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CVE-2026-63119 (GCVE-0-2026-63119)

Vulnerability from cvelistv5 – Published: 2026-07-29 19:12 – Updated: 2026-07-29 19:28

Title

MCP Ruby SDK: Unbounded line buffer in stdio transports leads to memory exhaustion (DoS)

Summary

MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StdioTransport and MCP::Client::Stdio in the mcp gem use IO#gets without a byte limit, allowing a peer that sends data without a newline to exhaust process memory. This issue is fixed in version 0.23.0.

Severity

6.2 (Medium)


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

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator (v2.0.3)

CWE

CWE-400 - Uncontrolled Resource Consumption
CWE-770 - Allocation of Resources Without Limits or Throttling

Assigner

GitHub_M

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

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GHSA-7683-3W9X-CH42

Vulnerability from github – Published: 2026-07-30 14:41 – Updated: 2026-07-30 14:41

Summary

MCP Ruby SDK: Unbounded line buffer in stdio transports leads to memory exhaustion (DoS)

Details

Summary

The stdio transports in MCP::Server::Transports::StdioTransport and MCP::Client::Stdio read newline-delimited JSON-RPC frames using IO#gets with no limit argument. CRuby's IO#gets with no limit reads from the current position until the next separator (\n) with no upper bound on the returned string length. A peer that streams bytes without ever emitting a newline causes gets to accumulate the entire stream in a single Ruby String until the process is killed by the operating-system OOM killer.

This is the same vulnerability class tracked in sibling MCP SDKs as GHSA-74gp-qhv5-v493 (Kotlin), GHSA-wqgc-pwpr-pq7r (TypeScript), GHSA-655q-2283-6jgj (Python), and others. It was identified during a cross-SDK audit; ruby-sdk had no prior report.

Affected code

Verified at main @ cf44475c.

Server transport — `lib/mcp/server/transports/stdio_transport.rb`

# line 23
while @open && (line = $stdin.gets)        # <-- no limit argument
  response = @session.handle_json(line.strip)
  ...

# line 76
while @open && (line = $stdin.gets)        # <-- no limit argument
  begin
    parsed = JSON.parse(line.strip, symbolize_names: true)

$stdin is the raw process global (only set_encoding is applied at line 16); there is no wrapper imposing a length limit.

Client transport — `lib/mcp/client/stdio.rb`

# line 150
@stdin, @stdout, @stderr, @wait_thread = Open3.popen3(spawn_env, @command, *@args)

# line 228
line = @stdout.gets                         # <-- no limit argument
raise_connection_error!(method, params) if line.nil?
parsed = JSON.parse(line.strip)

@stdout is the raw IO returned by Open3.popen3. The @read_timeout guard at line 227 (wait_for_readable!) only gates the IO.select before gets is invoked; once bytes are flowing, gets blocks indefinitely accumulating into one string.

Impact

Denial of service via memory exhaustion. A peer that controls the byte stream delivered to the stdio transport can grow a single Ruby String until the process exhausts available memory.

Threat-model caveat (important): In the default stdio deployment, the peer process already holds local execution privileges equal to or greater than the victim: - On the server side (StdioTransport#open), the writer to $stdin is the parent process that spawned the server, which already holds Process.kill, environment control, and filesystem access over the child. This direction is a robustness defect rather than a security boundary in typical deployments. - On the client side (Client::Stdio#read_response), the writer is the third-party MCP server binary the host application chose to spawn via Open3.popen3. In an unsandboxed deployment that binary already has local code execution as the host user.

This issue is primarily a security concern for: - (a) host applications that sandbox the spawned MCP server (container, restricted user, seccomp) while piping its stdout to an unsandboxed host process — an unbounded gets lets a memory-capped sandboxed process exhaust the unconstrained host; - (b) HTTP-to-stdio bridge deployments that forward bytes from a remote peer to a stdio server's stdin (note: surveyed bridges re-frame JSON-RPC and emit their own newlines, mitigating this in practice); - (c) robustness against non-malicious servers that emit large unterminated output (misconfigured logging to stdout, runaway loops), which can crash the host process and all other MCP sessions it manages.

The bug fires before the JSON-RPC initialize handshake, since gets cannot return until \n arrives.

