Auth bypass in Lfprojects Mcp Ruby Sdk
CVE-2026-33946
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.9 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N.
EPSS exploit prediction
EPSS: 0.005 (41.2th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 41st percentile — 41.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.005 · newest: 0.005 · change: -0.001
Affected products
- Lfprojects Mcp Ruby Sdk
- Modelcontextprotocol Ruby-Sdk — versions < 0.9.2
Weakness classification (CWE)
References
- https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35 (Exploit, Vendor Advisory)
- https://github.com/modelcontextprotocol/ruby-sdk/commit/db40143402d65b4fb6923cec42d2d72cb89b3874 (Patch)
- https://hackerone.com/reports/3556146 (Permissions Required)
- https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L93-L97 (Patch)
- https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L281C1-L288C2 (Patch)
- https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py#L680-L685 (Patch)
- https://github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb (Patch)
- https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.9.2 (Product, Release Notes)
Frequently asked questions
- What is CVE-2026-33946?
- CVE-2026-33946 is a medium-severity vulnerability in Lfprojects Mcp Ruby Sdk, classified under Session Fixation. CVSS score: 5.9/10. Published 2026-03-27.
- How severe is CVE-2026-33946?
- Medium severity. CVSS v3 base score is 5.9 out of 10.