# CVE-2026-67431: MCP Ruby SDK: Ruby SSE Session Poisoning

> Attackers with stolen session IDs can execute tools in victim sessions due to missing session ownership binding in the Ruby MCP SDK.

- **Severity:** High 8.3 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N`)
- **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-5p9g-j988-pcwv

## What is CVE-2026-67431?

CVE-2026-67431 is a high-severity missing authorization 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 bind a session ID to a session owner, allowing an attacker with a stolen session ID to send tools/call requests that execute in the victim's session. This issue is fixed in version 0.23.0.

Weakness: CWE-284 (Improper Access Control)

## Which versions are affected by CVE-2026-67431?

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-2026-67431?

Upgrade mcp to 0.23.0 or later.

## Is CVE-2026-67431 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-67431 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-67431 and fix it if it is affected.

What it is: CVE-2026-67431 is a high-severity missing authorization 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-2026-67431 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-2026-67431/ (Markdown: https://insights.sbomify.com/CVE-2026-67431/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-67431
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-67431
- OSV: https://osv.dev/vulnerability/CVE-2026-67431

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-67431/
Summaries are generated and may contain mistakes; the official description above is the record.