proxy_set_host_upstream

THE RISK

Medium Risk

Why proxy_set_host_upstream needs a policy

This tool modifies proxy routing configuration by writing a per-host upstream proxy override. It creates or updates a routing rule that redirects traffic for a specific hostname through a designated proxy. This is a reversible configuration change (Write), not a read or destructive operation. Misuse could redirect sensitive traffic through an attacker-controlled proxy, making the severity medium-to-high.

From the tool's definition Set a per-host upstream proxy override. Traffic to this hostname will use the specified proxy instead of the specified proxy instead of the global one.

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

POLICY

How to control proxy_set_host_upstream

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

policy.json

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

proxy_set_host_upstream 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 Proxy — 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 Proxy tools

Destructive interceptor_docker_detach Remove proxy configuration from a Docker container and clean up injected files. Destructive proxy_clear_traffic Clear all captured traffic from the buffer. Destructive proxy_delete_session Delete a recorded session from disk. Destructive proxy_remove_host_upstream Remove a per-host upstream proxy override. Destructive proxy_remove_rule Delete an interception rule. Destructive proxy_session_stop Stop persistent on-disk capture and finalize the active session. Destructive interceptor_deactivate_all Kill ALL active interceptors across all types. Emergency cleanup — stops all browser insta Destructive interceptor_kill Kill a spawned process by target ID. Also retrieves final stdout/stderr output.

All 89 Proxy 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 proxy_set_host_upstream

What does the proxy_set_host_upstream tool do? +

Set a per-host upstream proxy override. Traffic to this hostname will use the specified proxy instead of the global one. It is categorised as a Write tool in the Proxy MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on proxy_set_host_upstream? +

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

What risk level is proxy_set_host_upstream? +

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

Can I rate-limit proxy_set_host_upstream? +

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

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

proxy_set_host_upstream is provided by the Proxy MCP server (yfe404/proxy-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Proxy tool call.

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

CHECK YOUR STACK →