6.9

CVE-2026-63118: modelcontextprotocol ruby-sdk Origin Validation Error

CVSS: 6.9v4.0MediumDeferred
AliasesGHSA-rjr6-rcgv-9m7m
Published 29-07-2026Last modified 29-07-2026
Open JSON

Vulnerability summary

CVE-2026-63118 is a medium (CVSS 6.9) Origin Validation Error vulnerability in modelcontextprotocol ruby-sdk. EPSS estimates a 0.26% chance of exploitation in the next 30 days, higher than 16.1% of scored vulnerabilities.

Overall risk
28/100
0100
Medium · default modelHow is this calculated?
CVSS score
6.9/10
010
Medium · v4.0 (CVE.org)
EPSS score
0.26%
0%100%
Low · +0.06 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. 29-07-2026
    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 validate the HTTP Host or Origin request headers, which allows a malicious browser page to use DNS rebinding to reach a locally running MCP server and invoke exposed tools. This issue is fixed in version 0.23.0.

CVSS metrics

3 scores · 3 sources · 1 version

EPSS trend

Probability

0.26%

Chance of exploitation in the next 30 days

Percentile

16%

Versus all other CVEs

30-day change

+0.06 pp

From 0.19% on 20-09-2026

20-09-202620-10-2026
EPSS score
0.26%

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