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: crates.io: rmcp

### Summary The `rmcp` library does not validate the `resource` parameter in OAuth Protected Resource metadata (RFC 9728), allowing a malicious MCP server to redirect OAuth flows to a legitimate authorization server and steal the resulting access tokens.

### Details RFC 9728 specifies two MUST requirements for resource parameter validation: - Section 7.3: the client MUST ensure that the resource identifier URL it is using as the prefix for the metadata request exactly matches the `resource` value in the returned metadata document. - Section 3.3: if the `resource` value returned is not identical to the URL the client used, the data MUST NOT be used.

In the current implementation (`crates/rmcp/src/transport/auth.rs`), the `ResourceServerMetadata` struct (lines 390–394) does not include a resource field: ```rust struct ResourceServerMetadata { authorization_server: Option<String>, authorization_servers: Option<Vec<String>>, scopes_supported: Option<Vec<String>>, } ``` And discover_oauth_server_via_resource_metadata() (lines 1446–1465) proceeds without any resource URL validation.

#### Recommended fix 1. Add the `resource` field to the struct: ```rust struct ResourceServerMetadata { resource: Option<String>, // RFC 9728 REQUIRED field authorization_server: Option<String>, authorization_servers: Option<Vec<String>>, scopes_supported: Option<Vec<String>>, } ``` 2. Add validation logic after fetching metadata: ```r…

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