OWASP Cheat Sheet Series: OAuth2 Cheat Sheet

# OAuth 2.0 Protocol Cheatsheet

This cheatsheet describes the best current security practices for OAuth 2.0 as derived from its RFC. OAuth became the standard for API protection and the basis for federated login using OpenID Connect. OpenID Connect 1.0 is a simple identity layer on top of the OAuth 2.0 protocol. It enables clients to verify the identity of the end user based on the authentication performed by an authorization server, as well as to obtain basic profile information about the end user in an interoperable and REST-like manner.

**Note:** OAuth 2.0 supports different token types to address various security and implementation requirements. **Bearer tokens** (RFC 6750) provide simplicity and broad adoption. **Proof of Possession (PoP) tokens** offer advanced security through cryptographic binding between tokens and clients. The appropriate token type depends on your application's security requirements, threat model, and implementation constraints.

## Terminology

- **Client**: Generally refers to an application making protected resource requests on behalf of the resource owner and with its authorization. The term "client" does not imply any particular implementation characteristics (e.g., whether the application executes on a server, a desktop, or other devices).
- **Authorization Server (AS)**: Refers to the server issuing access tokens to the client after successfully authenticating the resource owner and obtaining consent from the resource owner.
- **Resource Owner (RO)**: Refers to an entity capable of granting access to a protected resource. When the resource owner is a person, it is referred to as an end user. It can also be an organization or system.
- **Resource Server (RS)**: Refers to the server hosting the protected resources, capable of accepting and responding to protected resource requests using access tokens.

- **Access Tokens**: Provide an abstraction, replacing different authorization constructs (e.g., username and password, assertion) by a single token understood by the resource server. This abstraction enables issuing access tokens valid for a short period, as well as removing the resource server's need to understand a wide range of authentication schemes. The most common type is the **bearer token** (RFC 6750), which is straightforward to implement and integrate, requiring only the token value for API access. Since anyone possessing a bearer token can use it, bearer tokens must be restricted to a single audience (Resource Server) to limit the impact of token leakage.
- **Refresh Tokens** are credentials used to obtain access tokens. These are issued to the client by the authorization server and are used to obtain a new access token when the current access token becomes invalid or expires or to obtain additional access tokens with identical or narrower scope (access tokens may have a shorter lifetime and fewer permissions than authorized by the resource owner). Refresh tokens should be protected using sender-constraining mechanisms (DPoP or mTLS) or refresh token rotation.

- **Proof of Possession (PoP) tokens**: Access tokens or refresh tokens that are cryptographically bound to clients through mechanisms like DPoP (RFC 9449) or mTLS-bound access tokens (RFC 8705). These tokens are bound to a private key owned by the client and the client must demonstrate possession of this private key in order to use the token. This approach provides additional protection in scenarios where token interception is a concern, with additional implementation requirements for key management and proof generation.

## OAuth 2.0 Essential Basics

1. Clients and Authorization Server must not expose URLs that forward the user's browser to arbitrary URIs obtained from a query parameter ("open redirectors") which can enable exfiltration of authorization codes and access tokens.
2. Clients have ensured that the Authorization Server supports PKCE may rely on the CSRF protection provided by PKCE. In OpenID Connect flows, the "nonce" parameter provides CSRF protection. Otherwise, one-time user CSRF tokens carried in the "state" parameter that are securely bound to the user agent must be used for CSRF protection.
3. When an OAuth Client can interact with more than one Authorization Server, Clients should use the issuer "iss" parameter as a countermeasure, or based on an "iss" value in the authorization response (such as the "iss" Claim in the ID Token in OpenID)
4. When the other countermeasure options for OAuth clients interacting with more than one Authorization Servers are absent, Clients may instead use distinct redirect URIs to identify authorization endpoints and token endpoints.
5. An Authorization Server avoids forwarding or redirecting a request potentially containing user credentials accidentally.

## PKCE - Proof Key for Code Exchange Mechanism

OAuth 2.0 public clients utilizing the Authorization Code Grant are susceptible to the authorization code interception attack. Proof Key for Code Exchange (PKCE, pronounced "pixy") is the technique used to mitigate against the threat of authorization code interception attack.

Originally, PKCE is intended to be used solely focused on securing native apps, but then it became a deployed OAuth feature. It does not only protect against authorization code injection attacks but also protects authorization codes created for public clients as PKCE ensures that the attacker cannot redeem a stolen authorization code at the token endpoint of the authorization server without knowledge of the code_verifier.

6. Clients are preventing injection (replay) of authorization codes into the authorization response by using PKCE flow. Additionally, clients may use the OpenID Connect "nonce" parameter and the respective Claim in the ID Token instead. The PKCE challenge or OpenID Connect "nonce" must be transaction-specific and securely bound to the client and the user agent in which the transaction was started. **Note:** PKCE protects authorization codes; use sender-constrained tokens to protect access and refresh tokens.
7. When using PKCE, Clients should use PKCE code challenge methods that do not expose the PKCE verifier in the authorization request. Otherwise, attackers who can read the authorization request can break the security provided by the PKCE. Authorization servers must support PKCE.
8. If a Client sends a valid PKCE "code_challenge" parameter in the authorization request, the authorization server enforces the correct usage of "code_verifier" at the token endpoint.
9. Authorization Servers are mitigating PKCE Downgrade Attacks by ensuring a token request containing a "code_verifier" parameter is accepted only if a "code_challenge" parameter is present in the authorization request.

## Implicit Grant (DEPRECATED — DO NOT USE)

The Implicit Grant (`response_type=token`) is **deprecated** by [RFC 9700 §2.1.2](https://datatracker.ietf.org/doc/html/rfc9700#section-2.1.2) and removed from OAuth 2.1. It exposes access tokens in the U