Reproduction

# poc_ruby_stdio_oom.rb — drives the real client transport against a producer that never emits \n
require "mcp/client/stdio"

# `yes` writes "y\n" — instead use a producer that never sends \n:
producer = %q{ruby -e 'STDOUT.sync=true; loop { print "A" * 65536 }'}

client = MCP::Client::Stdio.new(command: "bash", args: ["-c", producer])
client.start
# any request triggers read_response → @stdout.gets → unbounded String growth
client.send_request(method: "initialize", params: {})

Observed: process RSS grows linearly with bytes produced; gets never returns; process is OOM-killed.

Suggested fix

IO#gets accepts a second limit argument. Apply a configurable maximum line length (default suggested: 4 MiB — large enough for any realistic JSON-RPC frame including base64-embedded images) at all three call sites, and treat an over-limit line as a transport error that closes the connection:

MAX_LINE_BYTES = 4 * 1024 * 1024

while @open && (line = $stdin.gets("\n", MAX_LINE_BYTES))
  unless line.end_with?("\n")
    # gets returned because the limit was hit, not because a newline arrived
    raise MCP::TransportError, "stdio frame exceeds #{MAX_LINE_BYTES} bytes without newline"
  end
  ...
end

Apply the same pattern at stdio_transport.rb:76 and client/stdio.rb:228. Expose MAX_LINE_BYTES as a constructor option on both transports for callers with legitimate large-frame needs.

Credit

Identified during a cross-SDK audit of the stdio unbounded-buffer vulnerability class, prompted by GHSA-74gp-qhv5-v493 (reporter: tonghuaroot).

Severity

6.2 (Medium)


