GHSA-89vp-x53w-74fx 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 Streamable HTTP server transport has a DNS rebinding vulnerability
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 GHSA-89vp-x53w-74fx.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
GHSA-89vp-x53w-74fx plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,636 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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
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_hostsnow defaults to a loopback-only allowlist:["localhost", "127.0.0.1", "::1"].- All incoming HTTP requests pass through
validate_dns_rebinding_headers(), which parses theHostheader 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 validatesHost) viadisable_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
Hostheader is not one of your expected hostnames. - Do not bind the MCP server to
0.0.0.0without 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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | rmcp | all versions | 1.4.0cargo update -p rmcp --precise 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, 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 GHSA-89vp-x53w-74fx 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-89vp-x53w-74fx can be triaged on real exposure rather than presence alone.
Tailored to GHSA-89vp-x53w-74fx. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-89vp-x53w-74fx in your dependencies?
O3 Security finds GHSA-89vp-x53w-74fx across crates.io dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.