MCP Ruby SDK: Insufficient Session Binding Allows SSE Stream Hijacking via Session ID Replay

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Summary

CVECVE-2026-33946
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-03-27 22:16:21 UTC
Updated2026-03-30 19:16:26 UTC
DescriptionMCP 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.

Risk And Classification

Primary CVSS: v4.0 8.2 HIGH from [email protected]

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: 0.000480000 probability, percentile 0.148060000 (date 2026-04-01)

Problem Types: CWE-384 | CWE-639 | CWE-384 CWE-384: Session Fixation | CWE-639 CWE-639: Authorization Bypass Through User-Controlled Key

VersionSourceTypeScoreSeverityVector
4.0[email protected]Secondary8.2HIGHCVSS: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/C...
4.0CNADECLARED8.2HIGHCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

CVSS v4.0 Breakdown

Attack Vector
Network
Attack Complexity
Low
Attack Requirements
Present
Privileges Required
None
User Interaction
None
Confidentiality
High
Integrity
None
Availability
None
Sub Conf.
None
Sub Integrity
None
Sub Availability
None

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

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Modelcontextprotocol Ruby-sdk affected < 0.9.2 Not specified

References

ReferenceSourceLinkTags
github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/stre... [email protected] github.com
github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_h... [email protected] github.com
github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go [email protected] github.com
github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextPro... [email protected] github.com
github.com/modelcontextprotocol/ruby-sdk/commit/db40143402d65b4fb6923cec... [email protected] github.com
github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.9.2 [email protected] github.com
github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5... 134c704f-9b21-4f2e-91b3-4a467353bcc0 github.com
hackerone.com/reports/3556146 [email protected] hackerone.com
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