upload_file · S3 MCP: Risk & Policy

WHAT IT DOES

What upload_file does on S3

AI agents use upload_file to create or update resources in S3 — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your S3 environment.

WHY IT NEEDS A POLICY

Why upload_file needs a policy

Upload operations create new objects or overwrite existing ones in S3, which is a Write category action. Severity is high because an AI agent could upload malicious files, overwrite critical data, or consume significant storage/costs. Confidence is high because the description clearly indicates file creation/modification without deletion or financial transaction.

From the tool's definition Tool description states 'Uploads a file to an S3 object', which creates or modifies data in S3. The sibling tools include destructive operations (delete_object, delete_objects) and read operations (get_object, list_buckets), confirming this server's scope.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

upload_file Rate-limited

upload_file stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "upload_file": {
      "limits": [
        {
          "counter": "upload_file_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

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PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches S3, so you can run agents against it safely.

FAQ

Questions about upload_file

What does the upload_file tool do? +

Uploads a file to an S3 object. It is categorised as a Write tool in the S3 MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on upload_file? +

Register the S3 MCP server in PolicyLayer and add a rule for upload_file: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches S3. Nothing to install.

What risk level is upload_file? +

upload_file is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit upload_file? +

Yes. Add a rate_limit block to the upload_file rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.

How do I block upload_file completely? +

Set action: deny in the PolicyLayer policy for upload_file. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.

What MCP server provides upload_file? +

upload_file is provided by the S3 MCP server (konmam/s3-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.