How severe is CVE-2026-42559?

CVE-2026-42559 has a CVSS score of 8.8/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-42559?

CVE-2026-42559 affects the following packages: rmcp (crates.io). Ecosystems affected: crates.io.

How do I fix CVE-2026-42559?

Update rmcp to 1.4.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-42559 is resolved across your whole dependency graph.

How do I detect CVE-2026-42559 in my crates.io dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for rmcp, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-42559 if there is no patch (or I can't update yet)?

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.

Is CVE-2026-42559 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-42559 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-42559?

CVE-2026-42559 has an EPSS (Exploit Prediction Scoring System) score of 0.3%, placing it in the 19th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-42559?

CVE-2026-42559 is classified as CWE-346 (CWE-346), CWE-350 (CWE-350). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-42559 published, and has it been updated?

CVE-2026-42559 was published on May 14, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🦀

🦀 crates.io

Not in CISA KEV

HIGH severity

CVE-2026-42559 — rmcp

HIGHFix: modelcontextprotocol/rust-sdk@8e22aa2

CVE-2026-42559 is a high-severity (CVSS 8.8) CWE-346 vulnerability in rmcp. A fix is available for rmcp — see the affected versions and patch details below.

RMCP: DNS rebinding vulnerability in rmcp Streamable HTTP server transport

Also known asGHSA-89vp-x53w-74fx GHSA-fvh2-gm75-j4j7RUSTSEC-2026-0140RUSTSEC-2026-0189

Published

May 14, 2026

Updated

Oct 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

A successful exploit gives an attacker total control of the affected component, not partial access.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-42559.

EPSS Exploitation Probability

via FIRST.org ↗

0.3%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs19th percentile — riskier than 19% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

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

PR: https://github.com/modelcontextprotocol/rust-sdk/pull/764
Issue: https://github.com/modelcontextprotocol/rust-sdk/issues/815
Follow-up (Origin validation): https://github.com/modelcontextprotocol/rust-sdk/issues/822
MCP transport security guidance: https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#security-warning

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

Ecosystem	Package	Vulnerable range	Fix
🦀crates.io	`rmcp`	all versions	1.4.0`cargo update -p rmcp --precise 1.4.0`

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for rmcp, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update rmcp to 1.4.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-42559 is resolved across your whole dependency graph.
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.

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