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Keep an AI agent’s OAuth tokens safe by using a protected authorization flow, granting only the access it needs, storing tokens and keys outside the model and public-facing systems, and treating refresh, expiry, and revocation as routine lifecycle events. Where the provider supports it, add sender-constrained tokens or refresh-token rotation to reduce replay risk. None of these controls can protect credentials if an attacker compromises the host and its key material.
Start with a safe authorization flow
Use the OAuth authorization code flow with PKCE where appropriate. RFC 9700, the IETF’s OAuth 2.0 security best-current-practice standard published in January 2025, requires PKCE for public clients and recommends it for confidential clients. Use the S256 challenge method: RFC 9700 identifies it as the current PKCE method that does not expose the verifier in the authorization request.
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Generate a transaction-specific PKCE challenge and bind the authorization transaction to the client and user agent. Protect the redirect endpoint against cross-site request forgery (CSRF); if the client works with multiple authorization servers, implement a mix-up defense. Accept only registered, expected redirect destinations rather than destinations supplied in request parameters. Avoid the implicit flow and do not put access tokens in authorization-response URLs, where they are more exposed to leakage and replay.
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Limit the agent’s authority
Request only the scopes the agent’s workflow needs. Keep an access token’s audience to one resource server where possible, and require resource servers to check that the token is intended for them. If a workflow genuinely needs several APIs, use the smallest necessary set rather than treating one broad token as convenient access to every service.
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Bind refresh tokens to the scopes and resource servers the user approved. RFC 9700 explains that a refresh token is especially valuable because it can be exchanged for new access tokens; it must not become a way for a client to expand the user’s authorization. Narrow scopes and audiences limit the potential impact if a token is exposed.
Keep tokens and keys out of the agent’s conversational surface
Treat both access and refresh tokens as credentials. A practical server-side design keeps them in a private datastore with encryption at rest and strict access controls, rather than exposing the token store to the public internet. Keep raw token values out of prompts, logs, traces, crash reports, and analytics. Give the agent’s model or tool-planning layer only the operation results it needs, not a copy of reusable credentials.
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Google for Developers’ Best Practices | Authorization Resources advises storing tokens securely at rest and never transmitting them in plaintext; it also recommends encrypting server-side tokens when an application stores tokens for multiple users. Its examples of platform-specific secure storage include Android Keystore, Apple Keychain Services, and Windows Credential Locker. Choose storage appropriate to the deployment rather than assuming one platform mechanism applies everywhere. RFC 9700 also says resource servers must not store or transfer access tokens in plaintext.
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Protect the private keys or client certificates used for sender-constrained tokens as carefully as the tokens themselves. Appropriate platform protections or a hardware or software security module may fit an architecture, but the right mechanism depends on the environment and provider support. If an attacker gets both a token and its associated key material, sender constraint no longer provides the intended protection.
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Choose a replay defense the provider and deployment can support
A stolen bearer token can be replayed by whoever has it. RFC 9700 recommends sender-constrained access tokens, including DPoP or mutual TLS, where supported. For public clients, it requires refresh tokens to be sender-constrained or rotated. These options address different operational needs; provider and resource-server support must be verified for the actual integration.
| Option | How it constrains replay | Deployment considerations |
|---|---|---|
| DPoP | The client signs application-level proofs with a public/private key pair, demonstrating possession of the associated private key. It can be used by public clients and combined with confidential-client authentication. | Check authorization-server and resource-server support, and protect the private key. See RFC 9449, published September 2023. |
| Mutual TLS (mTLS) | Token use is bound to client-certificate key material and the TLS connection. | It suits deployments able to provision and maintain client certificates across the client and resource server. Check support and certificate lifecycle operations. See RFC 8705, published February 2020. |
| Refresh-token rotation | After a refresh, the authorization server issues a replacement and invalidates the previous refresh token. Reuse of the invalidated token can signal replay. | For a public client, RFC 9700 requires rotation or sender constraint. If reuse is detected, the server may revoke the active token; it cannot know whether the legitimate client or an attacker made the second request, so the user may need to authorize again. |
These mechanisms reduce particular replay risks; they are not interchangeable checkboxes or a defense against a fully compromised endpoint. Select based on client architecture, provider capabilities, where keys can be protected, and the team’s ability to operate key or certificate lifecycles—or handle reauthorization after a rotation event.
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Make refresh a controlled lifecycle operation
Do not assume a refresh token remains valid indefinitely. RFC 9700 recommends that authorization servers expire refresh tokens after inactivity, with timing determined by server policy and potentially by client or grant sensitivity. Servers may also revoke tokens after events such as a password change or logout. There is no universal token lifetime established by these sources; follow the policy and behavior of the authorization server in use.
When rotation is enabled, serialize refresh operations for a given grant so that concurrent work does not accidentally reuse an invalidated token. Store the replacement token as part of the refresh transaction and stop using the old value. This is an operational precaution based on rotation’s invalidation behavior, not a substitute for the authorization server’s replay handling.
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If refresh fails because a credential is expired, revoked, or otherwise invalid, stop using it. Avoid unbounded retry loops, which can worsen rate limits or token-reuse incidents. Clear or quarantine the stored credential according to the application’s policy, then request fresh user authorization when needed. Google’s guidance likewise advises applications to account for token invalidation or expiration and decide whether to prompt at the next sign-in or clean up associated data. Revoke and delete tokens when they are no longer needed.
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