8.3

CVE-2026-67431: modelcontextprotocol ruby-sdk Improper Access Control

CVSS: 8.3v4.0HighDeferred
AliasesGHSA-5p9g-j988-pcwv
Published 29-07-2026Last modified 30-07-2026
Open JSON

Vulnerability summary

CVE-2026-67431 is a high (CVSS 8.3) Improper Access Control vulnerability in modelcontextprotocol ruby-sdk. EPSS estimates a 0.48% chance of exploitation in the next 30 days, higher than 40% of scored vulnerabilities.

Overall risk
33/100
0100
Medium · default modelHow is this calculated?
CVSS score
8.3/10
010
High · v4.0 (CVE.org)
EPSS score
0.48%
0%100%
Low · +0.19 pp in 30 days
KEVNot listedIn no KEV catalog
RansomwareNone recordedNo known use recorded
Public exploits0No public exploits recorded

Timeline

  1. 29-07-2026
    PublishedVulnerability first disclosed.
  2. 30-07-2026
    Last modifiedVulnerability information updated.

Description

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.

CVSS metrics

3 scores · 3 sources · 1 version

EPSS trend

Probability

0.48%

Chance of exploitation in the next 30 days

Percentile

40%

Versus all other CVEs

30-day change

+0.19 pp

From 0.29% on 19-09-2026

19-09-202619-10-2026
EPSS score
0.48%

Known exploited

Compare KEV catalogs

Not in any of the 3 tracked catalogs.

Public exploits

0 found

No public exploits indexed.

Weaknesses

CWE · root cause

Affected systems

Modelcontextprotocol1 product
RubyGems1 product
  • mcp< 0.23.0

2 vendors · 2 products

References

5 links