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Vulnerability · Published Mar 27, 2026 · Assigned by GitHub_M

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

High 8.2

In plain English

A high-severity session fixation in ruby-sdk from modelcontextprotocol. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 41% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 4.0)8.2High
Exploit odds (30 days)0.5%top 58.9% of all CVEs
Exploited in the wildNo reportsCISA SSVC: poc
Attack toolingNoneno public tooling known

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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
RubyGemsmcp0.9.2GHSA-qvqr-5cv7-wh35

Affected products (as reported)

VendorProductVersions
modelcontextprotocol ruby-sdk < 0.9.2

Weakness: CWE-384, CWE-639 · Vector: 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

References