How severe is CVE-2026-67431?

No CVSS score has been assigned to CVE-2026-67431 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-67431?

CVE-2026-67431 affects the following packages: mcp (RubyGems). Ecosystems affected: RubyGems.

How do I fix CVE-2026-67431?

Update mcp to 0.23.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-67431 is resolved across your whole dependency graph.

How do I detect CVE-2026-67431 in my RubyGems dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mcp, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-67431 if there is no patch (or I can't update yet)?

Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Is CVE-2026-67431 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-67431 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-67431?

CVE-2026-67431 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 30th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-67431?

CVE-2026-67431 is classified as CWE-284 (CWE-284). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-67431 published, and has it been updated?

CVE-2026-67431 was published on July 29, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

💎 RubyGems

Not in CISA KEV

CVE-2026-67431 — mcp

Fix: modelcontextprotocol/ruby-sdk@3546660

CVE-2026-67431 is a CWE-284 vulnerability in mcp. A fix is available for mcp — see the affected versions and patch details below.

MCP Ruby SDK: Ruby SSE Session Poisoning

Also known asGHSA-5p9g-j988-pcwv

Published

Jul 29, 2026

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 29, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

Proof-of-concept exploit code exists

CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
A successful exploit gives an attacker total control of the affected component, not partial access.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-67431.

EPSS Exploitation Probability

via FIRST.org ↗

0.4%probability of exploitation in next 30 days

Lower Risk-0.12%

Lower risk than most CVEs30th percentile — riskier than 30% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

💎mcp

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.

Description

Summary

Vulnerability: Missing Session Ownership Validation in the Ruby MCP SDK's Streamable and SSE HTTP transport implementation. Any attacker with a stolen session ID can execute tools with the victim's session. This is a silent attack - the victim's session is compromised and being used for unauthorized actions, but it is hard to know for the victim

Details

https://github.com/modelcontextprotocol/ruby-sdk/blob/main/lib/mcp/server/transports/streamable_http_transport.rb#L260-L278

Victim starts a legitimate MCP session and receives session ID abc-123 Attacker obtains the session ID (various means - network sniffing, logs, etc. out of scope for this analysis) Attacker sends POST to /messages/abc-123 with a tool call Server accepts the request (no ownership validation!) Server executes the tool and sends response to victim's SSE stream Victim receives attacker's response, thinking it's legitimate

PoC

attacker_client.py legitimate_client.py

Prerequisites

Python 3.8+
Install dependencies: requests

Running the Demo

Terminal 1: Start the Ruby MCP Server

ruby streamable_http_server.rb

Makes use of https://github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb This server has a tool call notification_tool which the clients call

Terminal 2: Start Victim Client

python3 legitimate_client.py

Terminal 3 - Attacker Client:

Copy the session ID from Terminal 1 and run:

python3 attacker_client.py abc-123-def-456

Back to Terminal 2 - Victim sees the injected response:

Impact

Integrity: HIGH - Attacker can execute unauthorized tools and modify state
Availability: LOW - Attacker can disrupt victim's session with injected responses

Additional Details

Session Hijacking Protection in MCP Implementations

The MCP specification recommends - "MCP servers SHOULD bind session IDs to user-specific information".

User Binding - Comparison other SDKs

csharp-sdk

Go-sdk

Comparison with the Stream Replacement vulnerability

Stream Replacement https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35

Target: SSE stream connection
Attack Vector: GET /mcp
What happens to Victim: The connection is disconnected and doesn't receive reponses

Session Poisoning

Target: Tool execution
Attack Vector: POST /mcp
What happens to Victim: The connection stays connected and receives responses of tool calls by attacker

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
💎RubyGems	`mcp`	all versions	0.23.0`bundle update mcp --conservative`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mcp, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update mcp to 0.23.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-67431 is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Frequently Asked Questions

### Summary **Vulnerability**: Missing Session Ownership Validation in the Ruby MCP SDK's Streamable and SSE HTTP transport implementation. Any attacker with a stolen session ID can execute tools with the victim's session. This is a silent attack - the victim's session is compromised and being used for unauthorized actions, but it is hard to know for the victim ### Details https://github.com/modelcontextprotocol/ruby-sdk/blob/main/lib/mcp/server/transports/streamable_http_transport.rb#L260-L278 **Victim** starts a legitimate MCP session and receives session ID abc-123 **Attacker** obtains th

O3 Security · Impact-Aware SCA

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