Outpost24 identifies key OAuth risks & best practice solutions
An analysis by Outpost24 has examined seven of the most common vulnerabilities present in OAuth implementations and outlined recommended measures organisations can take to mitigate these risks.
OAuth, short for Open Authorization, is a widely used industry protocol that allows users to grant access to their data on one site to another site, without sharing their credentials directly. This delegation of authority involves issuing tokens that provide time-limited and scoped permissions to client applications on behalf of users.
Underlying complexity
Although OAuth helps reduce direct exposure of user credentials and supports fine-grained access control, its broad flexibility also creates significant opportunities for errors during implementation. The protocol's reliance on strict validation of parameters, endpoints and tokens, as well as correct management of application state, means that mistakes or oversights can introduce vulnerabilities that attackers can exploit.
Outpost24's analysis notes that OAuth is not inherently weak, but that its "power (delegated, token-based access) relies on numerous checks and balances. However, OAuth vulnerabilities often arise when developers or architects skip steps, like byte-for-byte URI validation, state verification, or signature checks on ID tokens. These oversights create exploitable gaps that attackers can target. So, OAuth itself isn't inherently 'weak'—but its flexibility and the proliferation of optional parameters and flows make it easy to misconfigure in ways that lead to real-world vulnerabilities."
Common vulnerabilities
The analysis identifies seven main areas where OAuth vulnerabilities commonly occur:
1. Open redirect and redirect URI manipulation: If the system does not strictly validate redirect URIs, attackers can manipulate authorisation flows to direct tokens or codes to endpoints they control, resulting in unauthorised access to user data.
2. Missing or weak Cross-Site Request Forgery (CSRF)/state protections: Failing to include a robust state parameter tied to each user's session enables attackers to trick users into completing authorisation requests that generate tokens for attacker-controlled clients.
3. Implicit flow and lack of Proof Key for Code Exchange (PKCE): The use of implicit flow, where access tokens are delivered directly via the browser, exposes tokens to interception. Without PKCE, even the more secure code flow can be susceptible if an attacker can access intermediate codes.
4. Inadequate scope validation and overly broad permissions: Applications may request excessive permissions, which can lead to abuse if an attacker acquires the access token. Users can be misled into granting high-privilege access.
5. Token leakage via insecure storage or transport: Storing tokens in browser storage areas accessible to client-side scripts, or transmitting them over insecure channels, can lead to theft through network compromise or browser vulnerabilities.
6. Missing or ineffective token revocation: Without appropriate means to revoke tokens, attackers or malicious clients may retain access indefinitely, even after a user believes they have rescinded authorisation.
7. Homegrown or outdated OAuth implementations: Custom or obsolete libraries may omit essential security checks, such as validating signature fields or all necessary request parameters, making exploitation feasible through replay or impersonation attacks.
Mitigation strategies
The analysis offers concrete recommendations to address each identified risk. For redirect URI threats, strict, exact matching of registered URIs is advised, along with enforcement of HTTPS. To defend against CSRF threats, the report urges clients to "generate a cryptographically random state value, store it in the user's session, and include it in the request. Strictly validate state on callback," and to make use of SameSite cookie attributes.
The deprecation of the implicit flow and the universal adoption of PKCE are recommended for public clients. The analysis recommends the "use of authorization code flow + PKCE for all public clients", which helps bind token requests to verified identifiers, limiting misuse. Limiting scope requests to the minimal set required, alongside server-side validation of access scope, are key principles for scope management.
Regarding token storage and transport, the advice is to "use secure, HttpOnly cookies for storing tokens" and to "enforce TLS everywhere… All endpoints (authorization, token, resource) must enforce HTTPS with strong ciphers." Short token lifetimes and refresh token rotation are also recommended to reduce the exposure following a token compromise.
For revocation, the report recommends implementing dedicated endpoints that can invalidate access and refresh tokens in accordance with relevant standards, with continuous verification at the resource server layer to ensure revoked tokens remain unusable.
On the issue of custom or outdated OAuth implementations, the recommendation is to "adopt well-maintained libraries and frameworks" and to "stay current with RFCs and security advisories," underscored by regular code reviews, threat modelling and attention to emerging IETF best practices.
Operational recommendations
To build a resilient OAuth deployment, enforce strict validation of redirect URIs, state parameters, and token signatures; adopt PKCE for all public clients; and adhere to least‐privilege scope requests. Ensure secure storage and transmission of tokens (favouring HttpOnly cookies over local storage) and implement token revocation with continuous introspection. Use community‐trusted OAuth libraries, keep up with evolving IETF/OAuth 2.1 guidelines, and maintain robust logging/monitoring to catch misuse quickly.
Outpost24's analysis points out that by addressing these common misconfigurations and implementation issues, organisations "significantly reduce the risk of credential theft, unauthorised API access, and large-scale data breaches arising from flawed OAuth integrations."
