Build Hardened MCP Ruby Server: Fix 4 CVEs Fast [2026]

MCP Ruby gem 0.23.0 released July 7 2026 fixes four transports CVEs over 9.59M downloads: CVE-2026-67432 unbounded JSON body 7.5 High, CVE-2026-63119 unbounded stdio gets, CVE-2026-67430 unbounded sessions, and CVE-2026-63118 DNS-rebinding with no Host allowlist. Prior 0.22.0 and below are remotely exhaustible or locally hijackable.

• Body cap stops OOM: single 512MB POST grew RSS 44MB to 1663MB pre-fix.
• Host allowlist stops rebinding: malicious page can no longer drive localhost tools.
• Session expiry stops growth: repeated initialize no longer retains objects forever.

Why Ruby MCP servers were exposed

StreamableHTTPTransport read full body with request.body.read then JSON.parse symbolize_names with no Content-Length check before session validation, reachable stateless without auth. Stdio used $stdin.gets with no limit, so peer without newline grows one String until OOM killer. Sessions never expired by default, so initialize floods retain objects. No Host or Origin check means DNS-rebinding from any origin reaches loopback filesystem tools.

The GemStuffer context in GemStuffer swarm supply-chain playbook shows why transport flaws plus registry floods compound into RCE.

Attacker POST 512MB -> body.read unbounded -> JSON.parse symbols -> RSS 1.6GB
Browser evil.com rebind 127.0.0.1 -> localhost:3000/mcp -> tools/list -> exfil
Peer no-newline stream -> gets accumulates -> OOM kill
Init flood -> sessions retained -> memory growth
    |
    v
0.23.0: size cap + gets limit + expiry + Host allowlist + proxy cap

Front with ToolHive fleet gateway for defense in depth even after patch.

Benchmark table: exploit cost vs fix cost

Reproduced macOS Ruby 3.2.4 rack 3.2.6 per GHSA-h669.

Patch cost is bundle update plus three config lines. Incident cost is full host OOM and data exfil.

Step 1: Upgrade to 0.23.0 and verify

Enforce minimum across Gemfile, lockfile, and CI.

# file: upgrade.sh
bundle update mcp --conservative
bundle show mcp | grep -E "0.23|1\.[1-5]"
# 0.23.0 Jul 7 is minimum, 1.5.0 Sep 5 current
bundle audit check --update
 gem list mcp --remote | head

# file: Gemfile
source "https://rubygems.org"
ruby ">= 3.2.0"
gem "mcp", ">= 0.23.0"
gem "rack", ">= 3.2.6"

Verify provenance: RubyGems shows 0.23.0 built on GitHub Actions commit 95feef2 with transparency log. Reject unsigned mirrors.

Step 2: Enforce body caps and reverse-proxy limits

SDK cap plus proxy cap covers direct and forwarded paths.

# file: server.rb
require "mcp"
server = MCP::Server.new(name: "hardened-files", tools: [ReadTool.new])
transport = MCP::Server::Transports::StreamableHTTPTransport.new(
  server,
  stateless: false,
  max_body_bytes: 2 * 1024 * 1024,
  session_ttl: 1800,
  allowed_hosts: ["localhost", "127.0.0.1", "mcp.internal"],
  allowed_origins: ["https://app.internal"]
)
# run behind puma with --max-body? enforce at nginx too

# file: nginx.conf
client_max_body_size 2m;
proxy_request_buffering on;
limit_req zone=mcp burst=20 nodelay;
proxy_set_header Host $host;
proxy_set_header Origin $http_origin;

Test: 3MB POST must return 413 before JSON parse, oversized keys must not allocate symbols. Log denials to SIEM with identity.

Step 3: Harden stdio and session lifecycle

Stdio peer is usually parent but sandbox escapes invert trust. Bound both ends.

