ToolHive's remote MCP server authentication discovery issues outbound HTTP requests to URLs the remote MCP server controls, with no private-IP or loopback guard and no restriction on redirects. ToolHive's core security model treats every MCP server as untrusted: the README states it "runs every MCP server in an isolated container" with "no local credentials," and it ships an egress proxy for network isolation. This discovery code runs host-side, in the ToolHive process, before and outside that per-server container sandbox. A malicious or compromised remote MCP server, added by a user through ToolHive's normal remote-server workflow, can therefore drive the ToolHive host itself to fetch arbitrary internal URLs, including cloud instance metadata, which bypasses the isolation ToolHive exists to provide. The user never selects a malicious target; they connect to a server they intend to use, and the attack is carried entirely in that server's discovery response. ToolHive already establishes this boundary in code. "ValidateRemoteURL" (cmd/thv-operator/pkg/validation/url_validation.go:61) rejects internal IPs and known internal hostnames for the configured remote URL, and "IsPrivateIP" (pkg/networking/utilities.go:105) blocks RFC1918, link-local, "169.254.0.0/16", and loopback for outbound requests. The discovery clients below never call either guard, and the attacker-supplied resource_metadata URL and its redirect target are validated by neither. This is a deviation from the project's intended behavior, not a configuration choice by the operator.