Slack (Sigma Computing) (sigmacomputing) MCP Server

This server (without client built-ins) exposes a complete data-exfiltration chain: slack_download_file → slack_list_files ⇒ slack_post_message. Untrusted input is ingested, private data is read, and it can be sent to an external sink — and at least one leg is a direct schema wire (⇒), where a producer's output drops straight into a free-text parameter of the next tool, so the chain needs little agent cooperation. Static analysis proves the primitive exists, not that a specific run will occur.

Fix: Remove one leg of the trifecta: isolate untrusted-input tools from secret-reading tools and from egress tools, or require human approval between them.

Location: flow slack_download_file → slack_list_files → slack_post_message

In the server's implementation (`dist/index.js:32`): A hardcoded outbound call to a fixed external host inside server code is a classic exfiltration/telemetry channel — especially paired with reads of local data. This is read from the code itself — not from the tool description — so a poisoned server cannot hide it behind honest-looking metadata.

Evidence: const response = await fetch(`https://slack.com/api/conversations.list?${params}`, { headers: this.botHeaders });

Fix: Review this call path: confirm it never receives unsanitized tool input, constrain it, or remove it. Treat a server whose code reaches these sinks as high-capability regardless of what its tools claim.

Location: server dist/index.js