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.
Affected products
lfprojects: mcp ruby sdk
References9
- github.com/modelcontextprotocol/ruby-sdk/security/advisories/GH… ExploitVendor Advisory
- github.com/modelcontextprotocol/ruby-sdk/security/advisories/GH… ExploitVendor Advisory
- github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelC… Patch
- github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable… Patch
- github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/se… Patch
- github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/str… Patch
- github.com/modelcontextprotocol/ruby-sdk/commit/db40143402d65b4… Patch
- github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.9.2 ProductRelease Notes
- hackerone.com/reports/3556146 Permissions Required
Metrics
- CVSS 3.1
-
5.9
MEDIUM
Score source: NVD
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N - CVSS 4.0
-
8.2
HIGH
Score source: CNA
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - EPSS — exploit probability
- 0% percentile: 41 · as of 12 November 2026
- CWE
- CWE-384CWE-639
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.