Best Practices for Storing Access Tokens in the Browser

When searching for the best way to store auth tokens for one of my pet projects, I was overwhelmed by the amount of information. Here’s a concise summary to help navigate this complex topic.

I chose JWT (JSON Web Tokens) because they provide a compact and secure way to transmit information between parties. They ensure data integrity via signature verification but lack encryption by default, making the content visible if intercepted.

After diving deep into various methods, I've found that using HTTP-only cookies is the most secure way to store tokens and sensitive data in the browser:

Local Storage: Easily accessible via JavaScript, making it highly susceptible to XSS attacks.
Session Storage: Similar to local storage, it’s vulnerable to XSS attacks. Although the data is cleared when the page session ends, it's still exposed to the same security risks.
In-Memory Storage: While offering some isolation, it loses data on page reloads and significantly affects user experience. Moreover, it remains vulnerable to XSS attacks.
IndexedDB: Though more secure than local storage, it is complex to implement and still vulnerable to XSS attacks, exposing stored tokens if an attacker injects malicious scripts.
Web Workers: Provide some isolation but can still be compromised by XSS attacks. Implementing secure communication between workers and the main thread adds complexity.

Setting Up Secure HTTP-Only Cookies:

HTTP-Only Flag: Ensure cookies are not accessible via JavaScript to prevent XSS attacks.
Secure Flag: Enforce transmission over HTTPS to prevent interception.
SameSite Attribute: Set to Strict or Lax to prevent CSRF attacks.
Encryption: Encrypt cookie data to add an additional layer of security.

Here’s a visual representation of the threat matrix to illustrate the risks and mitigations:

Why HTTP-Only Cookies Are Superior: HTTP-only cookies mitigate these risks by preventing JavaScript access, ensuring tokens are only sent over secure connections (HTTPS), and adding protection against CSRF through the SameSite attribute.