GHSA-33f5-2c5q-wgwj RMCP: Missing Resource Field Validation in OAuth Protected Resource Metadata Discovery

• crates.io rmcp: < 2.0.0 → fixed in 2.0.0

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, authorization_servers: Option>, scopes_supported: Option>, } ``` 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, // RFC 9728 REQUIRED field authorization_server: Option, authorization_servers: Option>, scopes_supported: Option>, } ``` 2. Add validation logic after fetching metadata: ```rust let Some(resource_metadata) = self .fetch_resource_metadata_from_url(&resource_metadata_url) .await? else { return Ok(None); }; // RFC 9728: validate that the resource identifier matches our target server if let Some(resource) = &resource_metadata.resource { if resource.trim_end_matches('/') != self.base_url.as_str().trim_end_matches('/') { return Err(AuthError::MetadataError(format!( "Resource metadata mismatch: expected '{}', got '{}'", self.base_url, resource ))); } } ``` PoC 1. Attacker sets up a malicious MCP server at `fake-mcp.com/mcp`. 2. At `fake-mcp.com/mcp/.well-known/oauth-protected-resource`, the attacker serves metadata declaring: • resource: `real-mcp.com/mcp` (the legitimate server) • authorization_servers: the legitimate authorization server(s) of `real-mcp.com/mcp` 3. Victim configures any MCP client using `rmcp` to connect to `fake-mcp.com/mcp`. 4. `rmcp` fetches the protected resource metadata and, without validating that the `resource` field (`real-mcp.com/mcp`) differs from the configured server (`fake-mcp.com/mcp`), initiates an OAuth flow with the legitimate authorization server. 5. The victim sees a legitimate authorization prompt and completes the flow. 6. The resulting access token — valid for `real-mcp.com/mcp` — is sent to `fake-mcp.com/mcp` in subsequent requests. 7. The attacker captures the token and can impersonate the victim on `real-mcp.com/mcp`. Impact This is an access token theft vulnerability via OAuth resource metadata spoofing. All MCP clients built on `rmcp` that rely on OAuth-protected MCP servers are affected. An attacker who tricks a user into connecting to a malicious MCP server can steal valid access tokens for any legitimate MCP server, enabling full impersonation of the victim. Credit Jian Cui, Minsun Shim, Zhou Li, Xiaojing Liao University of Illinois Urbana-Champaign (UIUC) University of California, Irvine (UCI)