LocalStorage vs Cookies: All You Need To Know About Storing JWT Tokens Securely in The Front-End
Tags: security, javascript, webdev
Published: 2020-07-21 18:20:34

####*JWT Tokens are awesome, but how do you store them securely in your front-end? We'll go over the pros and cons of localStorage and Cookies.*

____

We went over [how OAuth 2.0 works](https://blog.cotter.app/what-on-earth-is-oauth-super-simple-intro-to-oauth-20-access-tokens-and-how-to-implement-it-in-your-site/) in the last post and we covered how to generate access tokens and refresh tokens. The next question is: **how do you store them securely in your front-end?** 

##A Recap about Access Token & Refresh Token
**Access tokens** are usually short-lived JWT Tokens, signed by your server, and are included in every HTTP request to your server to authorize the request.

**Refresh tokens** are usually long-lived opaque strings stored in your database and are used to get a new access token when it expires.

##Where should I store my tokens in the front-end?
There are 2 common ways to store your tokens: in `localStorage` or cookies. There are a lot of debate on which one is better and most people lean toward cookies for being more secure.

Let's go over the comparison between `localStorage`. This article is mainly based on [Please Stop Using Local Storage](https://dev.to/rdegges/please-stop-using-local-storage-1i04) and the comments to this post.

###Local Storage
**Pros: It's convenient.**

* It's pure JavaScript and it's convenient. If you don't have a back-end and you're relying on a third-party API, you can't always ask them to set a specific cookie for your site.
* Works with APIs that require you to put your access token in the header like this: `Authorization Bearer ${access_token}`.

**Cons: It's vulnerable to XSS attacks.**

An XSS attack happens when an attacker can run JavaScript on your website. This means that the attacker can just take the access token that you stored in your `localStorage`.

An XSS attack can happen from a third-party JavaScript code included in your website, like React, Vue, jQuery, Google Analytics, etc. It's almost impossible not to include any third-party libraries in your site.

###Cookies
**Pros: The cookie is not accessible via JavaScript; hence, it is not as vulnerable to XSS attacks as `localStorage`.**

* If you're using `httpOnly` and `secure` cookies, that means your cookies cannot be accessed using JavaScript. This means, even if an attacker can run JS on your site, they can't read your access token from the cookie.
* It's automatically sent in every HTTP request to your server.

**Cons: Depending on the use case, you might not be able to store your tokens in the cookies.**

* Cookies have a size limit of 4KB. Therefore, if you're using a big JWT Token, storing in the cookie is not an option.
* There are scenarios where you can't share cookies with your API server or the API requires you to put the access token in the Authorization header. In this case, you won't be able to use cookies to store your tokens.

##About XSS Attack
Local storage is vulnerable because it's easily accessible using JavaScript and an attacker can retrieve your access token and use it later. However, while `httpOnly` cookies are not accessible using JavaScript, **this doesn't mean that by using cookies, you are safe from XSS attacks involving your access token.**

If an attacker can run JavaScript in your application, then they can just send an HTTP request to your server and that will automatically include your cookies. **It's just less convenient for the attacker because they can't read the content of the token** although they rarely have to. It might also be more advantageous for the attacker to attack using victim's browser (by just sending that HTTP Request) rather than using the attacker's machine.

##Cookies and CSRF Attack
CSRF Attack is an attack that forces a user to do an unintended request. For example, if a website is accepting an email change request via:

```http
POST /email/change HTTP/1.1
Host: site.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 50
Cookie: session=abcdefghijklmnopqrstu

email=myemail.example.com 
```

Then an attacker can easily make a `form` in a malicious website that sends a POST request to `https://site.com/email/change` with a hidden email field and the `session` cookie will automatically be included.

However, this can be mitigated easily using `sameSite` flag in your cookie and by including an [anti-CSRF token](https://owasp.org/www-community/Anti_CRSF_Tokens_ASP-NET).

##Conclusion
Although cookies still have some vulnerabilities, it's preferable compared to `localStorage` whenever possible. Why?

* Both `localStorage` and cookies are vulnerable to XSS attacks but it's harder for the attacker to do the attack when you're using httpOnly cookies.
* Cookies are vulnerable to CSRF attacks but it can be mitigated using `sameSite` flag and [anti-CSRF tokens](https://owasp.org/www-community/Anti_CRSF_Tokens_ASP-NET).
* You can still make it work even if you need to use the `Authorization: Bearer` header or if your JWT is larger than 4KB.
This is also consistent with the recommendation from the OWASP community:

>Do not store session identifiers in local storage as the data are always accessible by JavaScript. Cookies can mitigate this risk using the `httpOnly` flag.
- [OWASP: HTML5 Security Cheat Sheet](https://cheatsheetseries.owasp.org/cheatsheets/HTML5_Security_Cheat_Sheet.html)

##So, how do I use cookies to persists my OAuth 2.0 tokens?
As a recap, here are the different ways you can store your tokens:

* **Option 1:** Store your access token in `localStorage` : prone to XSS.
* **Option 2:** Store your access token in `httpOnly` cookie: prone to CSRF but can be mitigated, a bit better in terms of exposure to XSS.
* **Option 3:** Store the refresh token in `httpOnly` cookie: safe from CSRF, a bit better in terms of exposure to XSS.
We'll go over how **Option 3** works as it is the best out of the 3 options.

###Store your access token in memory and store your refresh token in the cookie
**Why is this safe from CSRF?**

Although a form submit to `/refresh_token` will work and a new access token will be returned, the attacker can't read the response if they're using an HTML form. To prevent the attacker from successfully making a `fetch` or `AJAX` request and read the response, this requires the Authorization Server's CORS policy to be set up correctly to prevent requests from unauthorized websites.

####So how does this set up work?
**Step 1: Return Access Token and Refresh Token when the user is authenticated.**

After the user is authenticated, the Authorization Server will return an `access_token` and a `refresh_token`. The `access_token` will be included in the Response body and the `refresh_token` will be included in the cookie.

Refresh Token cookie setup:

* Use the `httpOnly` flag to prevent JavaScript from reading it.
* Use the `secure=true` flag so it can only be sent over HTTPS.
* Use the `SameSite=strict` flag whenever possible to prevent CSRF. This can only be used if the Authorization Server has the same site as your front-end. If this is not the case, your Authorization Server must set CORS headers in the back-end or use other methods to ensure that the refresh token request can only be done by authorized websites.

**Step 2: Store the access token in memory**

Storing the token in-memory means that you put this access token in a variable in your front-end site. Yes, this means that the access token will be gone if the user switches tabs or refresh the site. That's why we have the refresh token.

**Step 3: Renew access token using the refresh token**

When the access token is gone or has expired, hit the `/refresh_token` endpoint and the refresh token that was stored in the cookie in step 1 will be included in the request. You'll get a new access token and can then use that for your API Requests.

This means your JWT Token can be larger than 4KB and you can also put it in the Authorization header.

##That's It!
This should cover the basics and help you secure your site. This post is written by the team at [Cotter](https://www.cotter.app/) – we are building lightweight, fast, and passwordless login solution for websites and mobile apps.

If you're building a login flow for your website or mobile app, these articles might help:

* [What On Earth Is OAuth? A Super Simple Intro to OAuth 2.0, Access Tokens, and How to Implement it in your Site](https://blog.cotter.app/what-on-earth-is-oauth-super-simple-intro-to-oauth-20-access-tokens-and-how-to-implement-it-in-your-site/)
* [Passwordless Login with Email and JSON Web Token (JWT) Authentication using Next.js](https://blog.cotter.app/passwordless-login-with-email-and-json-web-token-jwt-authentication-with-nextjs/)
* [Here's How to Integrate Cotter's Magic Link to Your Webflow Site in Less Than 15 minutes!](https://blog.cotter.app/integrate-cotter-magic-link-to-webflow-in-less-than-15-minutes/)

*** 

##References
We referred to several articles when writing this blog, especially from these articles:

* [Please Stop Using Local Storage](https://dev.to/rdegges/please-stop-using-local-storage-1i04)
* [The Ultimate Guide to handling JWTs on front-end clients (GraphQL)](https://hasura.io/blog/best-practices-of-using-jwt-with-graphql/#jwt_persist)
* [Cookies vs Localstorage for sessions – everything you need to know](https://supertokens.io/blog/cookies-vs-localstorage-for-sessions-everything-you-need-to-know)

***

###Questions & Feedback
If you need help or have any feedback, feel free to comment here or ping us on [Cotter's Slack Channel](https://join.slack.com/t/askcotter/shared_invite/zt-dxzf311g-5Mp3~odZNB2DwYaxIJ1dJA)! We're here to help.

###Ready to use Cotter?
If you enjoyed this post and want to integrate Cotter into your website or app, you can [create a free account](https://dev.cotter.app/) and [check out our documentation](https://docs.cotter.app/).