Fetch an image or video from a public HTTPS URL into Posterly storage. Returns a URL that can be used with create_post. You can also pass public HTTP(S) media URLs directly to create_post/update_post; use this tool when you want to upload once and reuse the returned Posterly URL.
AI agents use upload_media_from_url 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.
This tool creates new media resources in a storage system and returns references to them for later use. While reversible (media can presumably be deleted), it modifies the user's stored data and has side effects. It does not execute arbitrary code, delete data irreversibly, or move money, making Write the appropriate category.
From the tool's definition The tool 'uploads' media and 'returns a URL that can be used with create_post', indicating it creates and stores data in Posterly's storage system. The description states it fetches content 'into Posterly storage', which is a persistent modification operation.
Attacks that exploit this kind of access
Fetch an image or video from a public HTTPS URL into Posterly storage. Returns a URL that can be used with create_post. You can also pass public HTTP(S) media URLs directly to create_post/update_post; use this tool when you want to upload once and reuse the returned Posterly URL. 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.
Register the Posterly MCP server in PolicyLayer and add a rule for upload_media_from_url: 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.
upload_media_from_url is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
Yes. Add a rate_limit block to the upload_media_from_url 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.
Set action: deny in the PolicyLayer policy for upload_media_from_url. 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.
upload_media_from_url 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.
Every MCP server has a record like this.
Type a name, get the same breakdown: verified identity, auth posture, risk grade, capabilities, recommended policy.
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