upload_media · Posterly MCP: Risk & Policy

WHAT IT DOES

What upload_media does on Posterly

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

WHY IT NEEDS A POLICY

Why upload_media needs a policy

The tool creates and stores new media assets, making it a Write operation. Severity is medium rather than high because: (1) uploads are reversible (media can be deleted), (2) the blast radius is limited to media assets rather than financial or account-level changes, and (3) worst case involves storage quota exhaustion or inappropriate content being stored, not account compromise.

From the tool's definition Tool description states it 'Upload[s]...file to posterly storage. Returns a URL' — this creates new data (media files) in the system that persists in storage and can be subsequently used in posts.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

upload_media Rate-limited

upload_media 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_media": {
      "limits": [
        {
          "counter": "upload_media_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 Posterly, so you can run agents against it safely.

FAQ

Questions about upload_media

What does the upload_media tool do? +

Upload an image or video file to posterly storage. Returns a URL that can be used with create_post. Supports JPEG, PNG, GIF, WebP, MP4, MOV, WebM. Images up to 10MB, videos up to 50MB.\n\n. It is categorised as a Write tool in the Posterly 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_media? +

Register the Posterly MCP server in PolicyLayer and add a rule for upload_media: 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 Posterly. Nothing to install.

What risk level is upload_media? +

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

Can I rate-limit upload_media? +

Yes. Add a rate_limit block to the upload_media 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_media completely? +

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

upload_media is provided by the Posterly MCP server (posterly-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.