Vulnerability in rmcp (CVE-2026-42559)
Summary
Vulnerability in rmcp (CVE-2026-42559). Successful exploitation can lead to full system takeover. Exploitable via rmcp. Mitigation: upgrade to 1.4.0 or later.
Original description
The description as published by the source.
Prior to version 1.4.0, the rmcp crate's Streamable HTTP server transport did
not validate the incoming Host header.
This allowed a malicious public website, via a DNS rebinding attack, to send
requests to an MCP server running on the victim's loopback or private-network
interface.
An attacker who convinced a victim to visit a malicious page could enumerate and
invoke tools exposed by a locally running rmcp-based MCP server, read resources
and prompts, and trigger side effects limited by the tools exposed by that
server.
Non-HTTP transports such as stdio and child-process transports are not affected.
Patches
The issue was fixed in rmcp 1.4.0 by adding default loopback-only host
allowlist validation for the Streamable HTTP server transport. Incoming HTTP
requests now validate the Host header and return HTTP 403 when the host is not
allowed.
Users should upgrade to rmcp >= 1.4.0.
Workarounds
If upgrading is not possible, place the MCP server behind a reverse proxy
configured to reject requests whose Host header is not one of the expected
hostnames. Do not bind the MCP server to 0.0.0.0 without such validation.
Source: OSV.dev (Open Source Vulnerabilities) NVD GitHub Advisory OSV
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-42559 has been found in rmcp.
Attackers can target a specific entry point like rmcp over the network to misuse the product.
Successful exploitation can lead to full system takeover. CVSS score: 8.8/10.
What to do: upgrade rmcp to 1.4.0 or later.
If unsure, ask your IT team or search "rmcp CVE-2026-42559" on the vendor's site.
CVE-2026-42559 (rmcp) — CWE-346 / CVSS v3 8.8
Attack vector: remote (network-reachable) / unauthenticated
Attack surface: rmcp / Host
Patched: 1.4.0 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-346) exists in rmcp. Attackers reach the vulnerable code path via rmcp without authentication.
Check the vendor advisory for the affected versions of rmcp. Attack surface: rmcp / Host.
Severity: High (CVSS 8.8/10). Successful exploitation can lead to full system takeover.
Update to 1.4.0.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'rmcp' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'rmcp' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
rmcpを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade rmcp to 1.4.0ステージング環境で 1.4.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
References
- advisory github.com
- advisory rustsec.org
- advisory nvd.nist.gov
- advisory rustsec.org
- package github.com
- package crates.io
- package github.com
- package crates.io
- report github.com
- report github.com
- web github.com
- web modelcontextprotocol.io
- web github.com
- web github.com
- web github.com
- web github.com
- web github.com