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

published 2026-03-27 7 mo ago modified 2026-10-26 13 d ago also GHSA-qvqr-5cv7-wh35 , EUVD-2026-16866

cvss: 5.9; version 3.1, NVD
cisa kev: Not listed; not in CISA's catalog
epss: 0.5%; EPSS 0.5% chance of exploitation in 30 days, percentile 41%
public exploits: 0; none known

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.

Patch / fix

fixed in 2 links 6

rubygems mcp 0.9.2
lfprojects mcp_ruby_sdk 0.9.2

CISA SSVC

Exploitation: Proof of concept is public
Automatable: yes
Technical impact: partial
Assessed: 7 mo ago (2026-03-30)

Exploit prediction

0.47% chance of exploitation in the next 30 days, higher than 41% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

github.com https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by	version	score	vector
CNA	4.0	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
CNA	4.0	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/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
NVD	3.1	5.9	CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

Affected packages

ecosystem	package	affected	fixed in
rubygems	mcp	<0.9.2	0.9.2

Affected products

modelcontextprotocol ruby-sdk

Classification

Weakness

CWE-384 Session Fixation
CWE-639 Authorization Bypass Through User-Controlled Key

References

github.com /modelcontextprotocol/ruby-sdk/commit/db40143402d65b4fb6923cec42d2d72cb89b3874
web, patch
github.com /modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L93-L97
web, patch
github.com /modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L281C1-L288C2
web, patch
github.com /modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py#L680-L685
web, patch
github.com /modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb
web, patch
github.com /modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35
exploit, web, vendor-advisory
hackerone.com /reports/3556146
web, permissions-required
github.com /modelcontextprotocol/ruby-sdk/releases/tag/v0.9.2
web, product, release-notes
nvd.nist.gov /vuln/detail/CVE-2026-33946
advisory
github.com /modelcontextprotocol/ruby-sdk
package
github.com /rubysec/ruby-advisory-db/blob/master/gems/mcp/CVE-2026-33946.yml
web

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary