upload_image

THE RISK

Medium Risk

Why upload_image needs a policy

This tool creates new media assets on Substack but does not execute arbitrary code, delete irreversibly, move money, or read without side effects. It fits Write (creates/modifies data reversibly). Severity is medium because misuse could result in unauthorized content uploads to a user's account, but the blast radius is limited to image asset storage without direct publication or deletion impact.

From the tool's definition Tool description states 'Upload an image file' which creates/adds new content to Substack. The action is reversible (images can be deleted).

Documented attack patterns abuse exactly the kind of access upload_image gives an agent:

POLICY

How to control upload_image

PolicyLayer is an MCP gateway — it sits between your AI agents and Substack Mcp Plus, and nothing reaches the server without passing your rules. This is the rule we recommend for upload_image:

policy.json

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

upload_image 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.

Create a free account and register Substack Mcp Plus — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

LIMIT THIS TOOL →

Free to start. No card required.

EXPLORE

Related tools and policies

More Substack Mcp Plus tools

Destructive delete_draft Delete a draft post. IMPORTANT: You MUST ALWAYS ask the user to confirm deletion in a foll Execute publish_post Publish a draft post immediately to your Substack publication. This makes the post publicl Write create_formatted_post Create a new formatted draft post on Substack. Supports full markdown formatting. IMPORTAN Write duplicate_post Create a copy of an existing post as a new draft. Perfect for using posts as templates. IM Write update_post Update an existing Substack draft post. WARNING: This tool COMPLETELY REPLACES the specifi Read get_post_content Read the full content of a specific post (draft or published) with all its formatting. Ret Read get_sections Get a list of available sections/categories in your Substack publication. Sections help or Read get_subscriber_count Get the total number of subscribers to your Substack publication. Useful for tracking grow

All 12 Substack Mcp Plus tools →

Write tools on other servers

M-Team MCP Server download_torrent YouTube MCP Server generate_video_title Android Forensics ADB MCP Server combine_reports 0xarchive web3_signup

Go deeper

The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Argument validation → Checking tool call arguments against schema and policy before they run.
Scoped token → Per-person credentials that grant a defined subset of servers and tools.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about upload_image

What does the upload_image tool do? +

Upload an image file from your local computer to Substack. It is categorised as a Write tool in the Substack Mcp Plus 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_image? +

Register the Substack Mcp Plus MCP server in PolicyLayer and add a rule for upload_image: 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 Substack Mcp Plus. Nothing to install.

What risk level is upload_image? +

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

Can I rate-limit upload_image? +

Yes. Add a rate_limit block to the upload_image 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_image completely? +

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

upload_image is provided by the Substack Mcp Plus MCP server (ty13r/substack-mcp-plus). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Substack Mcp Plus tool call.

Start from Substack Mcp Plus, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →