s3_presign_put · AWS S3 MCP Server MCP: Risk & Policy

WHAT IT DOES

What s3_presign_put does on AWS S3 MCP Server

AI agents use s3_presign_put to create or update resources in AWS S3 MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your AWS S3 MCP Server environment.

WHY IT NEEDS A POLICY

Why s3_presign_put needs a policy

This tool enables creating or writing new objects to S3, which is reversible (objects can be deleted). While the tool itself doesn't directly upload data, it generates credentials that allow S3 uploads.

From the tool's definition Tool generates presigned URLs for uploading objects to S3. The description explicitly states 'uploading an object' which creates or modifies data in S3 buckets.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

s3_presign_put Rate-limited

s3_presign_put 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": {
    "s3_presign_put": {
      "limits": [
        {
          "counter": "s3_presign_put_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

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

FAQ

Questions about s3_presign_put

What does the s3_presign_put tool do? +

Generate a presigned URL for uploading an object to S3. It is categorised as a Write tool in the AWS S3 MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on s3_presign_put? +

Register the AWS S3 MCP Server MCP server in PolicyLayer and add a rule for s3_presign_put: 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 AWS S3 MCP Server. Nothing to install.

What risk level is s3_presign_put? +

s3_presign_put is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit s3_presign_put? +

Yes. Add a rate_limit block to the s3_presign_put 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 s3_presign_put completely? +

Set action: deny in the PolicyLayer policy for s3_presign_put. 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 s3_presign_put? +

s3_presign_put is provided by the AWS S3 MCP Server MCP server (khuynh22/aws-s3-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.