# 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.

- **Severity:** High 8.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H`)
- **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:** Yes
- **Published:** 14 May 2026
- **Updated:** 14 May 2026
- **Also known as:** GHSA-89vp-x53w-74fx, GHSA-fvh2-gm75-j4j7, RUSTSEC-2026-0140, RUSTSEC-2026-0189

## 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.

Weakness: CWE-350 (Reliance on Reverse DNS Resolution for a Security-Critical Action), CWE-346 (Origin Validation Error)

## Which versions are affected by CVE-2026-42559?

8 open source packages ship an affected version, including dynoxide-rs, rmcp and dynoxide.


| Package | Ecosystem | 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 |
| `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.

## 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:

- Docker lab + Python PoC for CVE-2026-42559 - DNS rebinding via unvalidated Host header in the rmcp (Rust MCP SDK) Streamable HTTP server transport (github_poc): https://github.com/joaovicdev/EXPLOIT-CVE-2026-42559

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-42559 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-42559 in the wild and no public exploit code.

Affected packages and fixed versions:
- dynoxide-rs (crates.io): affected from 0.9.3; fixed in 0.9.13
- dynoxide-rs (crates.io): affected from 0.9.3; fixed in 0.9.13
- dynoxide-rs (crates.io): affected from 0.9.3; fixed in 0.9.13
- rmcp (crates.io): affected all versions; fixed in 1.4.0
- rmcp (crates.io): affected all versions; fixed in 1.4.0
- rmcp (crates.io): affected from 0.0.0-0; fixed in 1.4.0
- dynoxide (npm): affected from 0.9.3; fixed in 0.9.13
- dynoxide (npm): affected from 0.9.3; fixed in 0.9.13

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-42559/ (Markdown: https://insights.sbomify.com/CVE-2026-42559/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-42559
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-42559
- OSV: https://osv.dev/vulnerability/CVE-2026-42559

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-42559/
Summaries are generated and may contain mistakes; the official description above is the record.