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Fencer Intel
high 8.3

CVE-2026-67431 json

MCP Ruby SDK: Ruby SSE Session Poisoning

published 3mo ago modified 18d ago also EUVD-2026-50501 , GHSA-5p9g-j988-pcwv

cvss
8.3
version 4.0, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 30%
public exploits
0
none known

Summary

MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not bind a session ID to a session owner, allowing an attacker with a stolen session ID to send tools/call requests that execute in the victim's session. This issue is fixed in version 0.23.0.

Patch / fix

  • rubygems mcp 0.23.0
  • lfprojects mcp_ruby_sdk 0.23.0

Is CVE-2026-67431 in your dependencies? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

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CISA SSVC

Exploitation
Proof of concept is public
Automatable
yes
Technical impact
total
Assessed
3mo ago

Exploit prediction

0.36% chance of exploitation in the next 30 days, higher than 30% of all scored CVEs (FIRST EPSS, 2026-11-11).

Exploits and checks

  • github.com https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 4.0 8.3 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N
CNA 4.0 8.3 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/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

Affected packages

ecosystem package affected fixed in
rubygems mcp
<0.23.0
0.23.0

Affected products

  • modelcontextprotocol ruby-sdk

Classification

Weakness

References

Sources

Fencer Intel 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.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.