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The least-privilege approach for an AI agent is deny by default, then grant only the narrowly defined tools, operations, data, identity and time window required for one task. Enforce that decision at an API gateway, policy service or tool-execution proxy on every call—not in a system prompt and not by asking the model to police itself. Separate read from write access, use short-lived task credentials, require independent approval for consequential actions, and isolate any code execution.
This model limits the damage an agent can cause when it misinterprets a request, follows a prompt injection, uses a compromised extension or produces a hallucinated tool call. It does not make model behavior inherently trustworthy; it creates a boundary that remains in force when the model is wrong.
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What least privilege means for an AI agent
For a conventional application, least privilege means giving a process only the permissions needed to perform its function. An agent adds a language model that can choose tools, construct parameters and chain actions, often while handling untrusted documents or websites. The security boundary therefore has to cover more than a role name.
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For every call, decide all of the following:
- Who: the initiating user or service, the agent identity and, where relevant, the tenant.
- What: the exact tool and operation, such as
read_invoicerather than a general database connection. - Which resource: a particular mailbox, project, record, repository or folder.
- With what data: the fields and sensitivity level the operation may access or return.
- Where and under which conditions: the approved environment, network and input-trust state.
- For how long: a task-scoped credential that expires or is revoked when the task ends, times out or is cancelled.
NIST’s tool-use taxonomy describes permission levels and whether the environment is trusted or untrusted, providing a useful vocabulary for designing these boundaries: NIST, “Lessons Learned from the Consortium: Tool Use in Agent Systems” (August 5, 2025).
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Permission levels: read-only, constrained write and write
Start by classifying the agent’s capability before selecting tools or credentials. A task that only needs retrieval should not inherit the authority to modify the same system.
| Level | Typical capability | Suitable examples | Controls to add |
|---|---|---|---|
| Read-only | Retrieve approved records or content; no external state changes | RAG, document search, research and summarization | User- or tenant-scoped identity, field filtering, rate limits and output monitoring |
| Constrained write | Perform a limited, reversible change on named resources | Drafting a ticket, updating a status field or creating a proposed code change | Allowlisted operations and parameters, validation, transaction limits and review before publication |
| Write | Change, delete, send, configure or administer external systems | Sending email, deploying code, changing access or issuing a refund | Separate credentials, independent authorization, exact-action approval, strong audit logs and rapid revocation |
“Write” should not be a default merely because an SDK or connector exposes it. Remove unused functions and, within retained tools, remove operations the task does not need. A read-only agent should receive a database identity that cannot update or delete, not just a prompt saying “do not write.”
Design the boundary outside the model
The model can propose a tool call, but a trusted component must decide whether that call is authorized. Put an API gateway, policy decision point or tool proxy between the agent runtime and every capability. The proxy should fail closed when identity, resource, operation, approval or policy context is missing.
Validate both syntax and context. A request can match a perfectly valid JSON schema and still be unauthorized—for example, a valid delete_record request aimed at another tenant. Use per-tool and per-operation allowlists, narrow parameter schemas, resource checks and policy evaluation on every invocation. Do not allow a previously approved token to be replayed with different parameters.
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A practical authorization record
Represent each grant as a structured decision containing the initiating principal, agent identity, tenant, tool, operation, target resource, permitted fields, environment, purpose, expiry and any approval identifier. The execution layer should compare the actual call with that record and reject deviations. Record the effective permission state with the call, while redacting secrets and unnecessary personal data.
Step-by-step implementation pattern
- Inventory capabilities. List every tool, operation, reachable data set, side effect, network path and execution environment. Mark whether each action reads, creates, changes, sends, deletes or administers.
- Remove excess functionality. Delete unused tools and split broad tools into specific operations. Prefer
get_customer_balanceover a general-purpose SQL or shell interface. - Set a deny-by-default policy. Permit a named operation only on a defined resource for a defined task. No matching rule means denial.
- Create task-scoped credentials. Issue the smallest practical scope and shortest useful lifetime. Expire or revoke it at completion, timeout, cancellation or detected abuse.
- Bind the user and tenant context. Carry the initiating user, agent identity and tenant through the call. Use the user’s scoped authorization where appropriate instead of a shared, broadly privileged service account.
- Separate read and write paths. Use distinct identities and endpoints for retrieval, proposal, commit, delete, send and administration. A read-only workflow must not be able to “upgrade” itself by selecting another operation.
- Validate at execution time. Check the operation, target, fields, parameters, policy state, token expiry and approval binding immediately before the tool acts.
- Gate high-impact actions. Keep proposing separate from executing. Require independent validation and, where appropriate, human approval for destructive, financial, administrative or externally visible actions.
- Constrain the environment. Use specific APIs instead of open-ended shell or code tools. When code is unavoidable, run it in a sandboxed, low-privilege process with only explicitly required filesystem and network access.
- Observe and test. Log calls, denials, approvals, effective scopes and tool chains without recording secrets. Monitor unusual volume or sequences and run adversarial tests after changes to prompts, tools, memory, retrieval or model providers.
How to handle approvals safely
Approval must describe the action that will actually occur. Bind it to the actor, agent, tool, target, complete parameter set, purpose and expiration. An approval to send one message should not authorize a later message with a changed recipient or attachment.
Use independent checks for high-impact operations:
- Destructive: deletion, irreversible migration or data purging.
- Financial: payments, refunds, transfers or purchases.
- Administrative: permission changes, account creation, key rotation or production configuration.
