MCP server risk comparison: Slack, Microsoft 365 and Google Drive for broker-dealers

Why the MCP server is the point of risk

An MCP server is the connection point between an AI assistant and external tools, APIs and data sources. OWASP notes that MCP servers operate with delegated user permissions, dynamic tool-based architectures and chained tool calls. Together these raise the potential impact of a single vulnerability compared with a traditional API integration.

For a broker-dealer, delegated permissions matter most. If the agent acts as a user, a tool call may carry that user's full access. A supervisor's account can reach far more client material than any single task requires. Chaining compounds the problem: content retrieved in one call can steer the next call, so a poisoned message, email or document can influence what the agent does with data it was never meant to touch.

OWASP names tool poisoning, prompt injection, memory poisoning and tool interference as risks of connecting models to third-party tools. These apply to all three platforms. What differs is the content an attacker can plant, the data the agent can reach, and the actions available to carry out an injected instruction. That is why the comparison below works at the level of tool-call capability, not vendor.

Risk matrix: six criteria applied to each connection

Use the matrix as a template. Confirm each cell against the specific MCP server and the scopes your firm would grant, because servers differ in which tools they expose and how narrowly access can be set.

• Data sensitivity: Slack: client communications and internal discussion, often informal and unstructured. Microsoft 365: email and shared documents, with client correspondence and attachments. Google Drive: stored documents, which may include client files and supervisory records.
• Permission scope: Slack: workspace and channel reach, including private channels the user belongs to. Microsoft 365: mailbox, calendar and file reach under the user's account. Google Drive: everything the user can open, including files shared with them. In every case, ask whether scopes can be set below the user's full access.
• Read versus write actions: Slack: reading history and search versus posting or editing messages. Microsoft 365: reading mail and files versus sending mail or modifying files. Google Drive: reading and searching versus creating, editing or deleting files. Writes are harder to reverse than reads.
• External sharing exposure: Slack: posting into channels that include external parties. Microsoft 365: sending to outside recipients or creating sharing links. Google Drive: link sharing and permission changes that open files to outside parties. This is the main route for data leakage.
• Authentication and token handling: For all three: how tokens are issued, where they are stored, how they are rotated and revoked, and whether the agent holds an identity separate from the user's. A stored token with broad scope is a standing exposure.
• Auditability of tool calls: For all three: whether each call can be recorded with the requested action, the policy decision, any approval and the data accessed, then retained in a form that supports books-and-records review.

Reading the matrix: how the risk differs by platform

Messaging, email and file storage fail in different ways. In Slack, the central concern is breadth of conversational content and the speed of outbound posting. A message written into the wrong channel is visible immediately, and retrieved messages are an easy carrier for injected instructions because anyone in a channel can write them.

In Microsoft 365, email adds the most direct route to a client. An agent that can read a mailbox and send mail can both collect sensitive information and transmit it, within one chain of calls. Attachments and shared documents widen the data in scope, so the pairing of read and send actions deserves the closest control.

In Google Drive, the exposure is mostly stored data and sharing. Search across a drive can return large volumes of documents, and a single permission change or link share can expose a file to outside parties without any message being sent. The risk is quieter, which makes evidence of what was accessed and shared more important.

Two patterns hold across all three. First, outbound actions carry more consequence than reads. Second, bulk retrieval followed by an outbound action is the sequence most worth interrupting.

Deciding which tool calls to allow, redact, block, rate limit or escalate

OWASP emphasizes least-privilege access and human-in-the-loop oversight. Applying that per tool call gives five practical outcomes.

Allow narrowly scoped reads tied to a defined task, such as retrieving a specific document or thread the user is already working on. Redact when a read is legitimate but the response contains customer information the task does not need, so the model never receives it. Rate limit search and listing calls, which are the usual way an agent sweeps a mailbox, workspace or drive. Escalate for human approval any call that posts, sends, creates a sharing link or changes permissions, particularly where recipients are outside the firm. Block deletion and permission-changing calls by default until a specific use case justifies them.

These defaults apply to all three platforms, but the classification of an individual call can differ. Posting an internal Slack message may warrant approval only when the channel includes outside parties. Sending an email to an external address almost always will. Define the rules by tool call and context, then test them against injected instructions in retrieved content, since the agent's own judgment cannot be the control.

The regulatory frame and the evidence an examiner would expect

FINRA states that existing obligations apply when firms use generative AI, and that policies should address technology governance, including model risk management, data privacy and integrity, and reliability. FINRA's oversight reports also note that cyber events involving customer information can affect compliance with Rules 3110, 3120 and 4370, Exchange Act Rules 17a-3 and 17a-4, Regulation S-P and Regulation S-ID.

Rule 30 of Regulation S-P requires written policies and procedures for administrative, technical and physical safeguards for customer records and information. The amendments adopted May 15, 2024 add an incident response program and notice to affected individuals after unauthorized access to or use of sensitive customer information, no later than 30 days after the firm becomes aware. As of November 2026, the compliance dates for larger entities (December 2, 2025) and smaller entities (June 3, 2026) have passed. An over-permissioned agent reading customer data is an access path that program has to detect and respond to.

The following evidence list is a practitioner expectation, not rule text. For each connection, a firm should be able to show the policy defining what the agent was permitted to do, each requested action, the policy decision on it, any human approval, and the data accessed. Retention should be set to meet the firm's books-and-records approach. Evidence generated at the moment of enforcement is more credible than records reconstructed afterward.

Where runtime policy enforcement fits

Reviewing agent activity after the fact tells a firm what went wrong. It does not stop a share or send from happening. Trussed enforces policy in the execution path across agents, tools and MCP servers, applying allow, redact, block, rate limit or escalate for human approval to individual tool calls. Because enforcement generates audit-ready evidence as it runs, the record of what the agent was allowed to do and what it did comes from the decision itself. Trussed supports self-managed deployment in the customer cloud, with configurable retention and data residency.

The tradeoff is operational. Per-call policy requires a firm to classify its tool calls, name approvers and accept some friction on outbound actions. That effort is the same whichever platform enforces the policy, and it is what makes a connection defensible to an examiner.