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DNS rebinding vulnerability in rmcp Streamable HTTP server transport

High
DaleSeo published GHSA-89vp-x53w-74fx Apr 29, 2026

Package

cargo rmcp (Rust)

Affected versions

< 1.4.0

Patched versions

1.4.0

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.

References

Related advisories (same class of vulnerability)

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

CVE ID

CVE-2026-42559

Weaknesses

Origin Validation Error

The product does not properly verify that the source of data or communication is valid. Learn more on MITRE.

Reliance on Reverse DNS Resolution for a Security-Critical Action

The product performs reverse DNS resolution on an IP address to obtain the hostname and make a security decision, but it does not properly ensure that the IP address is truly associated with the hostname. Learn more on MITRE.

Credits