- Externally visible: sending email, publishing content, filing a legal or regulatory submission, or deploying software.
Where practical, make the agent produce a preview or proposed diff, validate it with deterministic rules, then ask for approval. The executor—not the model—should verify that the approved parameters still match at commit time.
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Untrusted inputs require tighter limits
Documents, email, web pages, retrieved text, tool output and messages from other agents are data, not authority. They can contain direct or indirect prompt injection that attempts to make the agent reveal secrets, call an unintended tool or change its goal. Treat external content as untrusted and keep its instructions outside the policy channel.
When untrusted content enters a workflow, narrow permissions further: use read-only credentials, restrict reachable resources, disable outbound network access where possible and require review before any side effect. If sensitive data is introduced, reduce the set of tools that can see it and prevent unnecessary copying into prompts, logs or memory.
Delegated and subagent work
A subagent should receive a subset of the parent’s permissions, never the parent’s full token by default. Define the delegated task, target resources, allowed operations and expiry explicitly. A summarization subagent might read three specified files but have no mailbox, network or write capability. Revoke the subset when the delegation ends.
Code execution and open-ended tools
Open-ended shell, browser automation or arbitrary code tools combine broad functionality with difficult-to-predict effects. Prefer narrowly designed APIs that expose only the required operation and return only required fields.
If code execution is necessary, combine several controls:
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- Sandbox the process and run it as a non-administrator user.
- Mount only the required files or a temporary workspace.
- Allowlist network destinations and block metadata services and internal administration endpoints.
- Set CPU, memory, process, storage and execution-time limits.
- Separate credentials from the host and inject only task-scoped secrets, preferably through a broker.
- Scan outputs and require approval before moving results into a trusted system.
Why excessive agency is the failure mode
OWASP characterizes excessive agency as a weakness in which unexpected, ambiguous or manipulated model output leads to damaging action. Its common causes are excessive functionality, excessive permissions and excessive autonomy. For example, an email-summarization agent may have an extension that can also send messages, while its mailbox identity can access more accounts than the user intended.
The least-privilege correction is concrete: expose reading and summarization only, use a read-only user-scoped mailbox authorization, remove the send function and require review before any message is sent. OWASP’s LLM06:2025 Excessive Agency and AI Agent Security Cheat Sheet also discuss prompt injection, tool abuse, privilege escalation, data exfiltration, memory poisoning, goal hijacking and cascading failures.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Controls that support—but do not replace—least privilege
Logging, rate limits and monitoring are valuable, but none is a substitute for an authorization boundary. A rate limiter may slow an unauthorized deletion; it does not make the deletion permissible. Similarly, a complete audit trail helps investigation after a policy failure but cannot undo an external message or irreversible transaction.
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- Tool-chain limits: cap depth, duration and unusual combinations of tools.
- Input validation: reject malformed or out-of-policy values before execution.
- Output controls: redact secrets and sensitive fields before returning data to the model or user.
- Memory isolation: separate tenants and trust domains; do not let untrusted content silently become durable instructions.
- Adversarial testing: test direct and indirect injection, confused-deputy scenarios, cross-tenant access, replayed approvals and compromised extensions.
- Revocation drills: verify that credentials, sessions, queued jobs and delegated tokens stop working after cancellation.
These practices align with the controls in OWASP’s General Controls — AI Security and Privacy Guide and its Securing Agentic Applications Guide 1.0, dated July 27, 2025.
How to compare two agent designs
When more than one architecture is possible, compare designs against the same questions rather than choosing by convenience:
| Question | What a safer design demonstrates |
|---|---|
| Permission level | Read-only where possible; constrained write only when necessary; unrestricted write is exceptional. |
| Environment and input trust | Untrusted documents and web content trigger narrower scopes and stronger review. |
| Data and resource scope | Specific fields, records, folders or repositories—not an entire database or tenant. |
| Impact and reversibility | Irreversible, financial or public actions have independent checks and approval. |
| Credential identity and duration | User- or task-scoped identity, minimal breadth and automatic expiry. |
| Independent enforcement | A gateway or executor evaluates authorization on every call outside model reasoning. |
| Audit and revocation | Calls, denials, approvals and effective scopes are traceable, and access can be revoked quickly. |
Common mistakes and their fixes
Relying on a system prompt
“Never delete data” is guidance, not an enforceable permission. Remove the delete operation or deny it at the executor.
Giving a read task a shared service account
A shared identity often has write and cross-tenant authority. Replace it with a read-only, user- or tenant-scoped identity.
Using schema validation as authorization
Valid JSON proves only that a request is well formed. Add principal, tenant, resource, operation and policy checks.
Approving a vague plan
“Proceed with cleanup” is not an adequate approval. Require an exact list of targets, operations, parameters and expiry.
Leaving extensions enabled “just in case”
Every retained extension enlarges the attack surface. Remove unused functionality and grant each extension only the subset needed for its delegated task.
Quick Recap
A concise review checklist
- Does every tool call pass through an external policy enforcement point?
- Is the default decision deny?
- Are tools and operations minimized separately?
- Can a read-only task technically write, delete or send?
- Are user, agent, tenant, resource and operation bound to each call?
- Do credentials expire automatically and revoke on cancellation?
- Are high-impact actions independently validated and approved?
- Are code, browser and shell capabilities sandboxed and network-restricted?
- Are external content and peer-agent messages treated as untrusted?
- Can you reconstruct what happened without exposing secrets?
- Have you tested injection, confused-deputy, replay and cross-tenant cases?
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