Add advisory for rmcp DNS rebinding

Author: DaleSeo

crates/rmcp/RUSTSEC-0000-0000.md

@@ -0,0 +1,51 @@
+```toml
+[advisory]
+id = "RUSTSEC-0000-0000"
+package = "rmcp"
+date = "2026-04-29"
+url = "https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-89vp-x53w-74fx"
+cvss = "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H"
+keywords = ["dns-rebinding", "mcp", "http"]
+aliases = ["CVE-2026-42559", "GHSA-89vp-x53w-74fx"]
+related = ["RUSTSEC-2026-0140", "GHSA-fvh2-gm75-j4j7"]
+references = [
+    "https://github.com/modelcontextprotocol/rust-sdk/pull/764",
+    "https://github.com/modelcontextprotocol/rust-sdk/issues/815",
+    "https://github.com/modelcontextprotocol/rust-sdk/issues/822",
+    "https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#security-warning",
+]
+
+[versions]
+patched = [">= 1.4.0"]
+```
+
+# DNS rebinding vulnerability in rmcp Streamable HTTP server transport
+
+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.