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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 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 · score source

EPSS

• CVE-2026-33946: 0.47% estimated exploitation activity in the next 30 days; percentile 41.00 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The Ruby SDK's streamablehttptransport.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. Details Root Cause The StreamableHTTPTransport implementation stores only one SSE stream object per session ID and lacks: - Session-to-user identity binding - Ownership validation when establishing SSE connections - Protection against multiple simultaneous connections to the same session PoC Vulnerable Code File: streamablehttptransport.rb - L336-L339: Attack Scenario Step 1: Legitimate Session Establishment Step 2: Session ID Compromise - An attacker obtains the session ID through various means (out of scope for this analysis) Step 3: Stream Hijacking Step 4: Data Interception - ALL subsequent tool responses/notifications go to the attacker - The legitimate user receives no data and has no indication of the hijacking Technical Details…

Published by the source

27 Mar 2026, 18:36 UTC

Last modified by the source

30 Mar 2026, 20:10 UTC

Source last checked

26 Oct 2026, 15:51 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-qvqr-5cv7-wh35

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# Patch Next advisory brief: CVE-2026-33946

Advisory: MCP Ruby SDK: Insufficient Session Binding Allows SSE Stream Hijacking via Session ID Replay
IDs: CVE-2026-33946, GHSA-qvqr-5cv7-wh35
Record: https://patchnext.com/advisories/cve-2026-33946 (revision 1, source checked 26 Oct 2026, 15:51 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mcp 0.9.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems mcp: affected <= 0.9.1; fixed in 0.9.2

## Suggested inspection (read-only)
- RubyGems mcp: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mcp.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-qvqr-5cv7-wh35 (GitHub-reviewed advisory GHSA-qvqr-5cv7-wh35 lists affected ranges: RubyGems mcp >=0 <=0.9.1)

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