# CVE-2026-67430: MCP Ruby SDK: Unbounded session retention in StreamableHTTPTransport allows memory exhaustion via initialize flood

> Attackers can exhaust server memory by flooding initialize requests due to unbounded session retention in the MCP Ruby SDK.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 29 Jul 2026
- **Updated:** 30 Jul 2026
- **Also known as:** GHSA-52jp-gj8w-j6xh

## What is CVE-2026-67430?

CVE-2026-67430 is a medium-severity denial of service vulnerability in modelcontextprotocol ruby-sdk, published on 29 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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 does not expire sessions by default, so repeated initialize requests retain unbounded ServerSession objects and can exhaust process memory. This issue is fixed in version 0.23.0.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling), CWE-401 (Missing Release of Memory after Effective Lifetime)

## Which versions are affected by CVE-2026-67430?

2 open source packages ship an affected version, including mcp.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mcp` | RubyGems | All versions | 0.23.0 |
| `mcp` | RubyGems | All versions | 0.23.0 |

## How do I fix CVE-2026-67430?

Upgrade mcp to 0.23.0 or later.

## Is CVE-2026-67430 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-67430 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-67430 and fix it if it is affected.

What it is: CVE-2026-67430 is a medium-severity denial of service vulnerability in modelcontextprotocol ruby-sdk, published on 29 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-67430 exists.

Affected packages and fixed versions:
- mcp (RubyGems): affected all versions; fixed in 0.23.0
- mcp (RubyGems): affected all versions; fixed in 0.23.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-67430/ (Markdown: https://insights.sbomify.com/CVE-2026-67430/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-67430
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-67430
- OSV: https://osv.dev/vulnerability/CVE-2026-67430

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-67430/
Summaries are generated and may contain mistakes; the official description above is the record.