Stopping an Agent from Sharing Client Files Through the Google Drive MCP Server

How file sharing is exposed through an MCP server

A Model Context Protocol server presents Drive to an agent as a set of callable tools. The agent does not browse Drive. It selects a tool, fills in parameters and receives a result. For client files, the tools that matter fall into four groups: reading content, sharing files, changing permissions, and moving or copying files. Each group can move confidential data outside its intended audience, and the sharing and permission tools do so without the agent ever reading the file.

Before designing controls, inventory the specific server you deploy. List every tool it exposes, mark which ones can alter who can access a file, and record which parameters each accepts, such as a recipient address, a permission role or a link visibility setting. Tool surfaces differ between implementations, so the inventory, not a generic assumption, determines which rules you need. Grant the agent only the tools its task requires and remove the rest from the server's registered set.

How an agent ends up sharing client files

Three paths account for most of the risk. The first is prompt injection. A document, file name, comment or message from another agent can carry instructions, and the agent may treat them as part of its task. Guidance on agentic systems is direct on this point: do not trust instructions, metadata or artefacts that originate from users or other agents, and do not let external content reach memory unvalidated.

The second is an over-permissioned token. If the agent holds broad sharing rights, any mistaken or manipulated call succeeds. The third is plain misjudgment. An agent asked to send a report to a client may pick the wrong recipient, grant editor rather than viewer access, or create a link open to anyone who has it. None of these requires an attacker, and none is stopped by a scope that permits sharing in general.

The design consequence is that any tool-call input derived from file content or other agents should be treated as untrusted before it influences a sharing decision.

Defining and sequencing policy rules for sharing calls

Order matters, because the first rule that resolves a call determines the outcome. Start with the cheapest and most decisive checks and finish with permissive paths. Conditions such as recipient domain, permission role, file label and request frequency are useful only where the tool exposes the corresponding parameter or the file can be classified, so inventory and label sensitive client files first. Write the rules down as a data usage policy that states how access is determined, authenticated and enforced.

• 1. Block external exposure outright: Block calls that make a file public or link-accessible, and calls that grant access to a recipient domain not on an approved list.
• 2. Escalate for human approval: Route sharing of labeled client files, or grants of editor-level roles, to a named approver before the call proceeds.
• 3. Redact on read: Where a read call returns content containing sensitive fields, redact those fields before the result reaches the agent or its downstream tools.
• 4. Rate limit bursts: Cap the number of sharing, permission and move calls per agent per interval. Treat this as a brake that slows bulk exposure, not as a boundary, since a determined actor can work around volume limits.
• 5. Allow the remainder: Permit calls that match the task's approved scope, such as internal recipients with a viewer role, and record each decision.

Evidence to retain and how long to keep it

An auditor or regulator will ask what the agent was allowed to do and what it did. Answer that with a record created at the moment of enforcement rather than reconstructed afterward. For each sharing-related call, retain the agent and session identity, the tool and its parameters, the file and its label, the policy rule that applied, the decision (allow, redact, block, rate limit or escalate), the approver and approval time where a human was involved, and the outcome returned by Drive. Keep the policy version in force at the time, so a decision can be explained against the rule as it then stood.

Retention needs its own decision. If your organization is subject to the FTC Safeguards Rule, 16 CFR Part 314, note that as of November 2026 its definition of authorized user covers agents, among other persons authorized to access information systems or data. The rule also calls for periodic review of the data retention policy to minimize unnecessary retention. Agent evidence should be covered by that review, and applicability to your organization should be confirmed with counsel.

Trussed enforces policy in the execution path for tool calls on MCP servers, with allow, redact, block, rate limit and escalate for human approval, and generates audit-ready evidence from that enforcement, mapped to HIPAA, NAIC, NYDFS, EU AI Act and NIST. It deploys self-managed in the customer cloud with configurable retention and data residency. The pattern itself, enforcement at the call and evidence produced by that enforcement, applies regardless of the tooling you choose.