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GHSA-89vp-x53w-74fx

HIGHFix: modelcontextprotocol/rust-sdk#764

GHSA-89vp-x53w-74fx is a high-severity (CVSS 8.8) CWE-346 vulnerability in rmcp. O3 Security confirms whether GHSA-89vp-x53w-74fx is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

rmcp Streamable HTTP server transport has a DNS rebinding vulnerability

Also known asCVE-2026-42559GHSA-fvh2-gm75-j4j7RUSTSEC-2026-0140RUSTSEC-2026-0189
Published
May 6, 2026
Updated
Jun 29, 2026
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed

Real-World Exposure

1 pkg affected
🦀rmcp

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io packages — download data is not available via public APIs for these ecosystems.

Description

Summary

Prior to version 1.4.0, the rmcp crate's Streamable HTTP server transport (crates/rmcp/src/transport/streamable_http_server/) did not validate the incoming Host header. This allowed a malicious public website, via a DNS rebinding attack, to send authenticated requests to an MCP server running on the victim's loopback or private-network interface — violating the MCP specification's transport security guidance.

Impact

An attacker who convinces a victim to visit a malicious page can:

  • Enumerate and invoke any tool exposed by a locally-running rmcp-based MCP server.
  • Read resources, prompts, and any state accessible via the MCP session.
  • Trigger side effects (file writes, shell execution, API calls, etc.) limited only by what tools the victim's server exposes.

Because MCP servers frequently run with the user's privileges and expose developer tooling (filesystems, shells, browser control, language servers, etc.), the practical impact can extend to arbitrary code execution on the victim's machine.

Affected Versions

rmcp < 1.4.0 — all prior releases of the Streamable HTTP server transport. Non-HTTP transports (stdio, child-process) are not affected.

Patched Versions

rmcp >= 1.4.0 (current: 1.5.1).

Patch

Fixed in PR #764 (commit 8e22aa2), released as v1.4.0 on 2026-04-09:

  • StreamableHttpServerConfig::allowed_hosts now defaults to a loopback-only allowlist: ["localhost", "127.0.0.1", "::1"].
  • All incoming HTTP requests pass through validate_dns_rebinding_headers(), which parses the Host header and returns HTTP 403 if the host is not on the allowlist.
  • Public deployments can configure an explicit allowlist via StreamableHttpService::with_allowed_hosts(...), or opt out (not recommended without an upstream reverse proxy that validates Host) via disable_allowed_hosts().

This fix validates the Host header only. Origin header validation is tracked as a defense-in-depth follow-up in #822 and is not required to block the DNS rebinding attack described here — the browser cannot forge the Host header sent to the rebound server.

Workarounds for Unpatched Users

  • Upgrade to rmcp >= 1.4.0.
  • If upgrade is not possible, place the MCP server behind a reverse proxy (e.g. nginx, Caddy) configured to reject requests whose Host header is not one of your expected hostnames.
  • Do not bind the MCP server to 0.0.0.0 without such a proxy.

Resources

Related advisories (same class of vulnerability)

  • TypeScript SDK: GHSA-w48q-cv73-mx4w
  • Python SDK: GHSA-9h52-p55h-vw2f
  • Go SDK: GHSA-xw59-hvm2-8pj6
  • Java SDK: GHSA-8jxr-pr72-r468

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
🦀crates.iormcpall versions1.4.0

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for rmcp. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

  2. Fix

    Update rmcp to 1.4.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-89vp-x53w-74fx is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

  4. How O3 protects you

    O3 pinpoints whether GHSA-89vp-x53w-74fx is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to GHSA-89vp-x53w-74fx. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

## Summary Prior to version 1.4.0, the `rmcp` crate's Streamable HTTP server transport (`crates/rmcp/src/transport/streamable_http_server/`) did not validate the incoming `Host` header. This allowed a malicious public website, via a DNS rebinding attack, to send authenticated requests to an MCP server running on the victim's loopback or private-network interface — violating the MCP specification's [transport security guidance](https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#security-warning). ## Impact An attacker who convinces a victim to visit a malicious page ca
O3 Security · Impact-Aware SCA

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