CVE-2026-63127 Details
Description
RMCP is an official Rust SDK for the Model Context Protocol. Prior to 2.0.0, the rmcp crate's OAuth implementation in crates/rmcp/src/transport/auth.rs omits the RFC 9728 resource field from ResourceServerMetadata and allows discover_oauth_server_via_resource_metadata to use protected-resource metadata without confirming that the returned resource identifier exactly matches the configured MCP server. A malicious MCP server can publish metadata for a different legitimate MCP resource and its authorization server, causing a victim who connects and completes the authorization flow to obtain a legitimate access token that the client subsequently sends to the malicious server. The attacker can capture the token and impersonate the victim against the legitimate MCP resource within the token's granted scopes. This issue is fixed in version 2.0.0.
A vulnerability exists in the RMCP Rust SDK's OAuth implementation, specifically in versions through 1.8.0. The issue arises because the SDK's handling of resource metadata does not validate the resource identifier against the server's URL, as required by RFC 9728. This oversight allows a malicious MCP server to spoof resource metadata, directing OAuth flows to a legitimate authorization server and stealing access tokens. When a victim completes the authorization process, the token can be used to impersonate the victim on the legitimate MCP resource, within the scopes granted by the token.
Users can upgrade to RMCP version 2.0.0, which includes the necessary validation of resource metadata to prevent spoofing.
Metrics
CVSS 4.0 Severity and Vector Strings:
VolerionCVSS 3.x Severity and Vector Strings:
No data available for CVSS Version 2.0 on this CVE.
Volerion
Assessed Sep 16, 2026CISA-ADP
Assessed Sep 16, 2026References to Advisories, Solutions, and Tools
By selecting these links, you will be leaving this site. These are references gathered from the official CVE record and are not endorsed by Volerion.
| URL | Source(s) | Tag(s) |
|---|---|---|
| https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj | CISA-ADP | AdvisoryExploitRemedyVendor |
| https://github.com/modelcontextprotocol/rust-sdk/commit/c1a8b29ff2cc45e7820b900dae42cbb4958089ec | [email protected] | Source CodeVendor |
| https://github.com/modelcontextprotocol/rust-sdk/pull/937 | [email protected] | Issue TrackingVendor |
| https://github.com/modelcontextprotocol/rust-sdk/releases/tag/rmcp-v2.0.0 | [email protected] | Release NotesVendor |
| https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-33f5-2c5q-wgwj | [email protected] | AdvisoryExploitRemedyVendor |
Weakness Enumeration
| CWE-ID | CWE Name | Source |
|---|---|---|
| CWE-345 | Insufficient Verification of Data Authenticity | [email protected] |
Affected Products
| Product | Versions |
|---|---|
| Modelcontextprotocol RMCP | <= 1.8.0 (semver) |
CPE
Remediation
| |
Change History
2 change records found show changes
| Date | Action | Recorded By |
|---|---|---|
| Sep 16, 2026 | CVE Modified | CISA-ADP |
| Sep 16, 2026 | New CVE Received | [email protected] |