Understanding Access Tokens and Refresh Tokens in Web Applications
Modern web applications rely on tokens to manage authentication and authorization securely. Tokens are essentially pieces of encoded information that allow users to access the resources, databases, and functionalities of a website or application at an individual level. Instead of repeatedly verifying user credentials for every request, applications use tokens to confirm a user's identity.
In most industry-level web and application systems, two types of tokens are commonly used:
Access Token
Refresh Token
These tokens work together to maintain both security and usability in authentication systems.
Access Token
An access token contains user-related information such as _id, email, username, and other necessary details. It works like a digital identity card that allows the system to recognize the user and grant access to the tasks or resources they are permitted to use.
However, access tokens come with a security risk. Since they often contain sensitive user information, if an attacker manages to obtain an access token, they could potentially gain access to the user's data and perform unauthorized actions. To reduce this risk, access tokens are designed to have a limited validity period, known as an expiration time.
For example, consider an access token that is generated at time t and is valid until t + 10 units of time. If a cyber attacker obtains this token at t + 5, they would only be able to use it for the remaining 5 units of time. After the expiration time passes, the token becomes invalid and cannot be used further. This limitation reduces the potential damage caused by token theft.
Although token expiration improves security, it introduces another challenge:
How can the system generate a new access token after the current one expires without asking the user to log in again every time?
Refresh Token
To solve this problem, authentication systems use a refresh token.
A refresh token is also generated when the user logs into the application. However, unlike an access token, it contains much less information, typically only a unique identifier such as the user's _id. This identifier allows the backend system to map the token to the corresponding user in the database.
When the access token expires, the refresh token can be used to request a new access token without requiring the user to re-enter their username and password. The backend verifies the refresh token, retrieves the necessary user details from the database using the stored identifier, and then generates a new access token.
Because refresh tokens contain minimal information, the potential risk associated with them is lower compared to access tokens. Additionally, refresh tokens usually have a much longer expiration time than access tokens. This allows the system to generate multiple new access tokens during the refresh token's lifetime.
Once the refresh token itself expires, the user must log in again to generate a new pair of tokens: a fresh access token and a new refresh token.
Conclusion
Access tokens and refresh tokens together create a balanced authentication system. Access tokens provide short-term authorization for user actions, reducing security risks by expiring quickly. Refresh tokens, on the other hand, allow the system to issue new access tokens without repeatedly asking users to log in.
This combination ensures that applications remain both secure and user-friendly, limiting the impact of token theft while maintaining a smooth user experience.
Credit: This article is inspired by the explanation in a video by Chai Aur Code.