubject("subject") 
    .claim("name", "John Doe") 
    .signWith(SignatureAlgorithm.HS256, "secret".getBytes("UTF-8")) 
    .compact();