REST API Security Essentials

Security isn’t an afterthought. It has to be an integral part of any development project and also for REST APIs. There are multiple ways to secure a RESTful API e.g. basic auth, OAuth etc. but one thing is sure that RESTful APIs should be stateless – so request authentication/authorization should not depend on cookies or sessions. Instead, each API request should come with some sort authentication credentials which must be validated on the server for each and every request.

REST Security Design Principles

The paper “The Protection of Information in Computer Systems” by Jerome Saltzer and Michael Schroeder, put forth eight design principles for securing information in computer systems, as described in the following sections:

1. Least Privilege: An entity should only have the required set of permissions to perform the actions for which they are authorized, and no more. Permissions can be added as needed and should be revoked when no longer in use.
2. Fail-Safe Defaults: A user’s default access level to any resource in the system should be “denied” unless they’ve been granted a “permit” explicitly.
3. Economy of Mechanism: The design should be as simple as possible. All the component interfaces and the interactions between them should be simple enough to understand.
4. Complete Mediation: A system should validate access rights to all its resources to ensure that they’re allowed and should not rely on cached permission matrix. If the access level to a given resource is being revoked, but that isn’t reflected in the permission matrix, it would violate the security.
5. Open Design: This principle highlights the importance of building a system in an open manner—with no secret, confidential algorithms.
6. Separation of Privilege: Granting permissions to an entity should not be purely based on a single condition, a combination of conditions based on the type of resource is a better idea.
7. Least Common Mechanism: It concerns the risk of sharing state among different components. If one can corrupt the shared state, it can then corrupt all the other components that depend on it.
8. Psychological Acceptability: It states that security mechanisms should not make the resource more difficult to access than if the security mechanisms were not present. In short, security should not make worse the user experience.

Best Practices to Secure REST APIs

Below given points may serve as a checklist for designing the security mechanism for REST APIs.

Keep it Simple

Secure an API/System – just how secure it needs to be. Every time you make the solution more complex “unnecessarily”, you are also likely to leave a hole.

Always Use HTTPS

By always using SSL, the authentication credentials can be simplified to a randomly generated access token that is delivered in the username field of HTTP Basic Auth. It’s relatively simple to use, and you get a lot of security features for free.

If you use HTTP 2, to improve performance – you can even send multiple requests over a single connection, that way you avoid the complete TCP and SSL handshake overhead on later requests.

Use Password Hash

Passwords must always be hashed to protect the system (or minimize the damage) even if it is compromised in some hacking attempt. There are many such hashing algorithms which can prove really effective for password security e.g. MD5, SHA, PBKDF2, bcrypt and scrypt algorithms.

Never expose information on URLs

Usernames, passwords, session tokens, and API keys should not appear in the URL, as this can be captured in web server logs, which makes them easily exploitable.

https://api.domain.com/user-management/users/{id}/someAction?apiKey=abcd123456789  //Very BAD !!

Above URL exposes API key So, never use this form of security.

Consider OAuth

Though basic auth is good enough for most of the APIs and if implemented correctly, it’s secure as well – yet you may want to consider OAuth as well. The OAuth 2.0 authorization framework enables a third-party application to obtain limited access to an HTTP service, either on behalf of a resource owner by orchestrating an approval interaction between the resource owner and the HTTP service, or by allowing the third-party application to obtain access on its own behalf.

Consider Adding Timestamp in Request

Along with other request parameters, you may add a request timestamp as HTTP custom header in API request. The server will compare the current timestamp to the request timestamp, and only accepts the request if it is within a reasonable timeframe (1-2 minutes, perhaps).

This will prevent very basic replay attacks from people who are trying to brute force your system without changing this timestamp.

Input Parameter Validation

Validate request parameters on the very first step, before it reaches to application logic. Put strong validation checks and reject the request immediately if validation fails. In API response, send relevant error messages and example of correct input format to improve user experience.