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Authentication is necessary to secure an AI agent, but it does not make the agent’s actions safe. A credential can establish which user, agent, or service is making a request; a separate authorization check must decide whether that principal may perform this specific operation on this specific resource, with these parameters, now. Enforce that decision in a trusted tool, gateway, or downstream service—not in the model’s prompt or its own judgment.
Why a valid identity is not enough
Authentication answers, “Who or what presented this credential?” Authorization answers, “May that principal perform this operation on this resource?” An agent can be correctly authenticated and still attempt an action outside the user’s intent, the agent’s task, or the organization’s policy. OWASP’s MCP07:2025 – Insufficient Authentication & Authorization recommends validating tokens server-side and evaluating permissions on each request.
This distinction matters because agents can consume untrusted content and invoke tools with real-world effects. NIST’s CAISI article Strengthening AI Agent Hijacking Evaluations (January 2025) describes indirect prompt injection: malicious instructions hidden in ordinary-looking emails, files, or websites. In the scenarios CAISI tested, agents were frequently induced to follow instructions involving code execution, data exfiltration, or phishing. Those results describe the tested systems and scenarios; they do not establish a success rate for all agents.
If an agent has broad access, a manipulated goal can turn into an external action. Treating input as untrusted helps, but it cannot replace an independent authorization boundary. OWASP’s LLM06:2025 Excessive Agency groups the underlying risks as excessive functionality, excessive permissions, and excessive autonomy.
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Where the security boundary belongs
Put the decisive permission check at the point where a request can cause an effect: in a trusted API, policy service, tool-execution proxy, or downstream application. OWASP’s AI Agent Security Cheat Sheet states: “Enforce authorization in the execution component, outside the agent’s context.” The model may propose an action, but it must not be the component that grants itself permission to carry it out.
| Design choice | What it establishes | Security implication |
|---|---|---|
| Prompt or model refusal alone | The agent was instructed or appeared to decide not to make a call. | Not an access-control boundary: it does not prove that an unauthorized call would be blocked at execution. OWASP’s AI Agent Security Cheat Sheet and LLM06:2025 Excessive Agency recommend enforcing authorization outside the model. |
| Authorization at the tool, gateway, or downstream service | A trusted component evaluated the principal and the requested operation before execution. | Provides an enforceable boundary, provided the check covers the actual action and fails closed when the decision cannot be validated. OWASP MCP07:2025 recommends per-request authorization and deny-by-default. |
| Final response or refusal recorded | What the agent said after attempting—or declining—to act. | Does not by itself establish whether a side effect occurred or whether an unauthorized call was blocked. Record the decision and execution outcome, and test the boundary, as recommended by OWASP’s AI Agent Security Cheat Sheet. |
For each request, the enforcement component should evaluate the authenticated agent and any delegated user authority, the operation, the target resource, the relevant scope, and the action parameters. Do not treat a previous login, a broad session grant, or the agent’s statement of intent as a substitute for this decision. If identity, policy, or required approval cannot be validated, deny the action.
Rank #2
Build the authorization path around the action
- Establish the principal chain. Track the human who requested the work, the agent instance, the orchestrator, and the tool endpoint involved in each action. Do not trust caller identity supplied only as client-side metadata. OWASP MCP07:2025 identifies unverified caller identity and missing identity correlation in logs as risk indicators.
- Define narrow capabilities for each agent. Separate read access from write access, constrain which resources are available, and put high-privilege operations in distinct workflows. For example, an agent that reads email need not also have the ability to send or delete it. This follows OWASP’s excessive-agency guidance to minimize functionality and permissions.
- Authorize every request at execution. At the trusted enforcement point, check identity, delegated authority, operation, target, scope, and action parameters. Apply a default-deny policy when a request does not match an explicitly permitted action.
- Constrain credentials. Prefer attributable, revocable, short-lived credentials scoped to the task; avoid shared, long-lived tokens and generic high-privilege service identities. Where feasible, execute in the user’s authorized context. OWASP MCP07:2025, OWASP LLM06:2025, OWASP AISVS 1.0, and NIST’s Back to the Future: Why Agentic AI Needs a Strong Identity Foundation all address limiting or managing agent authority.
- Make high-impact actions require meaningful approval. Bind approval to the exact operation, target, and normalized parameters, and have the trusted executor validate it immediately before acting. If a material parameter changes, require a fresh decision. For critical or irreversible operations, consider step-up authentication and replay protection.
- Log and test what actually executed. Record identity and authority context, the tool request, authorization decision, target, and outcome. Test whether the executor blocks unauthorized calls, including calls proposed after indirect prompt injection.
Choose controls according to action risk
Approval is not equally useful for every tool call. Asking users to approve routine, low-risk steps can create consent fatigue; a vague repeated “allow” prompt also gives little assurance that the user understood the action. OWASP’s AI Agent Security Cheat Sheet and NIST’s identity guidance support risk-based approval rather than treating every interaction alike.
| Action type | Practical control |
|---|---|
| Read-only lookup already within the agent’s authorized scope | It may proceed without an interactive confirmation if the execution-side policy permits that resource and operation. |
| External message, deletion, or change to a user’s data | Require approval appropriate to the impact, showing the recipient or target and the exact proposed content or change. Bind the approval to those parameters. |
| Privilege change, movement of funds, or production deployment | Use stronger review and authorization; consider step-up authentication and replay protection for critical or irreversible operations. Revalidate the exact request immediately before execution. |
These are risk tiers, not universal classifications: an organization should define them for its own systems and consequences. The key is that approval must authorize the actual operation, not merely grant the agent a broad permission it can later apply to different targets or parameters.
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A model that refuses a dangerous request in a test conversation is not evidence that the system prevents unauthorized side effects. Test the enforcement boundary with requests that vary identity, scope, target, action, and approval, and verify the outcome at the tool or downstream service.
- Submit calls with invalid or missing identity, expired or revoked credentials, and caller identity asserted only in client-supplied metadata.
- Try an operation outside the agent’s scope, a permitted operation against an unauthorized target, and a write action using a read-only capability.
- Attempt to reuse an approval after changing the operation, target, or material parameters; confirm that stale or replayed approval is rejected where protection is required.
- Include indirect-injection tasks using untrusted emails, files, or websites, then check whether proposed out-of-scope tool calls are blocked at execution.
- Inspect logs to confirm they connect the human, agent, orchestrator, authorization decision, request, and outcome.
NIST CAISI recommends adaptive, task-specific evaluation and notes that testing across multiple attempts can better reflect risk. Its published discussion does not provide a numerical success rate in the cited passage, so the qualitative findings should not be turned into a general percentage or guarantee. Evaluate your own system’s execution controls against the actions and threat scenarios relevant to its deployment.
Rank #4
The implementation standard
Use authentication to identify the caller, then make an independent, fail-closed authorization decision for every action where it can take effect. Keep the agent’s credentials and capabilities limited to its task, attach consequential approvals to exact requests, and retain evidence of the decision and outcome. That is how to stop an authenticated agent from taking an action it was never authorized to take.
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