MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protection
Summary
| CVE | CVE-2026-63118 |
|---|---|
| State | PUBLISHED |
| Assigner | GitHub_M |
| Source Priority | CVE Program / NVD first with legacy fallback |
| Published | 2026-07-29 20:17:10 UTC |
| Updated | 2026-07-30 19:30:33 UTC |
| 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. |
Risk And Classification
Primary CVSS: v4.0 6.9 MEDIUM from [email protected]
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:H/VI:N/VA:N/SC:H/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
EPSS: 0.001980000 probability, percentile 0.098600000 (date 2026-08-01)
Problem Types: CWE-346 | CWE-350 | CWE-346 CWE-346: Origin Validation Error | CWE-350 CWE-350: Reliance on Reverse DNS Resolution for a Security-Critical Action
| Version | Source | Type | Score | Severity | Vector |
|---|---|---|---|---|---|
| 4.0 | [email protected] | Secondary | 6.9 | MEDIUM | CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:H/VI:N/VA:N/SC:H/SI:N/SA:N/E:X/C... |
| 4.0 | CNA | DECLARED | 6.9 | MEDIUM | CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:H/VI:N/VA:N/SC:H/SI:N/SA:N |
CVSS v4.0 Breakdown
Attack Vector
NetworkAttack Complexity
LowAttack Requirements
PresentPrivileges Required
NoneUser Interaction
ActiveConfidentiality
HighIntegrity
NoneAvailability
NoneSub Conf.
HighSub Integrity
NoneSub Availability
NoneCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:H/VI:N/VA:N/SC:H/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
Vendor Declared Affected Products
| Source | Vendor | Product | Version | Platforms |
|---|---|---|---|---|
| CNA | Modelcontextprotocol | Ruby-sdk | affected < 0.23.0 | Not specified |
References
| Reference | Source | Link | Tags |
|---|---|---|---|
| github.com/modelcontextprotocol/ruby-sdk/commit/ba543083a7594e7892b29464... | [email protected] | github.com | |
| github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-rjr6-r... | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | github.com | |
| github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0 | [email protected] | github.com | |
| CVE Program record | CVE.ORG | www.cve.org | canonical |
| NVD vulnerability detail | NVD | nvd.nist.gov | canonical, analysis |
No vendor comments have been submitted for this CVE.
There are currently no legacy QID mappings associated with this CVE.