What is the security difference between API Keys and the client credentials flow of OAuth?

Question

Consider an API that a client accesses directly (machine to machine) and that doesn't require user-specific authentication. The way I understand it, in client_credentials, the client must store a client_id and client_secret that it uses to acquire and refresh tokens. With an API key, the client just stores the key. What makes OAuth more secure in this case? It would appear to me that if the API key is never compromised, no attacker could pose as the intended client. And if the API key is compromised, it is effectively the same as compromising the client_id and client_secret, which an attacker would be able to use to obtain tokens and access the data in the API, posing as the client.

edit: clarified this is a machine-to-machine call

Answer 1

TLDR;

The difference comes down to direct access vs. delegated access.

OAuth allows you to make delegated access. The benefits of delegated access don't change if there is a user involved or not. The same arguments that make the OAuth Authorization code flow attractive for user-to-machine access, apply to the OAuth Client credentials flow for machine-to-machine access.

Ask yourself, do you want the resource server to handle client credentials or not?

On confidential clients for machine-to-machine access, the cost of delegated access vs. direct access may very well outweigh the benefits. That's why so many APIs still use API keys. You'll have to decide that for your individual use case.

Differences

In the OAuth client credentials flow, the client sends an access token to the resource server, which it got beforehand by the authorization server after presenting its client ID and secret. The resource server never sees the client secret. With an API key, the client sends the key with every request.

OAuth adds an additional layer of indirection with the authorization server, such that the credentials themselves never get transmitted to the resource server. This allows the authorization server to give the client only access for a limited amount of time or with limited permissions, without ever needing to change the actual client credentials. It also allows to revoke access tokens without revoking the credentials themselves. For multiple instances of a client this allows you to revoke access for some but not all.

Of course this all comes at the cost of a more complex implementation, and an additional roundtrip from the client to the authorization server.

I won't touch on transmission (URL, header, body, etc.) or format (random string, signed JWT, etc.), since these can be the same for access tokens just as for API keys.

Another, maybe not so obvious, advantage of OAuth is having a clear spec that libraries, documentation and discussions can be based on. With direct access there is no single best practice and different people may understand different things when referring to direct access methods like API keys.

Answer 2

With client credential flow your Client Id and Client Secret are sent to the authorization server to get back an access token. For all subsequent request to the API/resource servers, you pass the access token and not the client credentials themselves. The access token is usually a JWT, which is a set of encoded claims including the token expiry (exp), not before (nbf), token issuer (iss), authorized party (azp), roles, permissions, etc.

This has a number of advantages over a simple API Key approach. e.g.

• If the access token (which is included in requests to the API/resource server) is compromised, it's only valid until it expires (which is typically ~1 day for M2M tokens). If an API Key is compromised, it can be used indefinitely or until it's explicitly blocked by the API/resource server.
• JWT access tokens are encoded JSON objects that contains a number of fields (a.k.a. claims) that can be used for fine grained authorization e.g. roles, permissions, grant type, authorized party etc. An API Key is generally opaque and is all or nothing when it comes to auth.
• You machine tokens can get validated and authorized on the API/resource servers the same way as your user tokens, so you don't end up with multiple auth implementations on the back-end.

Answer 3

OAuth Client Credentials Flow

What is the security difference between API Keys and the client credentials flow of OAuth?

OAuth client credentials flow is not meant to be used by public clients, just between machines.

From auth0.com/docs:

Client Credentials Flow

With machine-to-machine (M2M) applications, such as CLIs, daemons, or services running on your back-end, the system authenticates and authorizes the app rather than a user. For this scenario, typical authentication schemes like username + password or social logins don't make sense. Instead, M2M apps use the Client Credentials Flow (defined in OAuth 2.0 RFC 6749, section 4.4), in which they pass along their Client ID and Client Secret to authenticate themselves and get a token.

So, I am not sure what is your scenario, but I will assume in my reply that you are referring to public clients.

If it is in the public client code, then it is public

The way I understand it, in client_credentials, the client must store a client_id and client_secret that it uses to acquire and refresh tokens.

Yes, it needs to be stored in the client code for the client to be able to obtain the OAuth token.

If you use the client_secret from a web app or mobile app you are making it public, therefore not a secret anymore.

Extracting secrets from public clients

For example, in a web app all it takes to extract the client_secret is to hit F12 in the browser and search for it, thus how much time can this take?

Now, in a mobile app, some may think it's secure because they are compiled into a binary but is almost as easy as it is in the browser, because we have several open-source tools that can help us with this task, like the MobSF framework, and on Linux, you can even achieve this with the strings command. Using the MobSF to perform static binary analysis on the mobile app binary allows for anyone without hacking knowledge to easily extract the client_secret in minutes, just like I show in my article How to Extract an API key from a Mobile App with Static Binary Analysis:

The range of open source tools available for reverse engineering is huge, and we really can't scratch the surface of this topic in this article, but instead, we will focus in using the Mobile Security Framework(MobSF) to demonstrate how to reverse engineer the APK of our mobile app. MobSF is a collection of open-source tools that present their results in an attractive dashboard, but the same tools used under the hood within MobSF and elsewhere can be used individually to achieve the same results.

So, the process of extracting the api-key in my article is the same you will use to extract the client_secret or any other string of your interest in the mobile app binary.

OAuth or API Key?

What makes OAuth more secure in this case? It would appear to me that if the API key is never compromised, no attacker could pose as the intended client. And if the API key is compromised, it is effectively the same as compromising the client_id and client_secret, which an attacker would be able to use to obtain tokens and access the data in the API, posing as the client.

If used from a public client neither are secure, because if read my linked article, you understand by now how easy is to bypass an API Key or extract the client_secret and client_id.

So, if your client is public you should not use the OAuth client credential flow, thus you need to go with the insecure API key approach or you can be more diligent and try to apply defence-in-depth approaches, but this will depend if the API clients are only web apps or mobile apps or both.

If your API clients are only web apps I invite you to read my answer to the question Secure API data from calls out of the app, especially the section dedicated to Defending the API Server.

In the case the API clients are only mobile apps then I recommend you to read this answer I gave to the question How to secure an API REST for mobile app?, especially the sections Securing the API Server and A Possible Better Solution.

On the other hand, if your API clients are both a web app and a mobile app I recommend you to apply the security measures more relevant to you from both answers linked above.

Remember that security is always about adding as many layers of defences as you can afford or it's required by law. Even in the past century, the castles were built with a lot of different security defence layers, thus this is nothing new to the digital era.

Do You Want To Go The Extra Mile?

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For APIS

OWASP API Security Top 10

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OWASP Mobile Security Project - Top 10 risks

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OWASP - Mobile Security Testing Guide:

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The Web Security Testing Guide:

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