CVE-2026-42559 RMCP: DNS rebinding vulnerability in rmcp Streamable HTTP server transport
Attackers can use DNS rebinding to send authenticated requests to local MCP servers via the Rust SDK's HTTP transport.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-42559?
CVE-2026-42559 is a high-severity server-side request forgery vulnerability in modelcontextprotocol rust-sdk, published on 14 May 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
RMCP is an official Rust SDK for the Model Context Protocol. 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. This vulnerability is fixed in 1.4.0.
Which versions are affected by CVE-2026-42559?
8 open source packages ship an affected version, including dynoxide-rs, rmcp and dynoxide.
| Package | Affected | Fixed in |
|---|---|---|
dynoxide-rs crates.io | From 0.9.3 | 0.9.13 |
dynoxide-rs crates.io | From 0.9.3 | 0.9.13 |
dynoxide-rs crates.io | From 0.9.3 | 0.9.13 |
rmcp crates.io | All versions | 1.4.0 |
rmcp crates.io | All versions | 1.4.0 |
rmcp crates.io | From 0.0.0-0 | 1.4.0 |
2 more packages
dynoxide npm | From 0.9.3 | 0.9.13 |
dynoxide npm | From 0.9.3 | 0.9.13 |
How do I fix CVE-2026-42559?
Upgrade dynoxide-rs to 0.9.13 or later.
Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.
Is CVE-2026-42559 being exploited?
There is no known exploitation of CVE-2026-42559 in the wild and no public exploit code.
Public exploit code: 1 proofs of concept on GitHub.
Sources
Also known as GHSA-89vp-x53w-74fx, GHSA-fvh2-gm75-j4j7, RUSTSEC-2026-0140, RUSTSEC-2026-0189.