# CVE-2026-63119: modelcontextprotocol ruby-sdk memory exhaustion in Stdio transports

Source: https://junglewise.ai/threats/cve-2026-63119-modelcontextprotocol-ruby-sdk-memory-exhaustion-in-stdio  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-29. Updated: 2026-07-30  
License: CC BY 4.0 with a link back

CVE-2026-63119 · Severity: medium · CVSS 6.2 · Published 2026-07-29

The Model Context Protocol (MCP) Ruby SDK is used to build applications that communicate with AI models and tools. A vulnerability in how it handles data input allows a connected process to send an endless stream of data without a line break, causing the application to consume all available system memory. This can lead to a denial-of-service (DoS) where the application or the entire host system crashes due to memory exhaustion.

Technologies: [mcp (RubyGems)](https://junglewise.ai/threats/technologies/rubygems-mcp). Vendors: [RubyGems](https://junglewise.ai/threats/vendors/rubygems).

## Quick answers

**Is CVE-2026-63119 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-63119 being exploited in the wild.

**How severe is CVE-2026-63119?** Medium severity, with a CVSS score of 6.2, published 29 July 2026.

**Which products and versions are affected?** mcp (RubyGems) ECOSYSTEM:introduced=0,fixed=0.23.0.

**Is there a fix for CVE-2026-63119?** Yes. mcp (RubyGems): upgrade to 0.23.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-63119 attacked?** It is attacked with local access, in a software library.

**Am I affected by CVE-2026-63119?** Check whether you run mcp (RubyGems) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- mcp (RubyGems): upgrade to 0.23.0 or later

  ```sh
  bundle update mcp
  ```


## Technical details

The vulnerability exists in `MCP::Server::Transports::StdioTransport` and `MCP::Client::Stdio` due to the use of Ruby's `IO#gets` without a `limit` argument. Because `gets` continues to accumulate bytes into a single Ruby String until a newline character is encountered, a malicious or misconfigured peer can stream data indefinitely to exhaust process memory. This is particularly critical in sandboxed environments where a restricted child process could crash an unrestricted host process by piping unbounded data to its stdout. The issue was resolved in version 0.23.0 by introducing a configurable maximum line length (defaulting to 4 MiB) and treating over-limit lines as transport errors.

## Affected products

- modelcontextprotocol mcp <= 0.22.0

## Identifiers

- CVE-2026-63119
- GHSA-7683-3w9x-ch42

## Timeline

- 2026-07-08: patched: Fixed in version 0.23.0
- 2026-07-29: advisory: NVD publication date
- 2026-07-30: disclosed: GitHub Advisory published

## References

- [https://api.github.com/users/tonghuaroot](https://api.github.com/users/tonghuaroot)
- [https://github.com/tonghuaroot](https://github.com/tonghuaroot)
- [https://api.github.com/users/tonghuaroot/gists%7B/gist_id%7D](https://api.github.com/users/tonghuaroot/gists%7B/gist_id%7D)
- [https://api.github.com/users/tonghuaroot/repos](https://api.github.com/users/tonghuaroot/repos)
- [https://avatars.githubusercontent.com/u/23011166?v=4](https://avatars.githubusercontent.com/u/23011166?v=4)
- [https://api.github.com/users/tonghuaroot/events%7B/privacy%7D](https://api.github.com/users/tonghuaroot/events%7B/privacy%7D)

## Related threats

- [Model Context Protocol Ruby SDK memory exhaustion in StreamableHTTPTransport](https://junglewise.ai/threats/cve-2026-67432-model-context-protocol-ruby-sdk-memory-exhaustion-in)
- [modelcontextprotocol ruby-sdk session hijacking in StreamableHTTPTransport](https://junglewise.ai/threats/cve-2026-67431-modelcontextprotocol-ruby-sdk-session-hijacking-in)
- [Model Context Protocol Ruby SDK memory exhaustion in StreamableHTTPTransport](https://junglewise.ai/threats/cve-2026-67430-model-context-protocol-ruby-sdk-memory-exhaustion-in)
- [modelcontextprotocol ruby-sdk DNS rebinding in StreamableHTTPTransport](https://junglewise.ai/threats/cve-2026-63118-modelcontextprotocol-ruby-sdk-dns-rebinding-in)
- [MCP Ruby SDK: Insufficient Session Binding Allows SSE Stream Hijacking via Session ID Replay](https://junglewise.ai/threats/cve-2026-33946-mcp-ruby-sdk-insufficient-session-binding-allows-sse-stream)