CVE-2026-63119

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

# 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)

# 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)

# 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: {})

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.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2026-63119 has a CVSS score of 6.2 (Medium). The vector is requires local access, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.23.0); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

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.

1. What is CVE-2026-63119? CVE-2026-63119 is a medium-severity uncontrolled resource consumption vulnerability in mcp (rubygems), affecting versions <= 0.22.0. It is fixed in 0.23.0. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-63119? CVE-2026-63119 has a CVSS score of 6.2 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of mcp are affected by CVE-2026-63119? mcp (rubygems) versions <= 0.22.0 is affected.
4. Is there a fix for CVE-2026-63119? Yes. CVE-2026-63119 is fixed in 0.23.0. Upgrade to this version or later.
5. Is CVE-2026-63119 exploitable, and should I be worried? Whether CVE-2026-63119 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-63119 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-63119? Upgrade mcp to 0.23.0 or later.

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