CVE-2026-63127

A flaw was found in rmcp, a Rust SDK for the Model Context Protocol. The OAuth implementation in rmcp omits the RFC 9728 resource field from ResourceServerMetadata. This allows a malicious Model Context Protocol (MCP) server to publish metadata for a different legitimate MCP resource. Consequently, a victim connecting to the malicious server may obtain a legitimate access token, which the client then sends to the malicious server. This enables the attacker to capture the token and impersonate the victim against the legitimate MCP resource within the token's granted scopes, leading to unauthorized access and information disclosure.

This is an Important flaw in the rmcp Rust SDK, which is utilized by the goose component in Red Hat products. The vulnerability allows a malicious Model Context Protocol (MCP) server to facilitate token impersonation by omitting a critical resource field during OAuth metadata discovery. This can lead to a client unknowingly sending a legitimate access token to the malicious server, resulting in unauthorized access and potential information disclosure within the token's granted scopes. Exploitation requires user interaction, but the attack complexity is low.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.