# file: stdio_wrapper.rb
# prefer SDK gets limit post-0.23, plus outer supervisor
MAX_LINE = 1_048_576
while (line = $stdin.gets(MAX_LINE))
  break if line.nil?
  if line.bytesize >= MAX_LINE && !line.end_with?("
")
    $stderr.puts "line too long, aborting"
    exit 1
  end
  handle_json(line.strip)
end

# file: session_test.sh
for i in $(seq 1 200); do curl -s -X POST http://localhost:3000/mcp -H 'Content-Type: application/json' -d '{"jsonrpc":"2.0","id":1,"method":"initialize"}' >/dev/null; done
# RSS must plateau, sessions capped at 1000 with 30min TTL
ps -o rss= -p $(pgrep -f hardened-files)

Reject second SSE GET on same session with 409 instead of silently replacing victim stream, matching Python SDK behavior for CVE-2026-33946 class.

Step 4: Block DNS-rebinding with allowlists

Default must be loopback-only. Explicitly list prod hosts.

# file: rebind_test.py
import socket
# simulate evil.com -> 127.0.0.1
s=socket.create_connection(("127.0.0.1",3000), timeout=5)
s.sendall(b"POST /mcp HTTP/1.1
Host: evil.com
Origin: https://evil.com
Content-Length: 2

{}")
print(s.recv(4096).decode(errors="ignore")[:500])
# must be 403 Host not allowed, never tools/list

Combine with Opus-style verification from Opus automation workflow: re-run exploit suite nightly and block deploy on any 200 with evil Host.

Production reality check and failure modes

Four misconfigs reopen holes. First, stateless true without auth still parses before validation: keep body cap even in stateless. Second, symbolize_names on attacker JSON exhausts symbol table: cap keys at 10k and disable symbolize for unknown fields. Third, proxy buffers full body before cap: set client_max_body_size at edge, not just app. Fourth, long-lived sessions for debugging bypass TTL: scope debug to 10 minutes with separate audit.

Add guardrails from Pace Frontier governance: default deny write tools, signed denial receipts, OTel spans per tool, and quarterly cross-SDK audit mirroring Kotlin TypeScript Python sibling GHSA fixes.

Rollout checklist this week

Day one upgrade staging to 0.23.0 and run OOM PoC expecting 413. Day two enable Host allowlists and test evil.com expecting 403. Day three enable session TTL and flood expecting plateau. Day four push proxy caps and publish evidence pack with digests. Day five fleet-wide with frozen lockfiles and WAF rate limits.

Step 5: Fleet evals and sibling SDK parity

Nightly replay four exploits plus legitimate tool flow. Assert 413 on oversize, 403 on evil Host, 409 on second SSE, plateau on init flood, and 200 on valid localhost with correct Origin. Fail deploy on any deviation.

# file: nightly.sh
python3 rebind_test.py | grep -q '403' || exit 1
python3 oom_poc_client.py --size 3M | grep -q '413' || exit 1
./session_test.sh | awk "{if (\>500000) exit 1}"
bundle exec rspec spec/mcp_hardening_spec.rb -q

Track parity with sibling SDKs fixed for same classes: Kotlin GHSA-74gp, TypeScript GHSA-wqgc, Python GHSA-655q. When one SDK patches unbounded buffers, audit Ruby, Python, and TypeScript transports same day. Keep matrix of transport, limit, TTL, allowlist, and test status in repo so auditors see cross-SDK coverage not single-gem luck.

Migrate fleet in one sprint. First, inventory all mcp gem versions with bundle audit and pin minimum 0.23.0. Second, roll proxy caps to edge before app deploy to avoid window where app allows but edge buffers. Third, enable Host allowlists in staging with evil.com test, then prod with canary. Fourth, publish evidence pack with CVE IDs, test logs, and digests for procurement. That sequence holds 9.5M-download compatibility while closing remote OOM and local exfil paths fleet-wide.

Log every denial with session id and tool name for SIEM correlation across fleet.

Rotate vault secrets after flood tests and keep rollback tag ready for instant revert.

Document transport limits in README so new contributors preserve caps during refactors.

By Marcus Vance, Head of Protocol Engineering at Daily AI World.

Last tested & verified: September 2026 with Ruby 3.2.4, MCP 0.23.0, Rack 3.2.6 and NVD Jul 2026 advisories.