CVE-2026-42559 is a high-severity (CVSS 8.8) CWE-346 vulnerability in rmcp. O3 Security confirms whether CVE-2026-42559 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
RMCP: DNS rebinding vulnerability in rmcp Streamable HTTP server transport
Real-World Exposure
rmcpReal-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
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | rmcp | all versions | 1.4.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for rmcp. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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.
How O3 protects you
O3 pinpoints whether CVE-2026-42559 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-42559. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-42559 in your dependencies?
O3 detects CVE-2026-42559 across crates.io dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.