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Vulnerability · Published Sep 16, 2026 · Assigned by GitHub_M

CVE-2026-63127RMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery

High 8.2

In plain English

A high-severity insufficient verification of data authenticity in rust-sdk from modelcontextprotocol. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 9% of all known vulnerabilities).

Severity (CVSS 3.1)8.2High
Exploit odds (30 days)0.2%top 90.7% of all CVEs
Exploited in the wildNo reportsCISA SSVC: poc
Attack toolingNoneno public tooling known

Official 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.

Affected products (as reported)

VendorProductVersions
modelcontextprotocol rust-sdk < 2.0.0

Weakness: CWE-345 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

References