RMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery

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.

Source published

16 Sep 2026 14:50:54 UTC

Source modified

16 Sep 2026 15:39:26 UTC

Observed here

23 Oct 2026 15:48:27 UTC

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CVSS 3.1 · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N