CVE-2026-63118: 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 vali

unknownCVE-2026-63118

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.

Details

Source: NVD Recent CVEs (US · database · site)
Severity: unknown
Published: 2026-07-29
Last updated: 2026-07-30
Exploitation: Not in CISA KEV at last sync

Original advisory: https://nvd.nist.gov/vuln/detail/CVE-2026-63118

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-631180.20% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 10% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-63118	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

mediumGHSA-rjr6-rcgv-9m7m: MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protectionghsa

Recent advisories for MCP Ruby SDK

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

highGHSA-5p9g-j988-pcwv: MCP Ruby SDK: Ruby SSE Session Poisoningghsa · 2026-07-30
highGHSA-h669-8m4g-r2hc: MCP Ruby SDK: Unbounded JSON-RPC request body causes uncontrolled memory allocation in St…ghsa · 2026-07-30
mediumGHSA-52jp-gj8w-j6xh: MCP Ruby SDK: Unbounded session retention in StreamableHTTPTransport allows memory exhaus…ghsa · 2026-07-30
mediumGHSA-7683-3w9x-ch42: MCP Ruby SDK: Unbounded line buffer in stdio transports leads to memory exhaustion (DoS)ghsa · 2026-07-30
mediumGHSA-rjr6-rcgv-9m7m: MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protectionghsa · 2026-07-30
highCVE-2026-67432: MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to…nvd · 2026-07-29