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

Show details on source website

JSON

To clipboard

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  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.22.0"
      },
      "package": {
        "ecosystem": "RubyGems",
        "name": "mcp"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
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              "fixed": "0.23.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-63119"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-400",
      "CWE-770"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-30T14:41:58Z",
    "nvd_published_at": "2026-07-29T20:17:10Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\n\nThe stdio transports in `MCP::Server::Transports::StdioTransport` and `MCP::Client::Stdio` read newline-delimited JSON-RPC frames using `IO#gets` with no `limit` argument. CRuby\u0027s `IO#gets` with no limit reads from the current position until the next separator (`\\n`) with no upper bound on the returned string length. A peer that streams bytes without ever emitting a newline causes `gets` to accumulate the entire stream in a single Ruby `String` until the process is killed by the operating-system OOM killer.\n\nThis is the same vulnerability class tracked in sibling MCP SDKs as GHSA-74gp-qhv5-v493 (Kotlin), GHSA-wqgc-pwpr-pq7r (TypeScript), GHSA-655q-2283-6jgj (Python), and others. It was identified during a cross-SDK audit; ruby-sdk had no prior report.\n\n## Affected code\n\nVerified at `main` @ `cf44475c`.\n\n### Server transport \u2014 [`lib/mcp/server/transports/stdio_transport.rb`](https://github.com/modelcontextprotocol/ruby-sdk/blob/cf44475c/lib/mcp/server/transports/stdio_transport.rb)\n\n```ruby\n# line 23\nwhile @open \u0026\u0026 (line = $stdin.gets)        # \u003c-- no limit argument\n  response = @session.handle_json(line.strip)\n  ...\n\n# line 76\nwhile @open \u0026\u0026 (line = $stdin.gets)        # \u003c-- no limit argument\n  begin\n    parsed = JSON.parse(line.strip, symbolize_names: true)\n```\n\n`$stdin` is the raw process global (only `set_encoding` is applied at line 16); there is no wrapper imposing a length limit.\n\n### Client transport \u2014 [`lib/mcp/client/stdio.rb`](https://github.com/modelcontextprotocol/ruby-sdk/blob/cf44475c/lib/mcp/client/stdio.rb)\n\n```ruby\n# line 150\n@stdin, @stdout, @stderr, @wait_thread = Open3.popen3(spawn_env, @command, *@args)\n\n# line 228\nline = @stdout.gets                         # \u003c-- no limit argument\nraise_connection_error!(method, params) if line.nil?\nparsed = JSON.parse(line.strip)\n```\n\n`@stdout` is the raw `IO` returned by `Open3.popen3`. The `@read_timeout` guard at line 227 (`wait_for_readable!`) only gates the `IO.select` before `gets` is invoked; once bytes are flowing, `gets` blocks indefinitely accumulating into one string.\n\n## Impact\n\nDenial of service via memory exhaustion. A peer that controls the byte stream delivered to the stdio transport can grow a single Ruby `String` until the process exhausts available memory.\n\n**Threat-model caveat (important):** In the default stdio deployment, the peer process already holds local execution privileges equal to or greater than the victim:\n- On the **server** side (`StdioTransport#open`), the writer to `$stdin` is the parent process that spawned the server, which already holds `Process.kill`, environment control, and filesystem access over the child. This direction is a robustness defect rather than a security boundary in typical deployments.\n- On the **client** side (`Client::Stdio#read_response`), the writer is the third-party MCP server binary the host application chose to spawn via `Open3.popen3`. In an unsandboxed deployment that binary already has local code execution as the host user.\n\nThis issue is primarily a security concern for:\n- (a) host applications that **sandbox** the spawned MCP server (container, restricted user, seccomp) while piping its stdout to an unsandboxed host process \u2014 an unbounded `gets` lets a memory-capped sandboxed process exhaust the unconstrained host;\n- (b) **HTTP-to-stdio bridge** deployments that forward bytes from a remote peer to a stdio server\u0027s stdin (note: surveyed bridges re-frame JSON-RPC and emit their own newlines, mitigating this in practice);\n- (c) **robustness** against non-malicious servers that emit large unterminated output (misconfigured logging to stdout, runaway loops), which can crash the host process and all other MCP sessions it manages.\n\nThe bug fires before the JSON-RPC `initialize` handshake, since `gets` cannot return until `\\n` arrives.\n\n## Reproduction\n\n```ruby\n# poc_ruby_stdio_oom.rb \u2014 drives the real client transport against a producer that never emits \\n\nrequire \"mcp/client/stdio\"\n\n# `yes` writes \"y\\n\" \u2014 instead use a producer that never sends \\n:\nproducer = %q{ruby -e \u0027STDOUT.sync=true; loop { print \"A\" * 65536 }\u0027}\n\nclient = MCP::Client::Stdio.new(command: \"bash\", args: [\"-c\", producer])\nclient.start\n# any request triggers read_response \u2192 @stdout.gets \u2192 unbounded String growth\nclient.send_request(method: \"initialize\", params: {})\n```\n\nObserved: process RSS grows linearly with bytes produced; `gets` never returns; process is OOM-killed.\n\n## Suggested fix\n\n`IO#gets` accepts a second `limit` argument. Apply a configurable maximum line length (default suggested: 4 MiB \u2014 large enough for any realistic JSON-RPC frame including base64-embedded images) at all three call sites, and treat an over-limit line as a transport error that closes the connection:\n\n```ruby\nMAX_LINE_BYTES = 4 * 1024 * 1024\n\nwhile @open \u0026\u0026 (line = $stdin.gets(\"\\n\", MAX_LINE_BYTES))\n  unless line.end_with?(\"\\n\")\n    # gets returned because the limit was hit, not because a newline arrived\n    raise MCP::TransportError, \"stdio frame exceeds #{MAX_LINE_BYTES} bytes without newline\"\n  end\n  ...\nend\n```\n\nApply the same pattern at `stdio_transport.rb:76` and `client/stdio.rb:228`. Expose `MAX_LINE_BYTES` as a constructor option on both transports for callers with legitimate large-frame needs.\n\n## Credit\n\nIdentified during a cross-SDK audit of the stdio unbounded-buffer vulnerability class, prompted by GHSA-74gp-qhv5-v493 (reporter: tonghuaroot).",
  "id": "GHSA-7683-3w9x-ch42",
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    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/commit/267b8fa6285453525c81ce43db6b7dcd7a8a8c2f"
    },
    {
      "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:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "MCP Ruby SDK: Unbounded line buffer in stdio transports leads to memory exhaustion (DoS)"
}

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.

Detection rules are retrieved from Rulezet.

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.

Action not permitted

CVE-2026-63119 (GCVE-0-2026-63119)

GHSA-7683-3W9X-CH42

Summary

Affected code

Server transport — lib/mcp/server/transports/stdio_transport.rb

Client transport — lib/mcp/client/stdio.rb

Impact

Reproduction

Suggested fix

Credit

Tags

Sightings

Nomenclature

Server transport — `lib/mcp/server/transports/stdio_transport.rb`

Client transport — `lib/mcp/client/stdio.rb`