# CVE-2026-33946: MCP Ruby SDK: Insufficient Session Binding Allows SSE Stream Hijacking via Session ID Replay

> Attackers can hijack SSE streams and intercept real-time data by replaying session IDs in the MCP Ruby SDK.

- **Severity:** High 8.2 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/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:** 27 Mar 2026
- **Updated:** 30 Mar 2026
- **Also known as:** GHSA-qvqr-5cv7-wh35

## What is CVE-2026-33946?

CVE-2026-33946 is a high-severity authentication bypass vulnerability in modelcontextprotocol ruby-sdk, published on 27 Mar 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 version 0.9.2, the Ruby SDK's streamable_http_transport.rb implementation contains a session hijacking vulnerability. An attacker who obtains a valid session ID can completely hijack the victim's Server-Sent Events (SSE) stream and intercept all real-time data. Version 0.9.2 contains a patch.

Weakness: CWE-639 (Authorization Bypass Through User-Controlled Key), CWE-384 (Session Fixation)

## Which versions are affected by CVE-2026-33946?

2 open source packages ship an affected version, including mcp. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mcp` | RubyGems | All versions | 0.9.2 |
| `mcp` | RubyGems | All versions | 0.9.2 |

Products: Lfprojects Mcp Ruby Sdk.

## How do I fix CVE-2026-33946?

Upgrade mcp to 0.9.2 or later.

## Is CVE-2026-33946 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33946 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33946 and fix it if it is affected.

What it is: CVE-2026-33946 is a high-severity authentication bypass vulnerability in modelcontextprotocol ruby-sdk, published on 27 Mar 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-2026-33946 exists.

Affected packages and fixed versions:
- mcp (RubyGems): affected all versions; fixed in 0.9.2
- mcp (RubyGems): affected all versions; fixed in 0.9.2

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-2026-33946/ (Markdown: https://insights.sbomify.com/CVE-2026-33946/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33946
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33946
- OSV: https://osv.dev/vulnerability/CVE-2026-33946

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33946/
Summaries are generated and may contain mistakes; the official description above is the record.